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Optimum insulation thicknesses of pipes with respect to different insulation materials, fuels and climate zones in Turkey
Energy 113 (2016) 991e1003
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Optimum insulation thicknesses of pipes with respect to different insulation materials, fuels and climate zones in Turkey Mustafa Ertürk Department of Electricity and Energy, Balıkesir Vocational School, Balıkesir University, 10145, Balıkesir, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 March 2016 Received in revised form 20 July 2016 Accepted 22 July 2016 Available online 3 August 2016
This article is about the effects of insulation thickness on the life cycle costs of steel pipes inside the pit (channel, duct) or above-ground structures with different diameters and sensitivity to economic parameters. The heating degree-day and life cycle cost procedures are used for the optimization and sensitivity analyses. For Afyon province in Turkey the fuels are coal, natural gas and fuel-oil. The insulation materials are rock wool, EPS and XPS. The results show that consideration of all the physical and economic conditions appear with the optimum insulation thickness that varies from 5 cm to 16 cm. Both lifetime and unit cost of insulation are sensitive to the insulation thickness. Additionally, the increased discount rate and cost per unit volume of insulation material lowers the optimum insulation thickness. In conclusion, it is expected that this study will provide a guide for engineers, where insulation is used for large diameter pipes. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Thermal insulation Case study Pipe sizes Optimum insulation thickness Sensitivity
1. Introduction Currently, due the effects of limited energy resources, rapid energy consumption and overuse of resources, thermal insulation has gained great interest and importance for many countries. In the industry, there are complicative and costly pipe installation and insulation applications. On the one hand, insulation should reduce energy consumption in buildings, mechanical installations and industrial systems. It also plays an important role in reducing energy loss from the transmission portion of heating and cooling loads in buildings and industrial systems. Insulation appears as one of the most important methods to reach environmental sustainability and energy efficiency in modern cities and systems [1e3]. Large-scale heating systems provide heating and hot water needs for industrial complexes, community housings, neighborhoods in the cities by carrying heat production from one or more energy resources through pipe systems. The important cost components in a district heating system are transmission and distribution pipe configurations, which frequently range from 40% to 60% of the total project cost [4]. The majority of these costs are related to the installation in the pipe system. The use of uninsulated pipes allows reduction in pipe material costs by more than 50% [4]. If insulation material is not used properly the heat losses increase,
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.energy.2016.07.115 0360-5442/© 2016 Elsevier Ltd. All rights reserved.
which lead to the increases in the system capacity (boiler, pipe, pump) by increasing both the initial investment costs and the operation expenses. Increase in the system flow rates or compensation may lead to large pumping costs. It is important to choose the most efficient and appropriate material in the practical application. One way to increase the energy efficiency and cost reduction in such a system is to reduce heat loses from transmission/distribution by more appropriate and efficient district heating design. The most effective way to reduce heat losses is to choose pipe and insulation materials, and then to determine pipe diameter and insulation thickness [5]. The most important difference in insulating pipes, compared to buildings is the temperature levels that are encountered in pipes are much higher than temperature levels in buildings. As a result, good insulation for installations may provide much greater energy saving in comparison to buildings. Depending on the fluid temperature within the pipe, errors in material and thickness choice may cause many problems leding to condensation and freezing. Insulation lowers boiler capacity, numbers of radiators and pipe diameters [6]. In the mechanical installations and industry, especially pipe insulation allows larger heat loss/gain and financial gains. As insulation thickness increases, savings increase and investment costs also increase. Optimum insulation thickness calculations should be completed in the type and thickness determination of the heat insulation material.
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Recent studies of optimization in pipe insulation thickness have increased (e.g. Refs. [7e24]). This is due to the fact that pipes, tanks, depots, air conditioning ducts, valves and fittings in buildings, mechanical installations and industry form a large source of energy consumption. Accordingly, researchers expended efforts in order to reduce the heat losses. At the same time, it may reduce the environmental effects of fuels that are used to produce electricity. Under optimization conditions, insulation thickness is one of the most effective ways to conserve energy. In the previous studies, Ito et al. [7] and Boehm [8] used multipurpose functions and analytical methods to reduce heat loss and insulation amount in pipe systems. Zaki and Al-Turki [9] completed an economic analysis for thermal insulation of double-layer insulated petrol pipelines (nominal pipe sizes of 100e273 mm) by using different insulation materials (rockwool and calcium silicate) with an explicit non-linear function. Sahin [10] numerically modeled variations in insulation thickness to maximize pipe outer surface temperature under extraterrestrial radiation conditions. Another study by Sahin and Kalyon [11] analytically modeled the variation in insulation thickness of pipes under convection and radiation conditions. Ozturk et al. [12] presented four different thermo-economic methods to optimize pipe diameter and insulation thickness based on heat losses, insulation costs, total costs and exergy efficiency for optimum design in hot water pipe systems. The method that produces the best result is reported to be the one with exergy and cost parameters. Karabay [13] used exergo-economic analysis to determine optimum insulation thickness of hot water distribution pipes (nominal pipe sizes of 100e1000 mm). The effects of mass flow rate, annual operating time, amortization period, and water temperature on the optimum pipe diameter and insulation thickness are also researched by Soponpongpipat et al. [14], who worked to determine the optimum insulation thickness based on the thermo-economic method for double-layer insulation of air conditioning ducts (circular galvanized steel duct of 500 mm). The calculations are completed for insulation material such as rubber and fiber glass based on annual equivalent full load cooling time of the air conditioning system. Bahadori and Vuthaluru [15] estimated optimum insulation thickness with a simple correlation as a function of thermal conduction coefficient of steel pipes and equipment diameter of insulation material at 100 C, 300 C, 500 C and 700 C. Another study by the same authors [16] helps to calculate a simple correlation for estimation of heat flow through insulation, thermal resistence and insulation thickness for flat and cylindrical surfaces. Kecebas et al. [17] modified life cycle cost analysis to determine optimum insulation thickness for different pipe diameters in a district heating pipeline network (nominal pipe sizes of 50e200 mm). They calculated the amount of heat carried by fluid within the pipe using the degree-day method. Later, they completed life cycle cost analysis by consideration of the heat loss from the pipe surface. For calculations, as insulation material the rock wool, coal, natural gas, fuel oil and geothermal energy are used as fuel sources. They reported optimum insulation thicknesses between 8.5 cm and 22.8 cm. They also reported that as greater savings from large diameter pipes should be considered separately. Basogul and Kecebas [18] used modified life cycle cost analysis to investigate environmental assessment of insulation pipes in district heating pipelines. They used different fuel types, a variety of insulation materials and nominal pipe diameters for economic and environmental assessment. They reported that the largest optimum insulation thickness, fuel savings and emissions are valid for 200 mm nominal pipe size. Apart from the models used by Ozturk et al. [12] and Karabay [13], Kecebas [19] suggested another method that combines exergy analysis and modifies life cycle cost analysis to determine optimum insulation thickness for
pipes. The analysis investigated the effect of combustion parameters such as excess air, stack gas temperature, and combustion chamber temperature on optimum insulation thickness, which reduces waste gases and combustion chamber temperature with increasing fuel entry temperature and decreasing stack. In addition, the optimum insulation thicknesses obtained by exergoeconomic analysis are reported as higher than those obtained by energo-economic analysis. Kayfeci [20] used the method recommended by Kecebas et al. [17] for estimation of the optimum insulation thickness in various heating pipe diameters with different insulation materials like EPS, fiberglass, foamboard, rockwool and XPS. The highest energy saving for small diameter pipes is obtained by the fiber glass as insulation material, while for large diameter pipes the EPS yields the highest energy saving. The artificial neural network (ANN) is employed by Kayfeci et al. [21] used to estimate optimum insulation thickness easily for pipe insulation applications. All parameters related to thermal insulation optimization are obtained from insulation markets and then it is determined by a life cycle cost analysis modification. The ANN model can easily and accurately calculate optimum insulation thickness for any pipe diameter. Kaynakli [22] reviewed the previous studies to determine economic optimum insulation thickness for pipes and ducts. They investigated heat transfer equations, optimization procedures and economic analyses methods for cylindrical pipes and ducts by comparison. More recently, Yildiz and Ersoz [23] determined the optimum insulation thickness for pipe lines in an air conditioning system with VRF using R-410A as cooling fluid both in heating and cooling modes. In heating mode, the optimum insulation thicknesses for high pressure gas and low pressure fluid pipelines are calculated as 1.6 cme2.0 cm and from 1.1 cm 1.3 cm, respectively. In cooling mode, the thickness varies from 0.7 to 0.8 cm. Another study by the same authors [24] researched to the variation in optimum insulation thickness with wind speed within HVAC ducts. For fuel types such as coal, fuel-oil, LPG and natural gas and insulation materials such as fiberglass and rockwool, life cycle costs are optimized, and they reported that at high wind speeds insulation is necessary. In the literature, there are many studies on the optimum insulation thickness of pipes and ducts based on the modified life cycle cost analysis that is suggested by Kecebas et al. [17]. These studies researched to the effects of different parameters such as fuel types, insulation materials, pipe diameters, heat degree-days, working fluid, combustion and economic parameters on optimum insulation thickness. However, apart these studies, the current study concentrates on the following points. (i) Determination of the optimum insulation thickness for large diameter pipes, (ii) Investigation of the variation in optimum insulation thickness, energy saving and payback period depending on variations in pipe diameters, (iii) Completion of the sensitivity analysis on the parameters of optimum insulation thickness, energy saving and payback period. This study is expected to provide the most important contribution to thermal insulation thickness analysis in case of for large diameter pipes under optimization conditions and the economic parameters.
2. Material and method Generally, heat insulation is a precaution taken to prevent heat loss and gains caused by differences in temperature. It is, therefore, installed in buildings, mechanical installations and industries.
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Depending on the hot or cold forms of the fluid passing through pipes, tanks, depots, air conditioning ducts, valves and fittings forming mechanical installations, it is necessary that they should be insulated with appropriate material characteristics and thickness. The insulation application for mechanical installations considered in this study for pipe insulation and its layers is shown in Fig. 1. The following assumptions are considered during life cycle cost (LCC) analysis: - The pipe has a smooth cross-section per unit length inside the pit (channel, duct) or above-ground structures, - Heat losses from the pipe surface can be determined by heating degree-days, - Ambient and fluid temperatures are constant, - Fluid (hot water) has fixed speed within the pipe, - Air speeds are constant inside the pit (channel, duct) or aboveground structures, - There are steady state flow controlled volume conditions within the pipe, - Temperature and pressure drops are ignored along the pipe, - Insulation resistence noise and fire are negligible in the corrosion applications, - Insulation material has high value for steam diffusion resistance coefficient.
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For un-insulated and insulated pipe layers, it may be written, respectively, as follows:
Uunins ¼
Uins ¼
1 Rp;unins
1 Rp;ins
(2b)
Total internal resistence in un-insulated pipe layers (inner fluid þ pipe þ outer fluid) is given as,
ln
Rp;unins ¼
Qp ¼ UAðTad To Þ
where Tad and To are average design temperatures of inside fluid and outside air, respectively; U is the total heat transfer coefficient.
(3)
with Ai¼2p L r0 and Ao¼2p L rn. For the total internal resistence of insulated pipe layers (inner fluid þ pipe þ insulation material þ outer fluid), the following expression is valid.
Rp;ins ¼
(1)
r1
r0 1 1 þ þ hi Ai ho Ao 2pLk1
ln
The heat losses from the pipe may be described as below.
(2a)
r1 r0
ln
r2
r1 1 1 þ þ þ hi Ai 2pLk1 2pLkins ho A0o
(4)
where A0o ¼ 2p L r2 , k1 is the heat transfer coefficient of pipe material and ki is the thermal conductivity of insulation material. The convection heat transfer coefficients for the interior and exterior of the pipe system may be calculated, respectively as [25],
Fig. 1. The hot water pipeline with insulation and definition of various parameters.
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hi ¼ 0:023 Re0:8 Pr0:4
ki 2r0
(5)
processing plants. The annual heat amount transfer for these thermal systems may be determined by using heating degree-days [17e21]. Annual heating degree-day (HDD) value can be calculated on the basis of hourly data according to,
and
ho ¼ 11:58ð1=dÞ0:2 ½2=ððTms To Þ 546:3Þ0:181 ðTms To Þ0:266 ð1 þ 2:86 Vair Þ0:5
Table 1 Parameters used in calculation.
(6)
HDD ¼ ð1 yearÞ
365 X ðTb Tsa Þ
(7)
1
Parameters/Technical characteristics
Values
Heating degree-days (HDD) Mean hot water temperature [(70 þ 90)/2] Outside air temperature Mean outside surface temperature Velocity of inside fluid Velocity of outside air Fuel types Various insulation materials Nominal pipe sizes of stainless steel pipe Discount rate (i) Increase rate (d) Lifetime (N)
2328 C-days 80 C 15 C 93 C 0.8 m/s 0.2 m/s See Table 2 See Table 3 See Table 4 13% 6.5% 10 years
Here, only the positive value of temperature difference is noted, and the temperature difference is zero if Tb < Tsa. In this study, for the basic temperature of 18 C in Afyon province (Turkey), the heating degree-day value is adapted from Buyukalaca et al. [27] and given in Table 1. It is necessary to conserve the heat transfer by pipe, and hence, the use of insulation becomes necessary. The annual heat losses from heat carried along the pipe may be calculated as,
QA ¼ 86; 400 HDD U
(8)
The annual fuel consumption amount to provide the amount of heat to move the pipe is calculated as, Table 2 Some properties of fuels and their system used in the study [28]. Fuels
Pricea
Hu
h
Coal Natural gas Fuel-oil
0.3129 ($/kg) 0.4453 ($/m3) 0.7935 ($/kg)
29.260 106 (J/kg) 34.485 106 (J/m3) 41.278 106 (J/kg)
65% 95% 80%
a
Assuming 1 $ ¼ 2.6843 Turkish Liras (TL) as of June 2015.
where according to [26] d ¼ 2(r1þd), Tms is the mean outside surface temperature and Vair is the air velocity. Information about the amount of heat transfer or fluid storage inside the cylindrical pipes/depots/tanks is vital for thermal systems like district heating/cooling, industrial and chemical
mf ¼
86; 400 HDD U Hu h s
(9)
where Hu is lower heating value of the fuel and hs is the efficiency of the heating system. The total annual cost, Cf, of energy spent to heat transportation in the pipe is determined as,
Cf ¼ mf CF
(10)
where CF is unit cost of any fuel as $/kg, $/m3, $/kW h. On the other hand, the total cost of insulation, Cins, material at pipe insulation application is calculated as,
Table 3 Some properties of insulation materials used in the study. Insulation material
Standard
Density kg/m3
Conductivity kins, W/m-K
Cost CI, $/m3
Rockwool EPS XPS
TS EN 13162 TS EN 13163 TS EN 13164
100 16 28e30
0.040 0.036 0.031
44 32 62
Table 4 Some properties of stainless steel pipe used in the study. Nominal pipe size
Outer diameter, r1
Wall thickness, t
(mm)
(inch)
(mm)
(mm)
50 100 200 400 600 800 1000
2 4 8 16 24 32 40
60.3 114.3 219.1 406.4 610.0 813.0 1016.0
3.91 6.02 8.18 9.53 9.53 9.53 9.53
Weight class
Sch No
Unit weight
STD STD STD STD STD STD STD
40 40 40 30 20 10 e
5.44 16.07 42.55 93.27 141.12 188.82 236.53
(kg/m)
Note: For stainless steel pipe (ANSI B 36.10), the density, melt temperature and conductivity are 7.99 g/cm3, 1371e1399 C and 16.2 W/m-K, respectively.
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Fig. 2. Effect of insulation thickness on annual cost at (a) 50 mm and (b) 1000 mm NPSs (for coal and rockwool).
Cins ¼ CI V
(11)
where V ¼ pðr22 r12 Þ L is the volume of insulation material and CI is the cost of insulation material per unit volume as $/m3. After the energy or fuel and insulation costs calculations, the LCC analysis is conducted and it is used to assess the insulation economy in insulation technologies and projects and in this study,
the P1-P2 method is used. Life cycle energy ratio P1 and life cycle expenditure ratio (initial and additional capital investments) P2 may be defined according to the following expressions.
P1 ¼
N if i ¼ d 1þi
Fig. 3. Effect of insulation thickness on energy saving for different nominal pipe sizes at (a) coal and (b) natural gas and (c) fuel-oil fuels (rockwool).
(12a)
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Fig. 4. Effect of insulation thickness on energy saving for various fuel types at (a) 50 mm and (b) 1000 mm NPSs (rockwool).
" # 1 1þi N 1 P1 ¼ if isd ðd iÞ 1þd
(12b)
and
P2 ¼ 1 þ P1 Ms
Rv ð1 þ dÞN
(13)
where i is discount rate, d is increase rate, N is lifetime, Ms is the ratio of the annual maintenance and operation cost to the original first cost and Rv is the ratio of the resale value to the first cost. There are no maintenance and operating costs at insulation application, the P2 value is taken as 1. The total cost, Ct, expended for heat transfer by insulated pipes and the energy saving S is calculated as,
Ct ¼ P1 Cf þ P2 Cins
(14)
and
S ¼ P1 Cf0 P2 Cins :
(15)
where Cf0 is the difference between the energy cost for the uninsulated and insulated pipes. In order to determine the optimum insulation thickness, the outermost diameter of the insulated pipe layer is determined by
minimizing Eq. (14). For this purpose, the differential of the total cost, Ct, is obtained and brought to zero according to outer diameter, r2. Hence, the optimum insulation thickness is found as dins¼r2r1. For the payback period, an appropriate P1 is chosen as one of the cases of Eq. (12a) or Eq. (12b) for P2 ¼ 1. Here, appropriate P1 is Eq. (12b) for isd due to discount rate, i, and increase rate, d, which are given in Table 1 as 13% and 6.5%, respectively. Subsequently, Eq. (12b) is substituted into Eq. (15) and brought to zero, and hence, the payback period, Np, is calculated. With the use of the parameters in Table 1, all calculation procedures are completed with the MATLAB optimization Toolbox. The characteristics of fuel types, insulation material types and stainless steel pipe diameters are given in Tables 2e4, respectively. 3. Results and discussion When the fuel type coal and insulation material of mineral wool are used, the variation in pipe insulation thickness annual costs for different diameter pipes is shown in Fig. 2, which implies that insulation pipes annual fuel costs begin to reduce and at this point insulation costs start to increase. However, the total annual cost including annual fuel and insulation costs falls with the use of insulation material but increases after a certain insulation thickness. This point is called as the optimum insulation thickness in insulation applications. In Fig. 2, a comparison is given between the insulation thickness annual costs for the optimum insulation
Fig. 5. Effect of insulation thickness on energy saving for various insulation materials at (a) 50 mm and (b) 1000 mm NPSs (coal).
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Fig. 6. Effect of insulation thickness on payback period for different nominal pipe sizes at (a) coal, (b) natural gas and (c) fuel-oil fuels (rockwool).
thickness of the smallest (50 mm in Fig. 2a) and largest (1000 mm in Fig. 2b) and the pipe diameter. The optimum insulation thicknesses for 50 mm and 1000 mm pipe diameters are found as 7.38 cm and 11.70 cm, respectively. The annual total costs equivalent to these values are 5.40 $/m-yr and 45.06 $/m-yr. As large diameter pipes are required to transfer more heat than small diameter pipes, annual cost due to heat loss is a large value. Additionally, in Fig. 2b the insulation cost curve is linear for large diameter pipes, while for small diameter pipes Fig. 2a shows a non-
linear curve, which is due to the geometry of the pipe. The more energy one needs to conserve, the more money should be paid for adding extra thickness of insulation. However, above a certain insulation thickness (optimum insulation thickness), the potential energy savings do not justify the extra cost. This is because of more insulation addition by increasing thickness with diminishing returns on the energy savage. Fig. 3 shows the variation of energy savings in pipes with different diameters for coal, natural gas and fuel-oil fuels. In the three graphics, the amount of
Fig. 7. Effect of insulation thickness on payback period for various fuel types at (a) 50 mm and (b) 1000 mm NPSs (rockwool).
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Fig. 8. Effect of insulation thickness on energy saving for various insulation materials at (a) 50 mm and (b) 1000 mm NPSs (coal).
energy saving increases with increasing insulation thickness. However, after a certain point it starts to fall. This value, where energy saving is at the maximum, is called as the optimum insulation thickness. Furthermore, in all graphics as pipe diameter increases, the energy saving value also increases. As a result, it is possible to obtain greater energy savings with large diameter pipes. The energy saving values obtained for fuel-oil in Fig. 3c is greater than those for the other two fuels. This due to the fact the unit cost of the fuel. In other words, the energy savings are equally high from the use of high-cost fuels. At the optimum insulation thickness of
50 mm diameter pipe, the energy savings with coal, natural gas and fuel-oil are 10.72, 8.68 and 16.23 $/m-yr, respectively, while at that value of 1000 mm pipe, they are calculated as 163.17, 138.05 and 231.04 $/m-yr, respectively. Depending on the amount of energy saving, the comparisons of different pipe diameters and various fuel types are given in Fig. 4. The higher cost of the fuel directly affects the amount of energy saving. According to this situation, the fuel with most saving is fueloil, which is followed by coal and natural gas. For all fuel types in Fig. 4, as the diameter of the insulated pipes increases, the energy
Fig. 9. The change of the optimum insulation thickness depending on the nominal pipe sizes for the parameters such as (a) rockwool, (b) EPS, (c) XPS and (d) coal.
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Fig. 10. The change of the energy saving depending on the nominal pipe sizes for the parameters such as (a) rockwool, (b) EPS, (c) XPS and (d) natural gas.
saving also increases. In the small diameter pipes, as insulation thickness increases, the saving curves fall rapidly, while this situation does not apply for large diameter pipes. There is a similar energy saving at the optimum insulation thickness and all insulation thicknesses above this value for large diameter pipes. It is observed that the amount of energy saving in small diameter pipes falls faster than that of large diameter pipes. The reason is due to the fact that the cost of insulation installation continues to increase non-linearly in small diameter pipes, while it increases as close-tolinear in large diameter pipes. The effect on energy saving of insulation materials in small and large diameter pipes is shown in Fig. 5. At the optimum insulation thickness, it is observed that the energy saving values of all insulation materials are similar. For example, the energy savings for rockwool, EPS and XPS insulation materials at a 50 mm pipe are 10.72, 11.39 and 11.15 $/m-yr, respectively, while for a 1000 mm diameter pipes these values are 163.17, 160.59 and 151.54$/m-yr, respectively. As stated previously, the energy saving curves for insulation materials at small diameter pipes decreases rapidly, while this situation does not apply to large diameter pipes. As a result, the type of insulation material does not matter for large diameter pipes. The ratio between the cost of the insulation material and the thermal conductivities are equal for different insulation materials. The payback period of costs for energy systems is an important projection for many investors, implementers and users. The correlation between insulation thickness and payback period is given in Fig. 6. By using rock wool insulation, the payback periods for coal, natural gas and fuel oil are presented in Fig. 6a, b and c, respectively.
In all graphs, depending on insulation thickness increase, the pipe diameter reduces with the increase in the payback period. For all large diameter pipes, the payback periods are nearly the same. For example, in Fig. 6a the payback period value for a 50 mm pipe of coal material is 0.79 years, while for a 1000 mm pipe it is 0.99 years. The same situation for natural gas in Fig. 6b is as 0.85 and 1.09 years, and it is 0.68 and 0.81 years for fuel oil in Fig. 6c. All pay-back period values are very close to each other. In Fig. 7 the variation is given in payback period against insulation thickness for various fuel types. As a result of increasing insulation thickness, the payback period increases. The payback period curves for small diameter pipes in Fig. 7a show a non-linear increase, while for large diameter pipes (Fig. 7b) these curves appear close-to-linear increase. The shortest payback period is for fuel oil, which is followed by coal and natural gas. The variation of payback period with insulation thickness is given in Fig. 8 for different insulation materials with similar results in Fig. 7. The XPS insulation material has the largest value of payback period for insulation materials and its lowest value is for EPS. In addition, energy savings are almost similar to Fig. 5b while the pay-back periods vary significantly in Fig. 8b. This is due to the difference in the cost of insulation materials per unit volume. The variation graphics of optimum insulation thickness are illustrated in Fig. 9 based on different parameters such as rock wool, EPS, XPS and coal according to pipe diameters. It is that increase in the optimum insulation thickness appears with increase in the pipe diameter. However, at large diameter pipes, the optimum insulation thickness begins to be closer to the same value. In other words, this means that, a fixed optimum insulation thickness may
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Fig. 11. The change of the payback period depending on the nominal pipe sizes for the parameters such as (a) rockwool, (b) EPS, (c) XPS and (d) fuel-oil.
be used for large diameter pipes, especially above 400 mm. For example, an optimum insulation thickness of 11.88 mm for 400 mm in the usages of coal and EPS is suitable for use in the other largescale pipes. As can be seen from all the graphics in Fig. 9, all insulation materials and fuel types provide similar results. This situation is related to the reduction in pipe outer surface area against flow capacity. This relationship reduces heat losses in the fluid flowing along the pipeline. The optimum insulation thicknesses belong to all fuel types, insulation materials and pipe diameters are given in Appendix 1 in detail. Fig. 10 shows the variation in energy saving with pipe diameters for the parameters of the previous figure. This figure shows that while small diameter pipes provide small amounts of energy saving, large diameter pipes (above 200 mm) provide comparatively larger energy savings. The fuel-oil and the highest fuel cost for different insulation materials provide greater savings than other fuels. However, depending on fuel type, the energy savings of insulation materials are similar (see Fig. 10d). Fig. 11 presents the variation in payback period according to pipe diameters. From the figure, the payback period value increases by a small amount as the pipe diameter increases. The payback period of natural gas is longer than the others. However, when considered in terms of years, there is a little difference in the range of 0.51 and 1.14 years per m and similar results are observed in between insulation materials. The results of the sensitivity analysis in the calculation method of optimum insulation thickness, energy saving and payback period for XPS, coal and pipe sizes with 50 and 1000 mm are graphically illustrated in Fig. 12. In the sensitivity analysis, the parameters are
interest rate, discount rate, lifetime, unit cost of fuel, unit volume cost of insulation and heating degree-day. With percentage increases in these parameters, the variations in optimum insulation thickness, energy saving and payback period are observed. Fig. 12a and b are for the lifetime and the unit volume cost of insulation and they are the most sensitive parameters for optimum insulation thickness. These parameters appear to be more effective for large diameter pipes. Additionally, increases in discount rate and insulation material unit volume cost lower the optimum insulation thickness. In Fig. 12c and d, the energy savings for 50 mm and 1000 mm pipes are affected mostly by lifetime and discount rate. For small diameter pipes, any percentage increase in lifetime and unit cost of fuel provides the highest energy savings, while for large diameter pipes both parameters and additionally heating degree-day are effective. For large diameter pipes, the parameters show the greatest effect on energy saving and the discount rate reduces energy savings. The sensitivity of the parameters on payback period is specified in Fig. 12e and f. They clearly show that the lifetime increases the payback period. From these figures, the payback period decreases with increasing heating degree-day. However, for large diameter pipes, the discount rate effect decreases slightly as in Fig. 12f. 4. Conclusions This study is related to the determination of optimum insulation thickness for different diameter pipes (50 mm, 100 mm, 200 mm, 400 mm, 600 mm, 800 mm and 1000 mm) using the life cycle cost (LCC) analysis over a 10 year life-time. The variations of optimum
M. Ertürk / Energy 113 (2016) 991e1003
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Fig. 12. The results of the sensitivity analysis for the optimum insulation thickness, energy saving and payback period.
insulation thickness, energy saving and payback period for different pipe diameters are presented after proper calculations. Additionally, a sensitivity analysis is considered in order to determine the effects of parameters such as interest rate, discount rate, lifetime, fuel unit cost, cost of insulation unit volume and heating degreeday on optimum insulation thickness, energy saving and payback period. The used fuels are coal, natural gas and fuel-oil and insulation materials such as rock wool, EPS and XPS for the heating degree-day values of the Afyonkarahisar province in Turkey. Some
of the concluding remarks from this study may be listed as follows. (a) When considered all parameters, the optimum insulation thickness varies from 5.18 cm to 15.80 cm. However, for all large diameter pipes the use a complete value is recommended for optimum insulation thickness. For example, the optimum insulation thickness of 10.52 cm should be used in pipes from 400 mm to 1000 mm diameter for rock wool and coal.
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(b) Due to pipe geometry, the insulation cost curve is not linear for small diameter pipes, while for large diameter pipes this linearity increases. This is related to the fact that in large diameter pipes the ratio of the exposed pipe outside surface area compared to the flow capacity is reduced. (c) Large diameter pipes provide greater energy savings compared to small diameter pipes. (d) The most energy saving fuel type related to fuel costs is found as fuel-oil, which is followed by coal and natural gas. In addition, the most saving among the insulation materials is the EPS. (e) Keeping other parameters constant, there is no large change in payback period as pipe diameter increases. (f) Among the evaluated parameters, the most sensitive parameters in the determination of optimum insulation thickness are the lifetime and unit cost of insulation. Additionally, the optimum insulation thickness decreases with increase in the discount rate and unit volume cost of insulation material. Acknowledgements The author is very grateful to the reviewers due their appropriate and constructive suggestions as well as their proposed corrections, which have been utilized in improving the quality of the paper. Nomenclature A Ai Ao A0o Cf Cf0 Cins CI CF Ct d hi ho Hu i ki kins
Total surface area of pipe (m2) Inside surface area of pipe (m2) Outside surface area of last layer of pipe (m2) Outside surface area of nth layer of pipe (m2) Annual energy cost ($) Difference between energy cost of the un-insulated and insulated pipes ($) Total insulation cost ($) Cost of insulation material per unit volume ($/m3) Unit cost of fuel ($/kg, $/m3, $/kW h) Total cost ($) Increase rate (%) Convection heat transfer coefficient for inside of pipe (W/ m2 K) Convection heat transfer coefficient for outside of pipe (W/m2 K) Lower heating value of the fuel (J/kg, J/m3, J/kW h) Discount rate (%) Heat transfer coefficient of fluid in the inside of pipe (W/ m K) Thermal conductivity of insulation material (W/m K)
L mf Ms
Length of pipe (m) Annual fuel consumption (kg, m3, kW h) Ratio of the annual maintenance and operation cost to the original first cost ($) N Lifetime (years) Np Payback period (years) Pr Prandtl number P1 Life cycle energy P2 Life cycle expenditures QA Annual heat losses occurring from heat carried along the pipe (W) Qp Heat losses from the inside of the pipe to the outside of the pipe through the pipe (W) Re Reynold number Rp Total internal resistance of piping system (K/W) Rp,ins Internal resistance of insulated pipe system (K/W) Rp,un-ins Internal resistance of un-insulated pipe system (K/W) Rv Ratio of the resale value to the first cost ($) r0,r1, etc. Radius of pipe system (m) S Energy savings ($) Tad Average design temperature of inside fluid (K) Tb Base temperature (K) Tms Mean outside surface temperature of pipe system (K) To Design temperature of outside air (K) Tsa Solar-air temperature for each hour (K) U Total heat transfer coefficient (W/m2 K) Uins Total heat transfer coefficient of insulated pipe (W/m2 K) Uun-ins Total heat transfer coefficient of un-insulated pipe (W/m2 K) V Volume of insulation material (m3) Vair Air velocity in the outside of pipe (m/s) Greek symbols dins Optimum insulation thickness (m) hs The efficiency of the heating system (%) Abbreviations ANN Artificial Neural Network EPS Expanded Polistiren HDD Heating Degree-Days HVAC Heating, Ventilation and Air-Conditioning VRF Variable Refrigerant Flow XPS Extrude Polistiren Appendix 1. The results of determining optimum insulation thickness (cm) for different pipe sizes of steel pipes at various insulation materials and fuel types.
Insulation materials
Fuel types
Nominal pipe sizes 50
100
200
400
600
800
1000
Rockwool
Coal Natural gas Fuel-oil Coal Natural gas Fuel-oil Coal Natural gas Fuel-oil
7.3792 6.6268 8.6592 7.6493 7.0398 8.9966 5.6429 5.1750 6.6763
8.1117 7.4383 10.2128 9.1164 8.3805 10.7405 7.3448 5.9175 8.1121
9.4191 8.6313 11.7876 10.5981 9.7161 12.5450 7.3931 6.7458 8.8240
10.5197 9.5759 13.0446 11.8819 10.8509 13.2634 8.1346 7.3880 9.7928
11.1255 10.0951 13.7491 12.6098 11.4816 14.5442 8.5195 7.7121 10.3201
11.4775 10.3897 14.1781 13.0457 11.8521 15.3085 8.7315 7.8853 10.6247
11.7048 10.5756 14.2289 13.3353 12.0942 15.7981 8.8619 7.9885 10.8209
EPS
XPS
M. Ertürk / Energy 113 (2016) 991e1003
References [1] Uygunoglu T, Kecebas A. LCC analysis for energy-saving in residential buildings with different types of construction masonry blocks. Energy Build 2011;43:2077e85. [2] Ristimaki M, Saynajoki A, Heinonen J, Junnila S. Combining life cycle costing and life cycle assessment for an analysis of a new residential district energy system design. Energy 2013;63:168e79. [3] Kayfeci M, Kecebas A, Gedik E. Determination of optimum insulation thickness of external walls with two different methods in cooling applications. Appl Therm Eng 2013;50:217e24. [4] Bloomquist RG. Geothermal district energy system analysis, design, and development, text-book of European summer school on geothermal energy applications. Romania: University of Oradea; 2001. p. 213e53. [5] Danielewicz J, Sniechowska B, Sayegh MA, Fidorow N, Jouhara H. Threedimensional numerical model of heat losses from district heating network pre-insulated pipes buried in the ground. 2016. http://dx.doi.org/10.1016/j. energy.2015.07.012. [6] Karakoc H, Turan O, Binyildiz E, Yildirim E. Tesisat yalıtımı. Yalıtım, 2012;98, http://www.yalitim.net/?pid¼26429, [in Turkish]. [7] Ito K, Akagi S, Nishikawa M. A multiobjective optimization approach to design problem of heat insulation for thermal distribution piping network systems. 1982. ASME paper 82-DET-57. [8] Boehm RF. Design analysis of thermal systems. New York: John Wiley and Sons; 1987. [9] Zaki GM, Al-Turki AM. Optimization of multilayer thermal insulation for pipelines. Heat Transf Enginering 2000;21:63e70. [10] Sahin AZ. Optimal insulation of ducts in extraterrestrial applications. Int J Energy Res 2004;28:195e203. [11] Sahin AZ, Kalyon M. Maintaining uniform surface temperature along pipes by insulation. Energy 2005;30:637e47. [12] Ozturk IT, Karabay H, Bilgen E. Thermo-economic optimization of hot water piping systems: a comparison study. Energy 2006;31:2094e107. [13] Karabay H. The thermo-economic optimization of hot-water piping systems: a parametric study of the effect of the system conditions. Strojniski Vestnik e J Mech Eng 2007;53:548e55.
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[14] Soponpongpipat N, Jaruyanon P, Nanetoe S. The thermo-economic analysis of the optimum thickness of double-layer insulation for air conditioning duct. Energy Res J 2010;1:146e51. [15] Bahadori A, Vuthaluru HB. A simple correlation for estimation of economic thickness of thermal insulation for process piping and equipment. Appl Therm Eng 2010;30:254e9. [16] Bahadori A, Vuthaluru HB. A simple method for the estimation of thermal insulation thickness. Appl Energy 2010;87:613e9. [17] Kecebas A, Alkan MA, Bayhan M. Thermo-economic analysis of pipe insulation for district heating piping systems. Appl Therm Eng 2011;31:3929e37. [18] Basogul Y, Kecebas A. Economic and environmental impacts of insulation in district heating pipelines. Energy 2011;36:6156e64. [19] Kecebas A. Determination of insulation thickness by means of exergy analysis in pipe insulation. Energy Convers Manag 2012;58:76e83. [20] Kayfeci M. Determination of energy saving and optimum insulation thicknesses of the heating piping systems for different insulation materials. Energy Build 2014;69:278e84. [21] Kayfeci M, Yabanova I, Kecebas A. The use of artificial neural network to evaluate insulation thickness and life cycle costs: pipe insulation application. Appl Therm Eng 2014;63:370e8. [22] Kaynakli O. Economic thermal insulation thickness for pipes and ducts: a review study. Renew Sustain Energy Rev 2014;30:184e94. [23] Yildiz A, Ersoz MA. Determination of the economical optimum insulation thickness for VRF (variable refrigerant flow) systems. Energy 2015;89: 835e44. [24] Yildiz A, Ersoz MA. The effect of wind speed on the economical optimum insulation thickness for HVAC duct applications. Renew Sustain Energy Rev 2016;55:1289e300. [25] Holman JP. Heat transfer. seventh ed. New York: McGraw-Hill Book Co; 1992. [26] Ashrae Handbook-Fundamentals. Heat transfer. Atlanta: ASHRAE; 198922.1e22.21 [Chapter 22]. [27] Buyukalaca O, Bulut H, Yilmaz T. Analysis of variable-base heating and cooling degree-days for Turkey. Appl Energy 2001;69:269e83. [28] TY, Teknik Yayincilik Inc. J Turkish Plumb 2015. Energy Technologies and HVAC Systems, http://www.tesisat.com.tr/yayin/yakit-fiyatlari/ [accessed on 25/11/2015].
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Optimum insulation thicknesses of pipes with respect to different insulation materials, fuels and climate zones in Turkey Mustafa Ertürk Department of Electricity and Energy, Balıkesir Vocational School, Balıkesir University, 10145, Balıkesir, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 March 2016 Received in revised form 20 July 2016 Accepted 22 July 2016 Available online 3 August 2016
This article is about the effects of insulation thickness on the life cycle costs of steel pipes inside the pit (channel, duct) or above-ground structures with different diameters and sensitivity to economic parameters. The heating degree-day and life cycle cost procedures are used for the optimization and sensitivity analyses. For Afyon province in Turkey the fuels are coal, natural gas and fuel-oil. The insulation materials are rock wool, EPS and XPS. The results show that consideration of all the physical and economic conditions appear with the optimum insulation thickness that varies from 5 cm to 16 cm. Both lifetime and unit cost of insulation are sensitive to the insulation thickness. Additionally, the increased discount rate and cost per unit volume of insulation material lowers the optimum insulation thickness. In conclusion, it is expected that this study will provide a guide for engineers, where insulation is used for large diameter pipes. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Thermal insulation Case study Pipe sizes Optimum insulation thickness Sensitivity
1. Introduction Currently, due the effects of limited energy resources, rapid energy consumption and overuse of resources, thermal insulation has gained great interest and importance for many countries. In the industry, there are complicative and costly pipe installation and insulation applications. On the one hand, insulation should reduce energy consumption in buildings, mechanical installations and industrial systems. It also plays an important role in reducing energy loss from the transmission portion of heating and cooling loads in buildings and industrial systems. Insulation appears as one of the most important methods to reach environmental sustainability and energy efficiency in modern cities and systems [1e3]. Large-scale heating systems provide heating and hot water needs for industrial complexes, community housings, neighborhoods in the cities by carrying heat production from one or more energy resources through pipe systems. The important cost components in a district heating system are transmission and distribution pipe configurations, which frequently range from 40% to 60% of the total project cost [4]. The majority of these costs are related to the installation in the pipe system. The use of uninsulated pipes allows reduction in pipe material costs by more than 50% [4]. If insulation material is not used properly the heat losses increase,
E-mail address: [email protected]. http://dx.doi.org/10.1016/j.energy.2016.07.115 0360-5442/© 2016 Elsevier Ltd. All rights reserved.
which lead to the increases in the system capacity (boiler, pipe, pump) by increasing both the initial investment costs and the operation expenses. Increase in the system flow rates or compensation may lead to large pumping costs. It is important to choose the most efficient and appropriate material in the practical application. One way to increase the energy efficiency and cost reduction in such a system is to reduce heat loses from transmission/distribution by more appropriate and efficient district heating design. The most effective way to reduce heat losses is to choose pipe and insulation materials, and then to determine pipe diameter and insulation thickness [5]. The most important difference in insulating pipes, compared to buildings is the temperature levels that are encountered in pipes are much higher than temperature levels in buildings. As a result, good insulation for installations may provide much greater energy saving in comparison to buildings. Depending on the fluid temperature within the pipe, errors in material and thickness choice may cause many problems leding to condensation and freezing. Insulation lowers boiler capacity, numbers of radiators and pipe diameters [6]. In the mechanical installations and industry, especially pipe insulation allows larger heat loss/gain and financial gains. As insulation thickness increases, savings increase and investment costs also increase. Optimum insulation thickness calculations should be completed in the type and thickness determination of the heat insulation material.
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M. Ertürk / Energy 113 (2016) 991e1003
Recent studies of optimization in pipe insulation thickness have increased (e.g. Refs. [7e24]). This is due to the fact that pipes, tanks, depots, air conditioning ducts, valves and fittings in buildings, mechanical installations and industry form a large source of energy consumption. Accordingly, researchers expended efforts in order to reduce the heat losses. At the same time, it may reduce the environmental effects of fuels that are used to produce electricity. Under optimization conditions, insulation thickness is one of the most effective ways to conserve energy. In the previous studies, Ito et al. [7] and Boehm [8] used multipurpose functions and analytical methods to reduce heat loss and insulation amount in pipe systems. Zaki and Al-Turki [9] completed an economic analysis for thermal insulation of double-layer insulated petrol pipelines (nominal pipe sizes of 100e273 mm) by using different insulation materials (rockwool and calcium silicate) with an explicit non-linear function. Sahin [10] numerically modeled variations in insulation thickness to maximize pipe outer surface temperature under extraterrestrial radiation conditions. Another study by Sahin and Kalyon [11] analytically modeled the variation in insulation thickness of pipes under convection and radiation conditions. Ozturk et al. [12] presented four different thermo-economic methods to optimize pipe diameter and insulation thickness based on heat losses, insulation costs, total costs and exergy efficiency for optimum design in hot water pipe systems. The method that produces the best result is reported to be the one with exergy and cost parameters. Karabay [13] used exergo-economic analysis to determine optimum insulation thickness of hot water distribution pipes (nominal pipe sizes of 100e1000 mm). The effects of mass flow rate, annual operating time, amortization period, and water temperature on the optimum pipe diameter and insulation thickness are also researched by Soponpongpipat et al. [14], who worked to determine the optimum insulation thickness based on the thermo-economic method for double-layer insulation of air conditioning ducts (circular galvanized steel duct of 500 mm). The calculations are completed for insulation material such as rubber and fiber glass based on annual equivalent full load cooling time of the air conditioning system. Bahadori and Vuthaluru [15] estimated optimum insulation thickness with a simple correlation as a function of thermal conduction coefficient of steel pipes and equipment diameter of insulation material at 100 C, 300 C, 500 C and 700 C. Another study by the same authors [16] helps to calculate a simple correlation for estimation of heat flow through insulation, thermal resistence and insulation thickness for flat and cylindrical surfaces. Kecebas et al. [17] modified life cycle cost analysis to determine optimum insulation thickness for different pipe diameters in a district heating pipeline network (nominal pipe sizes of 50e200 mm). They calculated the amount of heat carried by fluid within the pipe using the degree-day method. Later, they completed life cycle cost analysis by consideration of the heat loss from the pipe surface. For calculations, as insulation material the rock wool, coal, natural gas, fuel oil and geothermal energy are used as fuel sources. They reported optimum insulation thicknesses between 8.5 cm and 22.8 cm. They also reported that as greater savings from large diameter pipes should be considered separately. Basogul and Kecebas [18] used modified life cycle cost analysis to investigate environmental assessment of insulation pipes in district heating pipelines. They used different fuel types, a variety of insulation materials and nominal pipe diameters for economic and environmental assessment. They reported that the largest optimum insulation thickness, fuel savings and emissions are valid for 200 mm nominal pipe size. Apart from the models used by Ozturk et al. [12] and Karabay [13], Kecebas [19] suggested another method that combines exergy analysis and modifies life cycle cost analysis to determine optimum insulation thickness for
pipes. The analysis investigated the effect of combustion parameters such as excess air, stack gas temperature, and combustion chamber temperature on optimum insulation thickness, which reduces waste gases and combustion chamber temperature with increasing fuel entry temperature and decreasing stack. In addition, the optimum insulation thicknesses obtained by exergoeconomic analysis are reported as higher than those obtained by energo-economic analysis. Kayfeci [20] used the method recommended by Kecebas et al. [17] for estimation of the optimum insulation thickness in various heating pipe diameters with different insulation materials like EPS, fiberglass, foamboard, rockwool and XPS. The highest energy saving for small diameter pipes is obtained by the fiber glass as insulation material, while for large diameter pipes the EPS yields the highest energy saving. The artificial neural network (ANN) is employed by Kayfeci et al. [21] used to estimate optimum insulation thickness easily for pipe insulation applications. All parameters related to thermal insulation optimization are obtained from insulation markets and then it is determined by a life cycle cost analysis modification. The ANN model can easily and accurately calculate optimum insulation thickness for any pipe diameter. Kaynakli [22] reviewed the previous studies to determine economic optimum insulation thickness for pipes and ducts. They investigated heat transfer equations, optimization procedures and economic analyses methods for cylindrical pipes and ducts by comparison. More recently, Yildiz and Ersoz [23] determined the optimum insulation thickness for pipe lines in an air conditioning system with VRF using R-410A as cooling fluid both in heating and cooling modes. In heating mode, the optimum insulation thicknesses for high pressure gas and low pressure fluid pipelines are calculated as 1.6 cme2.0 cm and from 1.1 cm 1.3 cm, respectively. In cooling mode, the thickness varies from 0.7 to 0.8 cm. Another study by the same authors [24] researched to the variation in optimum insulation thickness with wind speed within HVAC ducts. For fuel types such as coal, fuel-oil, LPG and natural gas and insulation materials such as fiberglass and rockwool, life cycle costs are optimized, and they reported that at high wind speeds insulation is necessary. In the literature, there are many studies on the optimum insulation thickness of pipes and ducts based on the modified life cycle cost analysis that is suggested by Kecebas et al. [17]. These studies researched to the effects of different parameters such as fuel types, insulation materials, pipe diameters, heat degree-days, working fluid, combustion and economic parameters on optimum insulation thickness. However, apart these studies, the current study concentrates on the following points. (i) Determination of the optimum insulation thickness for large diameter pipes, (ii) Investigation of the variation in optimum insulation thickness, energy saving and payback period depending on variations in pipe diameters, (iii) Completion of the sensitivity analysis on the parameters of optimum insulation thickness, energy saving and payback period. This study is expected to provide the most important contribution to thermal insulation thickness analysis in case of for large diameter pipes under optimization conditions and the economic parameters.
2. Material and method Generally, heat insulation is a precaution taken to prevent heat loss and gains caused by differences in temperature. It is, therefore, installed in buildings, mechanical installations and industries.
M. Ertürk / Energy 113 (2016) 991e1003
Depending on the hot or cold forms of the fluid passing through pipes, tanks, depots, air conditioning ducts, valves and fittings forming mechanical installations, it is necessary that they should be insulated with appropriate material characteristics and thickness. The insulation application for mechanical installations considered in this study for pipe insulation and its layers is shown in Fig. 1. The following assumptions are considered during life cycle cost (LCC) analysis: - The pipe has a smooth cross-section per unit length inside the pit (channel, duct) or above-ground structures, - Heat losses from the pipe surface can be determined by heating degree-days, - Ambient and fluid temperatures are constant, - Fluid (hot water) has fixed speed within the pipe, - Air speeds are constant inside the pit (channel, duct) or aboveground structures, - There are steady state flow controlled volume conditions within the pipe, - Temperature and pressure drops are ignored along the pipe, - Insulation resistence noise and fire are negligible in the corrosion applications, - Insulation material has high value for steam diffusion resistance coefficient.
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For un-insulated and insulated pipe layers, it may be written, respectively, as follows:
Uunins ¼
Uins ¼
1 Rp;unins
1 Rp;ins
(2b)
Total internal resistence in un-insulated pipe layers (inner fluid þ pipe þ outer fluid) is given as,
ln
Rp;unins ¼
Qp ¼ UAðTad To Þ
where Tad and To are average design temperatures of inside fluid and outside air, respectively; U is the total heat transfer coefficient.
(3)
with Ai¼2p L r0 and Ao¼2p L rn. For the total internal resistence of insulated pipe layers (inner fluid þ pipe þ insulation material þ outer fluid), the following expression is valid.
Rp;ins ¼
(1)
r1
r0 1 1 þ þ hi Ai ho Ao 2pLk1
ln
The heat losses from the pipe may be described as below.
(2a)
r1 r0
ln
r2
r1 1 1 þ þ þ hi Ai 2pLk1 2pLkins ho A0o
(4)
where A0o ¼ 2p L r2 , k1 is the heat transfer coefficient of pipe material and ki is the thermal conductivity of insulation material. The convection heat transfer coefficients for the interior and exterior of the pipe system may be calculated, respectively as [25],
Fig. 1. The hot water pipeline with insulation and definition of various parameters.
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hi ¼ 0:023 Re0:8 Pr0:4
ki 2r0
(5)
processing plants. The annual heat amount transfer for these thermal systems may be determined by using heating degree-days [17e21]. Annual heating degree-day (HDD) value can be calculated on the basis of hourly data according to,
and
ho ¼ 11:58ð1=dÞ0:2 ½2=ððTms To Þ 546:3Þ0:181 ðTms To Þ0:266 ð1 þ 2:86 Vair Þ0:5
Table 1 Parameters used in calculation.
(6)
HDD ¼ ð1 yearÞ
365 X ðTb Tsa Þ
(7)
1
Parameters/Technical characteristics
Values
Heating degree-days (HDD) Mean hot water temperature [(70 þ 90)/2] Outside air temperature Mean outside surface temperature Velocity of inside fluid Velocity of outside air Fuel types Various insulation materials Nominal pipe sizes of stainless steel pipe Discount rate (i) Increase rate (d) Lifetime (N)
2328 C-days 80 C 15 C 93 C 0.8 m/s 0.2 m/s See Table 2 See Table 3 See Table 4 13% 6.5% 10 years
Here, only the positive value of temperature difference is noted, and the temperature difference is zero if Tb < Tsa. In this study, for the basic temperature of 18 C in Afyon province (Turkey), the heating degree-day value is adapted from Buyukalaca et al. [27] and given in Table 1. It is necessary to conserve the heat transfer by pipe, and hence, the use of insulation becomes necessary. The annual heat losses from heat carried along the pipe may be calculated as,
QA ¼ 86; 400 HDD U
(8)
The annual fuel consumption amount to provide the amount of heat to move the pipe is calculated as, Table 2 Some properties of fuels and their system used in the study [28]. Fuels
Pricea
Hu
h
Coal Natural gas Fuel-oil
0.3129 ($/kg) 0.4453 ($/m3) 0.7935 ($/kg)
29.260 106 (J/kg) 34.485 106 (J/m3) 41.278 106 (J/kg)
65% 95% 80%
a
Assuming 1 $ ¼ 2.6843 Turkish Liras (TL) as of June 2015.
where according to [26] d ¼ 2(r1þd), Tms is the mean outside surface temperature and Vair is the air velocity. Information about the amount of heat transfer or fluid storage inside the cylindrical pipes/depots/tanks is vital for thermal systems like district heating/cooling, industrial and chemical
mf ¼
86; 400 HDD U Hu h s
(9)
where Hu is lower heating value of the fuel and hs is the efficiency of the heating system. The total annual cost, Cf, of energy spent to heat transportation in the pipe is determined as,
Cf ¼ mf CF
(10)
where CF is unit cost of any fuel as $/kg, $/m3, $/kW h. On the other hand, the total cost of insulation, Cins, material at pipe insulation application is calculated as,
Table 3 Some properties of insulation materials used in the study. Insulation material
Standard
Density kg/m3
Conductivity kins, W/m-K
Cost CI, $/m3
Rockwool EPS XPS
TS EN 13162 TS EN 13163 TS EN 13164
100 16 28e30
0.040 0.036 0.031
44 32 62
Table 4 Some properties of stainless steel pipe used in the study. Nominal pipe size
Outer diameter, r1
Wall thickness, t
(mm)
(inch)
(mm)
(mm)
50 100 200 400 600 800 1000
2 4 8 16 24 32 40
60.3 114.3 219.1 406.4 610.0 813.0 1016.0
3.91 6.02 8.18 9.53 9.53 9.53 9.53
Weight class
Sch No
Unit weight
STD STD STD STD STD STD STD
40 40 40 30 20 10 e
5.44 16.07 42.55 93.27 141.12 188.82 236.53
(kg/m)
Note: For stainless steel pipe (ANSI B 36.10), the density, melt temperature and conductivity are 7.99 g/cm3, 1371e1399 C and 16.2 W/m-K, respectively.
M. Ertürk / Energy 113 (2016) 991e1003
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Fig. 2. Effect of insulation thickness on annual cost at (a) 50 mm and (b) 1000 mm NPSs (for coal and rockwool).
Cins ¼ CI V
(11)
where V ¼ pðr22 r12 Þ L is the volume of insulation material and CI is the cost of insulation material per unit volume as $/m3. After the energy or fuel and insulation costs calculations, the LCC analysis is conducted and it is used to assess the insulation economy in insulation technologies and projects and in this study,
the P1-P2 method is used. Life cycle energy ratio P1 and life cycle expenditure ratio (initial and additional capital investments) P2 may be defined according to the following expressions.
P1 ¼
N if i ¼ d 1þi
Fig. 3. Effect of insulation thickness on energy saving for different nominal pipe sizes at (a) coal and (b) natural gas and (c) fuel-oil fuels (rockwool).
(12a)
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Fig. 4. Effect of insulation thickness on energy saving for various fuel types at (a) 50 mm and (b) 1000 mm NPSs (rockwool).
" # 1 1þi N 1 P1 ¼ if isd ðd iÞ 1þd
(12b)
and
P2 ¼ 1 þ P1 Ms
Rv ð1 þ dÞN
(13)
where i is discount rate, d is increase rate, N is lifetime, Ms is the ratio of the annual maintenance and operation cost to the original first cost and Rv is the ratio of the resale value to the first cost. There are no maintenance and operating costs at insulation application, the P2 value is taken as 1. The total cost, Ct, expended for heat transfer by insulated pipes and the energy saving S is calculated as,
Ct ¼ P1 Cf þ P2 Cins
(14)
and
S ¼ P1 Cf0 P2 Cins :
(15)
where Cf0 is the difference between the energy cost for the uninsulated and insulated pipes. In order to determine the optimum insulation thickness, the outermost diameter of the insulated pipe layer is determined by
minimizing Eq. (14). For this purpose, the differential of the total cost, Ct, is obtained and brought to zero according to outer diameter, r2. Hence, the optimum insulation thickness is found as dins¼r2r1. For the payback period, an appropriate P1 is chosen as one of the cases of Eq. (12a) or Eq. (12b) for P2 ¼ 1. Here, appropriate P1 is Eq. (12b) for isd due to discount rate, i, and increase rate, d, which are given in Table 1 as 13% and 6.5%, respectively. Subsequently, Eq. (12b) is substituted into Eq. (15) and brought to zero, and hence, the payback period, Np, is calculated. With the use of the parameters in Table 1, all calculation procedures are completed with the MATLAB optimization Toolbox. The characteristics of fuel types, insulation material types and stainless steel pipe diameters are given in Tables 2e4, respectively. 3. Results and discussion When the fuel type coal and insulation material of mineral wool are used, the variation in pipe insulation thickness annual costs for different diameter pipes is shown in Fig. 2, which implies that insulation pipes annual fuel costs begin to reduce and at this point insulation costs start to increase. However, the total annual cost including annual fuel and insulation costs falls with the use of insulation material but increases after a certain insulation thickness. This point is called as the optimum insulation thickness in insulation applications. In Fig. 2, a comparison is given between the insulation thickness annual costs for the optimum insulation
Fig. 5. Effect of insulation thickness on energy saving for various insulation materials at (a) 50 mm and (b) 1000 mm NPSs (coal).
M. Ertürk / Energy 113 (2016) 991e1003
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Fig. 6. Effect of insulation thickness on payback period for different nominal pipe sizes at (a) coal, (b) natural gas and (c) fuel-oil fuels (rockwool).
thickness of the smallest (50 mm in Fig. 2a) and largest (1000 mm in Fig. 2b) and the pipe diameter. The optimum insulation thicknesses for 50 mm and 1000 mm pipe diameters are found as 7.38 cm and 11.70 cm, respectively. The annual total costs equivalent to these values are 5.40 $/m-yr and 45.06 $/m-yr. As large diameter pipes are required to transfer more heat than small diameter pipes, annual cost due to heat loss is a large value. Additionally, in Fig. 2b the insulation cost curve is linear for large diameter pipes, while for small diameter pipes Fig. 2a shows a non-
linear curve, which is due to the geometry of the pipe. The more energy one needs to conserve, the more money should be paid for adding extra thickness of insulation. However, above a certain insulation thickness (optimum insulation thickness), the potential energy savings do not justify the extra cost. This is because of more insulation addition by increasing thickness with diminishing returns on the energy savage. Fig. 3 shows the variation of energy savings in pipes with different diameters for coal, natural gas and fuel-oil fuels. In the three graphics, the amount of
Fig. 7. Effect of insulation thickness on payback period for various fuel types at (a) 50 mm and (b) 1000 mm NPSs (rockwool).
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M. Ertürk / Energy 113 (2016) 991e1003
Fig. 8. Effect of insulation thickness on energy saving for various insulation materials at (a) 50 mm and (b) 1000 mm NPSs (coal).
energy saving increases with increasing insulation thickness. However, after a certain point it starts to fall. This value, where energy saving is at the maximum, is called as the optimum insulation thickness. Furthermore, in all graphics as pipe diameter increases, the energy saving value also increases. As a result, it is possible to obtain greater energy savings with large diameter pipes. The energy saving values obtained for fuel-oil in Fig. 3c is greater than those for the other two fuels. This due to the fact the unit cost of the fuel. In other words, the energy savings are equally high from the use of high-cost fuels. At the optimum insulation thickness of
50 mm diameter pipe, the energy savings with coal, natural gas and fuel-oil are 10.72, 8.68 and 16.23 $/m-yr, respectively, while at that value of 1000 mm pipe, they are calculated as 163.17, 138.05 and 231.04 $/m-yr, respectively. Depending on the amount of energy saving, the comparisons of different pipe diameters and various fuel types are given in Fig. 4. The higher cost of the fuel directly affects the amount of energy saving. According to this situation, the fuel with most saving is fueloil, which is followed by coal and natural gas. For all fuel types in Fig. 4, as the diameter of the insulated pipes increases, the energy
Fig. 9. The change of the optimum insulation thickness depending on the nominal pipe sizes for the parameters such as (a) rockwool, (b) EPS, (c) XPS and (d) coal.
M. Ertürk / Energy 113 (2016) 991e1003
999
Fig. 10. The change of the energy saving depending on the nominal pipe sizes for the parameters such as (a) rockwool, (b) EPS, (c) XPS and (d) natural gas.
saving also increases. In the small diameter pipes, as insulation thickness increases, the saving curves fall rapidly, while this situation does not apply for large diameter pipes. There is a similar energy saving at the optimum insulation thickness and all insulation thicknesses above this value for large diameter pipes. It is observed that the amount of energy saving in small diameter pipes falls faster than that of large diameter pipes. The reason is due to the fact that the cost of insulation installation continues to increase non-linearly in small diameter pipes, while it increases as close-tolinear in large diameter pipes. The effect on energy saving of insulation materials in small and large diameter pipes is shown in Fig. 5. At the optimum insulation thickness, it is observed that the energy saving values of all insulation materials are similar. For example, the energy savings for rockwool, EPS and XPS insulation materials at a 50 mm pipe are 10.72, 11.39 and 11.15 $/m-yr, respectively, while for a 1000 mm diameter pipes these values are 163.17, 160.59 and 151.54$/m-yr, respectively. As stated previously, the energy saving curves for insulation materials at small diameter pipes decreases rapidly, while this situation does not apply to large diameter pipes. As a result, the type of insulation material does not matter for large diameter pipes. The ratio between the cost of the insulation material and the thermal conductivities are equal for different insulation materials. The payback period of costs for energy systems is an important projection for many investors, implementers and users. The correlation between insulation thickness and payback period is given in Fig. 6. By using rock wool insulation, the payback periods for coal, natural gas and fuel oil are presented in Fig. 6a, b and c, respectively.
In all graphs, depending on insulation thickness increase, the pipe diameter reduces with the increase in the payback period. For all large diameter pipes, the payback periods are nearly the same. For example, in Fig. 6a the payback period value for a 50 mm pipe of coal material is 0.79 years, while for a 1000 mm pipe it is 0.99 years. The same situation for natural gas in Fig. 6b is as 0.85 and 1.09 years, and it is 0.68 and 0.81 years for fuel oil in Fig. 6c. All pay-back period values are very close to each other. In Fig. 7 the variation is given in payback period against insulation thickness for various fuel types. As a result of increasing insulation thickness, the payback period increases. The payback period curves for small diameter pipes in Fig. 7a show a non-linear increase, while for large diameter pipes (Fig. 7b) these curves appear close-to-linear increase. The shortest payback period is for fuel oil, which is followed by coal and natural gas. The variation of payback period with insulation thickness is given in Fig. 8 for different insulation materials with similar results in Fig. 7. The XPS insulation material has the largest value of payback period for insulation materials and its lowest value is for EPS. In addition, energy savings are almost similar to Fig. 5b while the pay-back periods vary significantly in Fig. 8b. This is due to the difference in the cost of insulation materials per unit volume. The variation graphics of optimum insulation thickness are illustrated in Fig. 9 based on different parameters such as rock wool, EPS, XPS and coal according to pipe diameters. It is that increase in the optimum insulation thickness appears with increase in the pipe diameter. However, at large diameter pipes, the optimum insulation thickness begins to be closer to the same value. In other words, this means that, a fixed optimum insulation thickness may
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Fig. 11. The change of the payback period depending on the nominal pipe sizes for the parameters such as (a) rockwool, (b) EPS, (c) XPS and (d) fuel-oil.
be used for large diameter pipes, especially above 400 mm. For example, an optimum insulation thickness of 11.88 mm for 400 mm in the usages of coal and EPS is suitable for use in the other largescale pipes. As can be seen from all the graphics in Fig. 9, all insulation materials and fuel types provide similar results. This situation is related to the reduction in pipe outer surface area against flow capacity. This relationship reduces heat losses in the fluid flowing along the pipeline. The optimum insulation thicknesses belong to all fuel types, insulation materials and pipe diameters are given in Appendix 1 in detail. Fig. 10 shows the variation in energy saving with pipe diameters for the parameters of the previous figure. This figure shows that while small diameter pipes provide small amounts of energy saving, large diameter pipes (above 200 mm) provide comparatively larger energy savings. The fuel-oil and the highest fuel cost for different insulation materials provide greater savings than other fuels. However, depending on fuel type, the energy savings of insulation materials are similar (see Fig. 10d). Fig. 11 presents the variation in payback period according to pipe diameters. From the figure, the payback period value increases by a small amount as the pipe diameter increases. The payback period of natural gas is longer than the others. However, when considered in terms of years, there is a little difference in the range of 0.51 and 1.14 years per m and similar results are observed in between insulation materials. The results of the sensitivity analysis in the calculation method of optimum insulation thickness, energy saving and payback period for XPS, coal and pipe sizes with 50 and 1000 mm are graphically illustrated in Fig. 12. In the sensitivity analysis, the parameters are
interest rate, discount rate, lifetime, unit cost of fuel, unit volume cost of insulation and heating degree-day. With percentage increases in these parameters, the variations in optimum insulation thickness, energy saving and payback period are observed. Fig. 12a and b are for the lifetime and the unit volume cost of insulation and they are the most sensitive parameters for optimum insulation thickness. These parameters appear to be more effective for large diameter pipes. Additionally, increases in discount rate and insulation material unit volume cost lower the optimum insulation thickness. In Fig. 12c and d, the energy savings for 50 mm and 1000 mm pipes are affected mostly by lifetime and discount rate. For small diameter pipes, any percentage increase in lifetime and unit cost of fuel provides the highest energy savings, while for large diameter pipes both parameters and additionally heating degree-day are effective. For large diameter pipes, the parameters show the greatest effect on energy saving and the discount rate reduces energy savings. The sensitivity of the parameters on payback period is specified in Fig. 12e and f. They clearly show that the lifetime increases the payback period. From these figures, the payback period decreases with increasing heating degree-day. However, for large diameter pipes, the discount rate effect decreases slightly as in Fig. 12f. 4. Conclusions This study is related to the determination of optimum insulation thickness for different diameter pipes (50 mm, 100 mm, 200 mm, 400 mm, 600 mm, 800 mm and 1000 mm) using the life cycle cost (LCC) analysis over a 10 year life-time. The variations of optimum
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Fig. 12. The results of the sensitivity analysis for the optimum insulation thickness, energy saving and payback period.
insulation thickness, energy saving and payback period for different pipe diameters are presented after proper calculations. Additionally, a sensitivity analysis is considered in order to determine the effects of parameters such as interest rate, discount rate, lifetime, fuel unit cost, cost of insulation unit volume and heating degreeday on optimum insulation thickness, energy saving and payback period. The used fuels are coal, natural gas and fuel-oil and insulation materials such as rock wool, EPS and XPS for the heating degree-day values of the Afyonkarahisar province in Turkey. Some
of the concluding remarks from this study may be listed as follows. (a) When considered all parameters, the optimum insulation thickness varies from 5.18 cm to 15.80 cm. However, for all large diameter pipes the use a complete value is recommended for optimum insulation thickness. For example, the optimum insulation thickness of 10.52 cm should be used in pipes from 400 mm to 1000 mm diameter for rock wool and coal.
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(b) Due to pipe geometry, the insulation cost curve is not linear for small diameter pipes, while for large diameter pipes this linearity increases. This is related to the fact that in large diameter pipes the ratio of the exposed pipe outside surface area compared to the flow capacity is reduced. (c) Large diameter pipes provide greater energy savings compared to small diameter pipes. (d) The most energy saving fuel type related to fuel costs is found as fuel-oil, which is followed by coal and natural gas. In addition, the most saving among the insulation materials is the EPS. (e) Keeping other parameters constant, there is no large change in payback period as pipe diameter increases. (f) Among the evaluated parameters, the most sensitive parameters in the determination of optimum insulation thickness are the lifetime and unit cost of insulation. Additionally, the optimum insulation thickness decreases with increase in the discount rate and unit volume cost of insulation material. Acknowledgements The author is very grateful to the reviewers due their appropriate and constructive suggestions as well as their proposed corrections, which have been utilized in improving the quality of the paper. Nomenclature A Ai Ao A0o Cf Cf0 Cins CI CF Ct d hi ho Hu i ki kins
Total surface area of pipe (m2) Inside surface area of pipe (m2) Outside surface area of last layer of pipe (m2) Outside surface area of nth layer of pipe (m2) Annual energy cost ($) Difference between energy cost of the un-insulated and insulated pipes ($) Total insulation cost ($) Cost of insulation material per unit volume ($/m3) Unit cost of fuel ($/kg, $/m3, $/kW h) Total cost ($) Increase rate (%) Convection heat transfer coefficient for inside of pipe (W/ m2 K) Convection heat transfer coefficient for outside of pipe (W/m2 K) Lower heating value of the fuel (J/kg, J/m3, J/kW h) Discount rate (%) Heat transfer coefficient of fluid in the inside of pipe (W/ m K) Thermal conductivity of insulation material (W/m K)
L mf Ms
Length of pipe (m) Annual fuel consumption (kg, m3, kW h) Ratio of the annual maintenance and operation cost to the original first cost ($) N Lifetime (years) Np Payback period (years) Pr Prandtl number P1 Life cycle energy P2 Life cycle expenditures QA Annual heat losses occurring from heat carried along the pipe (W) Qp Heat losses from the inside of the pipe to the outside of the pipe through the pipe (W) Re Reynold number Rp Total internal resistance of piping system (K/W) Rp,ins Internal resistance of insulated pipe system (K/W) Rp,un-ins Internal resistance of un-insulated pipe system (K/W) Rv Ratio of the resale value to the first cost ($) r0,r1, etc. Radius of pipe system (m) S Energy savings ($) Tad Average design temperature of inside fluid (K) Tb Base temperature (K) Tms Mean outside surface temperature of pipe system (K) To Design temperature of outside air (K) Tsa Solar-air temperature for each hour (K) U Total heat transfer coefficient (W/m2 K) Uins Total heat transfer coefficient of insulated pipe (W/m2 K) Uun-ins Total heat transfer coefficient of un-insulated pipe (W/m2 K) V Volume of insulation material (m3) Vair Air velocity in the outside of pipe (m/s) Greek symbols dins Optimum insulation thickness (m) hs The efficiency of the heating system (%) Abbreviations ANN Artificial Neural Network EPS Expanded Polistiren HDD Heating Degree-Days HVAC Heating, Ventilation and Air-Conditioning VRF Variable Refrigerant Flow XPS Extrude Polistiren Appendix 1. The results of determining optimum insulation thickness (cm) for different pipe sizes of steel pipes at various insulation materials and fuel types.
Insulation materials
Fuel types
Nominal pipe sizes 50
100
200
400
600
800
1000
Rockwool
Coal Natural gas Fuel-oil Coal Natural gas Fuel-oil Coal Natural gas Fuel-oil
7.3792 6.6268 8.6592 7.6493 7.0398 8.9966 5.6429 5.1750 6.6763
8.1117 7.4383 10.2128 9.1164 8.3805 10.7405 7.3448 5.9175 8.1121
9.4191 8.6313 11.7876 10.5981 9.7161 12.5450 7.3931 6.7458 8.8240
10.5197 9.5759 13.0446 11.8819 10.8509 13.2634 8.1346 7.3880 9.7928
11.1255 10.0951 13.7491 12.6098 11.4816 14.5442 8.5195 7.7121 10.3201
11.4775 10.3897 14.1781 13.0457 11.8521 15.3085 8.7315 7.8853 10.6247
11.7048 10.5756 14.2289 13.3353 12.0942 15.7981 8.8619 7.9885 10.8209
EPS
XPS
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