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Heating and Cooling
151
Chapter 11
Heating and Cooling
11.1 Setting requirements The heating or cooling of culture water is a frequent systems requirement (see Appendix H). This may be to eliminate natural temperature variations and achieve longer periods of time at optimum growth temperature or to provide a number of precisely controlled temperatures for research purposes. If species exotic to the region are involved, heating or cooling may be essential for survival. Rapid changes in temperature, even for temperatures within the natural range of an organism, can easily prove fatal. Such fatal temperature changes could occur by turning something 'on' or 'off', or by the failure of a seawater heating or cooling system. The reliability and modes of failure of such systems can assume considerable importance. While the tolerance of marine organisms to temperature changes varies considerably, a rule of thumb is to limit temperature changes to a maximum of about 2~ (l~ Tolerances to momentary temperature changes are higher, although any rapid change will, at the least, be a source of stress. Fortunately, most culture organisms of interest are estuarine, with relatively high tolerances to temperature changes. Deep sea organisms and those from isothermal environments may prove much more sensitive to both water temperature values and fluctuations. Most of the requirements are for seawater heating rather than cooling. The two requirements are very similar. The principles, concepts, and some of the equipment would be common. The major difference would be in the need for a heat source for heating rather than a heat sink for cooling. It is often not appreciated that heating or cooling seawater involves moving considerable energy. The energy units most commonly used are the British Thermal Unit (BTU) and the gram-calorie. A BTU is defined as the energy necessary to raise the temperature of one pound of water one degree Fahrenheit at a water temperature of I~ The gram-calorie is the energy necessary to raise one gram of water one degree centigrade at 4~ In place of the gram-calorie, the kilogram-calorie or large calorie (1000 gram-calories) is often encountered in the literature. These heat-energy values for water can be assumed to be constant with temperature and salinity over the conditions encountered in aquatic systems. The efficiencies of the processes are defined as the energy moved divided by the energy input. For heating, this value must be less than one or 100%. For cooling, if a refrigeration cycle is used, the value is called the coefficient of performance and usually has a value greater than one. Energy flows (power), into or out of the seawater, can become substantial. See Appendix A for conversion factors between the various energy and power units. Thus to raise or lower 1 gpm I~ requires 8.5 BTU/min or 0.2 hp (149 W), assuming 100% efficiency. One lps raised or lowered I~ requires 1026 calories/s or 4300 W (5.8 hp). As an example, a winter
152 Seawater Temperature Increase (~ 10 20 1200
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Fig. ll.1. Seawater heating requirements. Based on a unit flow of one gallon per minute of seawater, fuel oil at 19,000 BTU/lb and specific gravity of 0.9, and several different overall thermal efficiencies. Lower efficiencies are due to heat losses at furnace, piping, and heat exchanger. Value will be determined primarily by degree of insulation provided to components. Also shown is the numerical example from Example 11.2.
heating requirement could be to raise 100 gpm 40~ This requires 800 hp to be transferred into the fluid. Typical fuel oil has a heat content of 19,000 BTU/lb, and this need would require about a gallon of fuel every 4.25 min or 339 gallons/day, assuming 100% efficiency. Fig. 11.1 provides information for a wider range of conditions. For higher flows, multiply the fight side results by the flow rate in gpm to get the required power or fuel oil inputs. The transfer efficiency will be determined primarily by the type of system and amount of thermal insulation provided. For high flow rates and large temperature increases, the energy costs can easily get out of hand, even at 100% efficiency. Seawater heating systems have been installed in test facilities and laboratories that could not be used to anywhere near their capabilities due to the impacts of high fuel costs on limited operating budgets. There are a few direct heating methods. One is the use of thermostatically controlled electric quartz or Teflon immersion units. While small ones are common in home aquariums, larger industrial units are available to very substantial power levels. Powers up to 1 kW are available in 120 volts and some as high as 3 kW may be available (see Example 11.1). Higher power units require higher voltages, which can substantially increase the risks around seawater to both culture organisms and operating personnel if not properly handled. These units typically have very high transfer efficiencies and can control the temperature to about a degree F (half degree C). However, they sometimes fail full 'on', resulting in an unintentional
153
Example 11.1. Electric immersion heat transfer Readily available electric power is limited to 120 V AC power. The highest readily available rating for a thermostatically controlled quartz immersion heater in 120 V AC is about 1 kW. How many gpm of seawater (30 ppt, 50~ can you heat to 80~ with a single immersion unit? Assume such electric resistance heating is 100% efficient. From Table A-3, average specific weight seawater (65~ is 63.8 lb/ft 3 Seawater is 63.8 lb/ft 3 or 7.48 gal/ft3 is 8.5 lb/gal Temperature change is 80~ - 50~ is 30~ Heat transfer in BTU/min is (flow in gpm)(8.5)(30) is 255(flow) Conversion factor, 1 BTU/min is 17.58 W 1000 W available is 56.88 BTU/min is heat transfer 255(flow) is 56.88 Seawater flow rate is 0.223 gpm per immersion heater Or directly from Fig. 11.1, 30~ 100%, read about 4.2 kW/gpm 1 kW/4.2 kW/gpm = 0.23 gpm
cooking of the culture organisms. Another possibility is direct steam injection. Some care would have to be taken as to its location in the system and, to our knowledge, it has never been used in culturing applications. Most heating or cooling processes need a heat source or heat sink. This is often also a fluid, which may or may not be seawater. If this fluid is in a closed loop and simply a carrier of heat from some other heat source or sink, it is called a working fluid. C o m m o n working fluids include steam, freshwater and a number of chemical refrigerants. Natural heat sinks and sources can be many and varied, and may be seasonal. They include lakes, the atmosphere, ocean, ground water, earth and rock. Any large mass with temperature above or below the culture water temperature can be utilized. The higher the heat capacity of the mass material the better. It may be possible to add heat to the mass in the summer and withdraw it in the winter. If these masses have temperatures that are higher (heating) or lower (cooling) than the desired culture temperatures, the processes can be simplified to simple heat transfer. If the heat sink is a pumpable fluid, it can be directly used through a heat exchanger. If the heat sink is a solid, a loop with a separate working fluid will be needed. If the heat source does not have quite high enough temperature, it can still be used with the addition of a heat pump. A heat pump is a refrigeration cycle run in reverse so that heat is removed from a 'cold' source and transferred to a 'warm' area. Depending on the temperatures of the heat sink and desired temperature, heat pumps can be designed to both heat and cool. Some care would have to be taken to prevent unintentional leakage of the working fluid into the culture system. Depending on the thermal fluid's properties and toxicity, this could be critical. The greater the temperature differences, the more attractive is the prospect of using natural heat sources or sinks. While such use is highly dependent on specific circumstances, when conditions permit, they often provide simple, reliable and inexpensive temperature control capabilities. If their temperatures are in the fight direction but not sufficient, they may still be used but powered assistance will be required.
154 For heating applications, the most common source of heat is an oil- or gas-fired boiler. These units are commercially available in a wide range of sizes and have excellent reliability. However, they generally have electrically powered controllers and thus will automatically shutdown on loss of electricity, even though they are run from an independent power source. This is an important factor in evaluations of overall system reliability and of need for backup electric power. Other sources of heat include waste heat from engines, power plants or solar collectors. Many of these heat sources may not be under the control of the culturist. Sudden loss or change in the heat supply could be catastrophic. Reliability and assurance of supply issues have precluded use of what might otherwise be a valuable resource. For cooling a refrigeration or chiller unit is generally used to cool a working fluid (usually a chemical in a sealed loop) to below the desired seawater temperature. These units usually include an evaporator, compressor, condenser, expansion valve, and control circuitry. They tend to be more complex and difficult to operate than heating units. A very common mistake is to enclose the refrigeration system in a small space without adequate ventilation. Since the heat removed from the working fluid must be discarded by the condenser, if ventilation is inadequate the space will quickly heat up and system performance will rapidly drop.
11.2 Heat exchangers Most heating and cooling processes will require the use of one or more heat exchangers. The heat exchanger separates the working fluid from the process water, while efficiently transferring heat between the two fluids. This prevents the corrosion of the heating or cooling equipment by seawater and contamination of the process water by potentially toxic working fluids. Due to the corrosive properties of seawater, considerable engineering efforts have been devoted to the design of marine heat exchangers. However, their use in aquatic culturing systems involves biological constraints not typically of other industrial seawater heat exchangers (see Sections 8.1 and 8.2). Heat exchanger types that have been successfully used in marine culturing applications are shown in Table 11.1. Some are for use with pressure on both the seawater and working fluid sides. Others can be used directly in open tanks, with only the working fluid being pressurized. In this case, the circulation in the tank has to be controlled to assure uniform heat transfer (see Fig. 16.1). Carbon and titanium units are particularly well suited to most marine culturing applications. Stainless steel (type 316) heat exchangers are readily available and common in marine applications, but should be used with caution in culturing systems (see Section 8.2). The symbol U shown in Table 11.1 is the overall heat transfer coefficient generally associated with the respective heat exchanger types and materials. It is an important parameter in sizing heat exchangers and is generally stated in equipment specifications. Heat transfer is a complex subject and is dependent on flow configuration, heat exchanger internal design, materials and fluid properties. Flow configurations include parallel, counter and cross flows. Counter flow is the most efficient in that it results in the smallest heat exchanger. It is also the type most likely to be encountered. In counter flow the seawater to be heated or cooled flows in the opposite direction to the working fluid. Total heat transfer must be integrated over the entire heat exchanger. Assuming steady flows, no phase changes,
155 TABLE 11.1 Industrial heat transfer devices for heating and cooling seawater (modified from Huguenin, 1976a) Material
Types available
Comments
Impervious carbon
Bayonet and plate immersion, crossbore and shell/tube, cascade coolers, etc.
Heat transfer areas of 0.5-12,000 ft 2, in order of increasing size-bayonet, plate, crossbore and shell/tube. Carbon has high thermal conductivity. For shell/tube water-water U = 150-250 BTU h -1 ft -2 ~
Glass
Coil, bayonet, cascade coolers, and shell/tube
Low thermal conductivity of glass compensated for by higher film coefficients and reduced fouling. Coils with 2-120 ft2 of heat transfer area are available. Shell/tube areas in multiples of 13.5 or 60 ft2. For shell/tube water-water U = 125-195 BTU h-1 ft-2 OF-1
Glass (electric heating only)
Bayonet-type quartz immersion heaters
Units of 300-1000 W with built-in thermostats will hold within 0.5~ Units of up to 36,000 W available in several configurations. Maximum of 3000 W possible with 120 volt power.
Teflon
Immersion coils and shell/tube
Low thermal conductivity but relatively high film coefficients and reduced fouling. High frictional loses across tubes. Heat transfer areas of 2-900 ft 2 and in small volumes. Shell/tube water-water U = 25-40 BTU h -1 ft -2 ~
Titanium
Heat transfer panels, plates and shell/tube
Variety of standard panel configurations with areas of 2-42 ft 2. Panel water-water U = 100-175 BTU h -1 ft -2 ~ value depends heavily on seawater circulation. Plate water-water U = 550-800 BTU h -1 ft -2 ~ -1 but high frictional loses. Titanium offered as optional material by some manufacturers of large industrial shell/tube heat exchangers. Shell/tube water-water U = 225-280 BTU h -1 ft -2 ~ -1
Stainless steel (Type 316)
Heat transfer panels, plates, shell/tube, etc.
Most manufacturers offer their units in 316 stainless. Very wide selection of sizes and types. Under some conditions 316 stainless steel can be incompatible with seawater and culturing applications. Panel water-water U = 100-175 BTU h -1 ft -2 ~ Shell/tube water-water U -- 225-280 BTU h -l ft -2 ~
c o n s t a n t specific heats, and n e g l i g i b l e h e a t losses, p r o v i d e s the e q u a t i o n s below. Q-
UA(LMTD)
L M T D --
(11.1)
DTA - DTB
(11.2)
ln(DTA/DTB) w h e r e Q is h e a t t r a n s f e r ( B T U / h ) , U is o v e r a l l coefficient o f h e a t t r a n s f e r ( B T U h -1 ft -2 ~
A is e f f e c t i v e a r e a o f h e a t e x c h a n g e r (ft2), L M T D is log m e a n t e m p e r a t u r e d i f f e r e n c e
o f b o t h fluids (~
DTA is t e m p e r a t u r e d i f f e r e n c e s b e t w e e n inputs (~
d i f f e r e n c e s b e t w e e n outputs (~
DTB is t e m p e r a t u r e
F o r c o u n t e r - f l o w w i t h b o t h fluids w a t e r and about equal flow
rates, DTA -- DTB and L M T D - DTA. Eq. 11.1 is a g o o d a p p r o x i m a t i o n for all t y p e s o f h e a t e x c h a n g e r s , a l t h o u g h a c o r r e c t i o n factor m u l t i p l i e r (less than 1) is n e e d e d for c o m p l e x m u l t i - p a s s units. T h u s for the c o m m o n
156
Example 11.2. Shell and tube heat exchanger sizing We have a winter heating requirement and wish to heat a maximum of l0 gpm of incoming seawater from 40~ to 70~ We wish to use a single pass carbon shell and tube heat exchanger. The heat exchanger will be connected in a counter-flow arrangement with its own circulator pump to a hot water system and it is expected to provide 20 gpm at 200~ (see Fig. 11.2). We need to size the heat exchanger, and for estimating operating costs, we need to estimate heat inputs and fuel oil requirements. From Table 11.1 for carbon shell and tube heat exchangers find a U-value of 150-250, so we will use U = 200 BTU h -l ft -2 ~ Heat input into the seawater can be obtained from Fig. 11.1 by using 100% efficiency. This results in a requirement of about 255 BTU/min per gpm. For 10 gpm, this means that 2550 BTU/min or 153,000 BTU/h must to transferred into the seawater. While the seawater in (40~ and out (70~ temperatures and the heating system in (200~ temperature are known, the hot water exit temperature may not be known. In this case the circulator flow rate is given and from knowing that the heat gained by the seawater must have come from the heating water, assuming no heat losses in the heat exchanger, the exit temperature must be about 185~ If the heating system has a high circulating flow, the temperature drop will be small. Even if one assumes no temperature drop of the hot input water, this will often not substantially change the required heat exchanger area. DTA = 2 0 0 - - 4 0 = 160~ DTB= 1 8 5 - 7 0 = l15~ LMTD = (160 - l15)/ln (160/115) = 136.3~ Q = 153,000 = (U)(A)(LMTD) = (200)(A)(136.3) A = 5.6 ft 2 of heat exchanger area From available options supplied by manufacturers, choose the next size above 5.6 ft 2. There will be some heat losses in the heating plant, heat exchanger, and piping runs. These losses will depend on the provided insulation. If we assume a 70% overall efficiency (30% thermal losses), going into Fig. 11.1 with a 30 degree temperature increase we find an estimated fuel oil requirement of about 3.8 gal/day per gpm or about 38 gal/day.
case of a s i m p l e counter-flow heat e x c h a n g e r , a s t r a i g h t f o r w a r d a p p r o x i m a t i o n is avai l abl e for e x c h a n g e r sizing (see E x a m p l e 11.2). A n e x a m p l e for h e a t i n g in an o p e n tank is given in E x a m p l e 11.3. T h e s e a w a t e r input and d e s i r e d output t e m p e r a t u r e s are usual l y k n o w n , as is the input t e m p e r a t u r e of the w o r k i n g fluid. Q can be c a l c u l a t e d f r o m the s e a w a t e r flow rate and r e q u i r e d t e m p e r a t u r e c h a n g e , l e a v i n g only the u n k n o w n h e a t - e x c h a n g e r h e a t - t r a n s f e r area. If n e e d e d , the w o r k i n g fluid output t e m p e r a t u r e can be c a l c u l a t e d f r o m fluid properties, a s s u m i n g that there are no heat losses out of the heat exchanger. C o r r e c t i o n factors for m u l t i - p a s s shell and tube units can be o b t a i n e d f r o m e n g i n e e r i n g texts such as M a r k ' s H a n d b o o k ( B a u m e i s t e r et al., 1978) or e q u i p m e n t m a n u f a c t u r e r s . A m o r e c o m p l e x shell and tube e x a m p l e , w i t h o u t s o m e of the s i m p l i f y i n g a s s u m p t i o n s p r e v i o u s l y m a d e , is p r e s e n t e d in E x a m p l e 11.4. W h i l e heat e x c h a n g e r s are not u s u a l l y cheap, they are not n o r m a l l y the d o m i n a n t cost c o m p o n e n t in a h e a t i n g or c o o l i n g system. B e c a u s e of m a n y uncertainties in r e q u i r e d c a p a c i t y or transfer coefficients, it is g o o d practice to c o n s e r v a t i v e l y size the heat exchanger. In this
157 way, the system's performance will be limited by the capacity of the heat source or sink, rather than by the heat exchanger. When money is limited, a number of inexpensive heat exchangers can be built, especially for temporary or small-scale applications. If a little rust is acceptable, epoxy painted cast iron radiators can be successfully used. Interestingly, a thin epoxy coat is better than a thicker one, because it is less likely to develop small cracks in the coating due to thermal stresses and will have higher heat-transfer rates. Even materials with poor thermal conductivity can be used, if the area is sufficiently large. Large coils of polyethylene pipe have been used as a heat exchanger in outdoor algae tanks. In this case, the heat source was brackish ground water at about 55~ (13~ It was sufficient to keep the algae ponds from freezing and in dramatically improving winter algal production for a commercial shellfish hatchery. 11.3 P r o b l e m areas
Cross-contamination between the working fluid and the process water can cause serious damage to both systems. Probably the biggest single risk is leakage through the heat exchanger or some of its associated fittings. Glass and carbon heat exchangers in particular are brittle and can crack due to miss handling during shipment or installation. Leakage of seawater into the working fluid can cause corrosion problems in the heating or chilling unit and dramatically alter the performance of the system. Since the working fluid is likely to be at a higher pressure than the process water, leakage into the culture system is more likely than the other way around. Such leakage of working fluid into the culture system can be serious, since many of the possible working fluids have significant toxicity to culture organisms. Even if the working fluid is fresh water, it may contain significant quantities of undesirable contaminates (rust inhibitors, metals, hydrocarbons, etc.). The heat source or sink may be the heating or cooling system of a building or industrial plant. This can work, but there are some potential problems in addition to cross-contamination. The building systems were probably not designed with culturing uses in mind. The loads on these systems from the culturing requirements and their consequences, as well as the mechanics of 'tapping-in', have to be investigated. From the culturing standpoint, it may be critical to understand the workings and timings of other demands and functions upon the system. The heat source/sink may be sufficiently variable in its temperature and flow as to make control of the culture unit's heating/cooling system very difficult. Resulting temperature variations could be lethal. The probability of total or partial failure also has to be considered. In addition, the plant being relied upon may have inconveniently planned and unplanned shutdowns over which the culturist has little control or influence. Some of these shutdowns may be for a number of possible reasons, such as maintenance, overhaul, or during holidays. Powering down during periods of school holidays is common at some universities. It may be cheaper and less risky to have a completely dedicated system to support culturing operations. There are a number of ways in which heating and cooling systems can fail. The heat source or sink can be cut off. Even momentary events may shutdown the system. Even when the power source is an on-site fuel, electric power losses to the controls can shutdown the system. On return of electric power, the system will often have to be manually restarted. Pressure losses, flame losses, insufficient fluid levels, etc. can also cause shutdown. If culture water continues to flow, the culture organisms may see a sudden and dramatic temperature change and the consequences could be lethal. It is usually better to immediately stop the flow, even though
158
Example 11.3. Open tank seawater heating A thin-walled titanium heat transfer panel is required to heat 5 gpm of 30 g/kg seawater from 30~ 70~ in a large open tank (8 ft long, 4 ft wide and 1 ft water depth) as shown in Fig. 16.1. This is the most demanding heating requirement that the system will experience. Hot water (200~ in large quantities is supplied from the building's hot water heating system. (A) Calculate the minimum panel heat transfer area required (being optimistic) and state the actual size that you would specify for this application. Table 11.1 gives a U-value of 100-175 BTU h -1 ft -2 ~ for this type of heat exchanger and states that the value is highly dependent on circulation. From Fig. 16.1 it is clear that circulation in the tank is artificially aided by a pump, greatly increasing convective heat transfer. The desired temperature increase is 70 ~ - 30 ~ = 40~ From Fig. 11.1, this requires an input of 345 BTU/min per gpm and 26 gal of fuel oil/gpm per day at 100% efficiency. If there is little opportunity for heat loss except between the two fluids across the heat exchanger, the efficiency will be high. At 100% and a flow of 5 gpm, the total requirement is for 1725 BTU/min or 130 gal of fuel oil per day or equivalent at the exchanger. Remember that this is a small research system and that the heating requirement is continuous seven days a week. The total fuel requirement would have to be adjusted for boiler inefficiencies and heat losses in the building's distribution system. Even though the panel is not completely a counter flow situation, the forced convection makes it a fair assumption and Eq. 11.1 can be used. The flow rate from the building's hot water system is not known but is stated to be high. The temperature drop of the hot water across the heat exchanger can therefore be assumed to be negligible. DTA = 2 0 0 - - 3 0 = 170~ DTB = 2 0 0 - 7 0 - - 130~ LMTD--(DTA--DTB)/ln(DTA/DTB) = (170 ~ 130~176 ~ = 150 U -- 175 BTU h-l ft-2 OF-1 (good circulation and being optimistic) Q = 1735 BTU/min = 103,500 BTU/h Q - - ( U ) ( A ) ( L M T D ) = 103,500 = 175 • a • 150 A -- 4
ft 2
(minimum panel heat transfer area, no losses, optimistic U)
A safety and contingency factor of 2 would not be inappropriate, making the area requirement 8 ft 2. The same operating conditions but using the lowest U (100), calculates out to 7 ft 2. The tank's water depth is 1 ft. If the panel is oriented in the preferred vertical plane (why would a horizontal orientation be less efficient?), this makes its maximum width to be 1 ft. For a required heat transfer area of 8 f t 2 and considering that the panel has two sides, results in a 1 x 4 ft heat transfer panel as shown in Fig. 16.1. (B) What are the potential problems inherent with this type of system? Discuss and quantify these problems and your approaches to reduce the risks and potential adverse consequences. 9
If the cold incoming seawater is saturated with dissolved gases, heating it will result in substantial levels of supersaturation. From Table 2.7, it can be seen that the saturation concentration of oxygen at 0~ is 11.9 mg/1 but only 7.5 mg/1 at 21~ producing a 158% supersaturation of oxygen. Similarly for the other dissolved gasses, especially for the more critical inert gases such as nitrogen. Fortunately this supersaturation takes place in a large open tank. The flow of 5 gpm (0.67 ft 3/min) and the tank's water volume of 32 ft 2, results in a residence time of 48 rain. The amount of gas removed in the tank will be limited, and some type of degassing system may be needed. For sensitive larval stages, degassing will be needed if hot and cold water are mixed even if both waters are saturated with dissolved gases. This is a result of the non-linearity of gas solubility with temperature.
159
E x a m p l e 11.3. (continued) 9
Another potential problem is loss of heat input, possibly due to heating plant shutdown or failure. It is vital to not expose the culture organisms to a rapid 40~ temperature drop. Even if the thermal shock is tempered by the water mass in the tank, it is almost certain to still be fatal. Prompt action is essential. Note in Fig. 16.1 that an alarm system is provided. If prompt restart of heating can not be accomplished, the cold water inflow must be stopped and water conservation measures (priority allocations and aeration) implemented. Auxiliary heating, such as immersion heaters, might be used to allow some cold water inflow. Electric heaters may not help much if the original failure is a loss of electric power to the building heating system's controls. In this case, a backup power source, even if limited, might prove critical. If the tank runs dry, the headbox pump must be turned off to prevent pump damage.
9
Another potential disaster is loss of seawater inflow. Since this heat exchanger is manually controlled, the water in the tank would be heated to levels well beyond those desired before being depleted. This type of failure would expose the culture organisms to a high temperature thermal shock followed by loss of flow. Prompt action is also required in this case. If water inflow can not be quickly restored or otherwise acquired, the heating must be turned off and the same water conservation measures previously mentioned imposed.
loss o f w a t e r m a y r e s u l t in d e c r e a s i n g d i s s o l v e d o x y g e n c o n c e n t r a t i o n or b u i l d u p o f a m m o n i a a n d o t h e r m e t a b o l i c s . T h e loss o f w a t e r c a n b e p a r t i a l l y c o m p e n s a t e d b y i n c r e a s i n g a e r a t i o n . H o w e v e r , this c a n b e d o n e o n l y if the f a i l u r e is i m m e d i a t e l y d e t e c t e d a n d a c t i o n taken. A s a c o n s e q u e n c e , h e a t - e x c h a n g e r c u l t u r e - w a t e r o u t p u t t e m p e r a t u r e s are a c o m m o n m o n i t o r i n g
Compressed Air Supply Hot W a t e r from Boiler Returnto ~,, Boiler ~~--7 ~
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~'t"~~ vNO~veRetu rn [
~
C o n t r ~
Signal.L,---~"T'~ I Temperature
1173 Con,ro,,er
& Aut~
I [~
Valve ~
~
) I
Warm ~ ) ~ , . . ~ ~ ' Seawater Out
J
Sensor
re
Heat Exchanger sC~
-
I ~
~ Drain
Fig. 11.2. Typical temperature control configuration using hot water heat source. Expanding and reducing fittings, circulator pump and line valves are not shown.
160
Example 11.4. Performance of given shell and tube heat exchanger You acquire use of a single pass counter-flow shell and tube titanium heat exchanger with 10 ft 2 of heat transfer area in a hot water loop as shown in Fig. 11.2. The dedicated hot water circulator provides 10 gpm at a temperature of 200~ at the heat exchanger. You have to maintain marine animals at 70~ during the winter. The coldest winter seawater temperature is 30~ the salinity is 30 ppt and you can assume negligible heat losses in and around the heat exchanger. (A) What is the maximum seawater flow rate you can count upon during the coldest part of the winter? The heat gained by the seawater must equal the heat lost by the hot water. The seawater inlet and outlet temperatures and hot water inlet temperature and flow are known. The seawater flow rate, hot water outlet temperature and the heat transfer (Q in BTU/h) are not known. The specific heat of seawater and freshwater are both about 1. The average specific weight of the seawater is estimated at 63.8 lb/fl 3 and 61.0 for the hot freshwater. F is the unknown seawater flow rate, T the hot water exit outlet temperature and 0.1337 the conversion factor (Table A-l) for gal to ft 3. The most pessimistic U of 225 from Table 11.1 for this type of heat exchanger is used in Eq. 11.1. Heat gained by seawater (BTU/h) = heat loss by hot water (BTU/h) A TswgswFsw = A THW)/I-IWFHW Q -- (60)(0.1337)(40)(61)F -- (60)(0.1337)(200 ~ - T)(63.8)(10) = 225(10)LMTD Q = 19,574F = 1,023,607-5,118T = 2,250(LMTD) Three equations and three unknowns, assuming a seawater flow rate, solving for T and LMTD and comparing to Q from Eq. 11.1, see below: Seawater flow, F (gpm)
Hot water outlet temperature, T
DTB, T - 7 0 (~
(~ 12 13 14 15
154.1 150.3 146.5 142.6
LMTD, 1 7 0 - DTB
Water balance, Q (BTU/h)
Eq. 11.1, Q (BTU/h)
234,888 254,462 274,036 293,610
274,651 269,090 263,475 257,625
In ( 170 \ 84.1 80.3 76.5 72.6
122.1 119.6 117.1 114.5
From inspection of the above and interpolation, the maximum seawater flow rate is about 13.6 gpm. (B) Assume that the boiler is relatively inefficient and the hot water distribution system is poorly insulated so that only half of the fuel oil's energy reaches the heat exchanger. If your seawater flow rate is the maximum, what is your worst case daily fuel consumption? From Fig. 11.1, temperature change is 40~ 50%, read 6.8 gal/day per gpm 6.8(13.6) = 92.5 gal/day
p o i n t o n a l a r m s y s t e m s . A f t e r the p r o b l e m c a u s i n g the s h u t d o w n is r e s o l v e d , the s y s t e m m u s t be c a r e f u l l y r e s t a r t e d to p r e v e n t t h e r m a l t r a n s i e n t s that m a y stress or kill the c u l t u r e o r g a n i s m s . Another major source of problems with heating and cooling systems involves problems w i t h the c o n t r o l s y s t e m . M u c h o f the a v a i l a b l e l i t e r a t u r e d e a l s w i t h t e m p e r a t u r e - c o n t r o l p r o b l e m s , e s p e c i a l l y for r e s e a r c h a p p l i c a t i o n s ( A p p e n d i x H). A t y p i c a l h e a t i n g a r r a n g e m e n t u s i n g h o t w a t e r f r o m a b o i l e r is s h o w n in Fig. 11.2. I f the p r o c e s s ( c u l t u r e ) w a t e r flow rate a n d
161 the ambient seawater temperature stay constant or change only slowly over a long period of time, this control system may be unnecessary (see Fig. 16.1). In this case the control system in the boiler, which maintained the hot water output from the boiler at a constant set temperature, and a heat-exchanger hot-water supply valve could be relied upon to accurately control the heat-exchanger culture-water output temperature. If there are tidal or daily ambient-seawater flow rate or temperature variations, a more active control system as shown in Fig. 11.2 will be required. This system monitors the process-water output temperature from the heat exchanger and controls a three way valve to bypass or use hot water from the boiler. If the 'dead band' on the heat-exchanger controller is set too wide, the resulting temperature fluctuations may stress the animals. This effect can be greatly reduced with higher water residence time in the culture units to dilute the temperature-controlled influent. If the band is too tight, the control system will be continuously operating, may overshoot and may become unstable, producing temperature pulses. If the power or air to the controller fails, the heating system will usually fail closed, which is the same as a heat loss. If it is configured to fail open, the culture organisms may be unintentionally cooked.
Chapter 11
Heating and Cooling
11.1 Setting requirements The heating or cooling of culture water is a frequent systems requirement (see Appendix H). This may be to eliminate natural temperature variations and achieve longer periods of time at optimum growth temperature or to provide a number of precisely controlled temperatures for research purposes. If species exotic to the region are involved, heating or cooling may be essential for survival. Rapid changes in temperature, even for temperatures within the natural range of an organism, can easily prove fatal. Such fatal temperature changes could occur by turning something 'on' or 'off', or by the failure of a seawater heating or cooling system. The reliability and modes of failure of such systems can assume considerable importance. While the tolerance of marine organisms to temperature changes varies considerably, a rule of thumb is to limit temperature changes to a maximum of about 2~ (l~ Tolerances to momentary temperature changes are higher, although any rapid change will, at the least, be a source of stress. Fortunately, most culture organisms of interest are estuarine, with relatively high tolerances to temperature changes. Deep sea organisms and those from isothermal environments may prove much more sensitive to both water temperature values and fluctuations. Most of the requirements are for seawater heating rather than cooling. The two requirements are very similar. The principles, concepts, and some of the equipment would be common. The major difference would be in the need for a heat source for heating rather than a heat sink for cooling. It is often not appreciated that heating or cooling seawater involves moving considerable energy. The energy units most commonly used are the British Thermal Unit (BTU) and the gram-calorie. A BTU is defined as the energy necessary to raise the temperature of one pound of water one degree Fahrenheit at a water temperature of I~ The gram-calorie is the energy necessary to raise one gram of water one degree centigrade at 4~ In place of the gram-calorie, the kilogram-calorie or large calorie (1000 gram-calories) is often encountered in the literature. These heat-energy values for water can be assumed to be constant with temperature and salinity over the conditions encountered in aquatic systems. The efficiencies of the processes are defined as the energy moved divided by the energy input. For heating, this value must be less than one or 100%. For cooling, if a refrigeration cycle is used, the value is called the coefficient of performance and usually has a value greater than one. Energy flows (power), into or out of the seawater, can become substantial. See Appendix A for conversion factors between the various energy and power units. Thus to raise or lower 1 gpm I~ requires 8.5 BTU/min or 0.2 hp (149 W), assuming 100% efficiency. One lps raised or lowered I~ requires 1026 calories/s or 4300 W (5.8 hp). As an example, a winter
152 Seawater Temperature Increase (~ 10 20 1200
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.
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Fig. ll.1. Seawater heating requirements. Based on a unit flow of one gallon per minute of seawater, fuel oil at 19,000 BTU/lb and specific gravity of 0.9, and several different overall thermal efficiencies. Lower efficiencies are due to heat losses at furnace, piping, and heat exchanger. Value will be determined primarily by degree of insulation provided to components. Also shown is the numerical example from Example 11.2.
heating requirement could be to raise 100 gpm 40~ This requires 800 hp to be transferred into the fluid. Typical fuel oil has a heat content of 19,000 BTU/lb, and this need would require about a gallon of fuel every 4.25 min or 339 gallons/day, assuming 100% efficiency. Fig. 11.1 provides information for a wider range of conditions. For higher flows, multiply the fight side results by the flow rate in gpm to get the required power or fuel oil inputs. The transfer efficiency will be determined primarily by the type of system and amount of thermal insulation provided. For high flow rates and large temperature increases, the energy costs can easily get out of hand, even at 100% efficiency. Seawater heating systems have been installed in test facilities and laboratories that could not be used to anywhere near their capabilities due to the impacts of high fuel costs on limited operating budgets. There are a few direct heating methods. One is the use of thermostatically controlled electric quartz or Teflon immersion units. While small ones are common in home aquariums, larger industrial units are available to very substantial power levels. Powers up to 1 kW are available in 120 volts and some as high as 3 kW may be available (see Example 11.1). Higher power units require higher voltages, which can substantially increase the risks around seawater to both culture organisms and operating personnel if not properly handled. These units typically have very high transfer efficiencies and can control the temperature to about a degree F (half degree C). However, they sometimes fail full 'on', resulting in an unintentional
153
Example 11.1. Electric immersion heat transfer Readily available electric power is limited to 120 V AC power. The highest readily available rating for a thermostatically controlled quartz immersion heater in 120 V AC is about 1 kW. How many gpm of seawater (30 ppt, 50~ can you heat to 80~ with a single immersion unit? Assume such electric resistance heating is 100% efficient. From Table A-3, average specific weight seawater (65~ is 63.8 lb/ft 3 Seawater is 63.8 lb/ft 3 or 7.48 gal/ft3 is 8.5 lb/gal Temperature change is 80~ - 50~ is 30~ Heat transfer in BTU/min is (flow in gpm)(8.5)(30) is 255(flow) Conversion factor, 1 BTU/min is 17.58 W 1000 W available is 56.88 BTU/min is heat transfer 255(flow) is 56.88 Seawater flow rate is 0.223 gpm per immersion heater Or directly from Fig. 11.1, 30~ 100%, read about 4.2 kW/gpm 1 kW/4.2 kW/gpm = 0.23 gpm
cooking of the culture organisms. Another possibility is direct steam injection. Some care would have to be taken as to its location in the system and, to our knowledge, it has never been used in culturing applications. Most heating or cooling processes need a heat source or heat sink. This is often also a fluid, which may or may not be seawater. If this fluid is in a closed loop and simply a carrier of heat from some other heat source or sink, it is called a working fluid. C o m m o n working fluids include steam, freshwater and a number of chemical refrigerants. Natural heat sinks and sources can be many and varied, and may be seasonal. They include lakes, the atmosphere, ocean, ground water, earth and rock. Any large mass with temperature above or below the culture water temperature can be utilized. The higher the heat capacity of the mass material the better. It may be possible to add heat to the mass in the summer and withdraw it in the winter. If these masses have temperatures that are higher (heating) or lower (cooling) than the desired culture temperatures, the processes can be simplified to simple heat transfer. If the heat sink is a pumpable fluid, it can be directly used through a heat exchanger. If the heat sink is a solid, a loop with a separate working fluid will be needed. If the heat source does not have quite high enough temperature, it can still be used with the addition of a heat pump. A heat pump is a refrigeration cycle run in reverse so that heat is removed from a 'cold' source and transferred to a 'warm' area. Depending on the temperatures of the heat sink and desired temperature, heat pumps can be designed to both heat and cool. Some care would have to be taken to prevent unintentional leakage of the working fluid into the culture system. Depending on the thermal fluid's properties and toxicity, this could be critical. The greater the temperature differences, the more attractive is the prospect of using natural heat sources or sinks. While such use is highly dependent on specific circumstances, when conditions permit, they often provide simple, reliable and inexpensive temperature control capabilities. If their temperatures are in the fight direction but not sufficient, they may still be used but powered assistance will be required.
154 For heating applications, the most common source of heat is an oil- or gas-fired boiler. These units are commercially available in a wide range of sizes and have excellent reliability. However, they generally have electrically powered controllers and thus will automatically shutdown on loss of electricity, even though they are run from an independent power source. This is an important factor in evaluations of overall system reliability and of need for backup electric power. Other sources of heat include waste heat from engines, power plants or solar collectors. Many of these heat sources may not be under the control of the culturist. Sudden loss or change in the heat supply could be catastrophic. Reliability and assurance of supply issues have precluded use of what might otherwise be a valuable resource. For cooling a refrigeration or chiller unit is generally used to cool a working fluid (usually a chemical in a sealed loop) to below the desired seawater temperature. These units usually include an evaporator, compressor, condenser, expansion valve, and control circuitry. They tend to be more complex and difficult to operate than heating units. A very common mistake is to enclose the refrigeration system in a small space without adequate ventilation. Since the heat removed from the working fluid must be discarded by the condenser, if ventilation is inadequate the space will quickly heat up and system performance will rapidly drop.
11.2 Heat exchangers Most heating and cooling processes will require the use of one or more heat exchangers. The heat exchanger separates the working fluid from the process water, while efficiently transferring heat between the two fluids. This prevents the corrosion of the heating or cooling equipment by seawater and contamination of the process water by potentially toxic working fluids. Due to the corrosive properties of seawater, considerable engineering efforts have been devoted to the design of marine heat exchangers. However, their use in aquatic culturing systems involves biological constraints not typically of other industrial seawater heat exchangers (see Sections 8.1 and 8.2). Heat exchanger types that have been successfully used in marine culturing applications are shown in Table 11.1. Some are for use with pressure on both the seawater and working fluid sides. Others can be used directly in open tanks, with only the working fluid being pressurized. In this case, the circulation in the tank has to be controlled to assure uniform heat transfer (see Fig. 16.1). Carbon and titanium units are particularly well suited to most marine culturing applications. Stainless steel (type 316) heat exchangers are readily available and common in marine applications, but should be used with caution in culturing systems (see Section 8.2). The symbol U shown in Table 11.1 is the overall heat transfer coefficient generally associated with the respective heat exchanger types and materials. It is an important parameter in sizing heat exchangers and is generally stated in equipment specifications. Heat transfer is a complex subject and is dependent on flow configuration, heat exchanger internal design, materials and fluid properties. Flow configurations include parallel, counter and cross flows. Counter flow is the most efficient in that it results in the smallest heat exchanger. It is also the type most likely to be encountered. In counter flow the seawater to be heated or cooled flows in the opposite direction to the working fluid. Total heat transfer must be integrated over the entire heat exchanger. Assuming steady flows, no phase changes,
155 TABLE 11.1 Industrial heat transfer devices for heating and cooling seawater (modified from Huguenin, 1976a) Material
Types available
Comments
Impervious carbon
Bayonet and plate immersion, crossbore and shell/tube, cascade coolers, etc.
Heat transfer areas of 0.5-12,000 ft 2, in order of increasing size-bayonet, plate, crossbore and shell/tube. Carbon has high thermal conductivity. For shell/tube water-water U = 150-250 BTU h -1 ft -2 ~
Glass
Coil, bayonet, cascade coolers, and shell/tube
Low thermal conductivity of glass compensated for by higher film coefficients and reduced fouling. Coils with 2-120 ft2 of heat transfer area are available. Shell/tube areas in multiples of 13.5 or 60 ft2. For shell/tube water-water U = 125-195 BTU h-1 ft-2 OF-1
Glass (electric heating only)
Bayonet-type quartz immersion heaters
Units of 300-1000 W with built-in thermostats will hold within 0.5~ Units of up to 36,000 W available in several configurations. Maximum of 3000 W possible with 120 volt power.
Teflon
Immersion coils and shell/tube
Low thermal conductivity but relatively high film coefficients and reduced fouling. High frictional loses across tubes. Heat transfer areas of 2-900 ft 2 and in small volumes. Shell/tube water-water U = 25-40 BTU h -1 ft -2 ~
Titanium
Heat transfer panels, plates and shell/tube
Variety of standard panel configurations with areas of 2-42 ft 2. Panel water-water U = 100-175 BTU h -1 ft -2 ~ value depends heavily on seawater circulation. Plate water-water U = 550-800 BTU h -1 ft -2 ~ -1 but high frictional loses. Titanium offered as optional material by some manufacturers of large industrial shell/tube heat exchangers. Shell/tube water-water U = 225-280 BTU h -1 ft -2 ~ -1
Stainless steel (Type 316)
Heat transfer panels, plates, shell/tube, etc.
Most manufacturers offer their units in 316 stainless. Very wide selection of sizes and types. Under some conditions 316 stainless steel can be incompatible with seawater and culturing applications. Panel water-water U = 100-175 BTU h -1 ft -2 ~ Shell/tube water-water U -- 225-280 BTU h -l ft -2 ~
c o n s t a n t specific heats, and n e g l i g i b l e h e a t losses, p r o v i d e s the e q u a t i o n s below. Q-
UA(LMTD)
L M T D --
(11.1)
DTA - DTB
(11.2)
ln(DTA/DTB) w h e r e Q is h e a t t r a n s f e r ( B T U / h ) , U is o v e r a l l coefficient o f h e a t t r a n s f e r ( B T U h -1 ft -2 ~
A is e f f e c t i v e a r e a o f h e a t e x c h a n g e r (ft2), L M T D is log m e a n t e m p e r a t u r e d i f f e r e n c e
o f b o t h fluids (~
DTA is t e m p e r a t u r e d i f f e r e n c e s b e t w e e n inputs (~
d i f f e r e n c e s b e t w e e n outputs (~
DTB is t e m p e r a t u r e
F o r c o u n t e r - f l o w w i t h b o t h fluids w a t e r and about equal flow
rates, DTA -- DTB and L M T D - DTA. Eq. 11.1 is a g o o d a p p r o x i m a t i o n for all t y p e s o f h e a t e x c h a n g e r s , a l t h o u g h a c o r r e c t i o n factor m u l t i p l i e r (less than 1) is n e e d e d for c o m p l e x m u l t i - p a s s units. T h u s for the c o m m o n
156
Example 11.2. Shell and tube heat exchanger sizing We have a winter heating requirement and wish to heat a maximum of l0 gpm of incoming seawater from 40~ to 70~ We wish to use a single pass carbon shell and tube heat exchanger. The heat exchanger will be connected in a counter-flow arrangement with its own circulator pump to a hot water system and it is expected to provide 20 gpm at 200~ (see Fig. 11.2). We need to size the heat exchanger, and for estimating operating costs, we need to estimate heat inputs and fuel oil requirements. From Table 11.1 for carbon shell and tube heat exchangers find a U-value of 150-250, so we will use U = 200 BTU h -l ft -2 ~ Heat input into the seawater can be obtained from Fig. 11.1 by using 100% efficiency. This results in a requirement of about 255 BTU/min per gpm. For 10 gpm, this means that 2550 BTU/min or 153,000 BTU/h must to transferred into the seawater. While the seawater in (40~ and out (70~ temperatures and the heating system in (200~ temperature are known, the hot water exit temperature may not be known. In this case the circulator flow rate is given and from knowing that the heat gained by the seawater must have come from the heating water, assuming no heat losses in the heat exchanger, the exit temperature must be about 185~ If the heating system has a high circulating flow, the temperature drop will be small. Even if one assumes no temperature drop of the hot input water, this will often not substantially change the required heat exchanger area. DTA = 2 0 0 - - 4 0 = 160~ DTB= 1 8 5 - 7 0 = l15~ LMTD = (160 - l15)/ln (160/115) = 136.3~ Q = 153,000 = (U)(A)(LMTD) = (200)(A)(136.3) A = 5.6 ft 2 of heat exchanger area From available options supplied by manufacturers, choose the next size above 5.6 ft 2. There will be some heat losses in the heating plant, heat exchanger, and piping runs. These losses will depend on the provided insulation. If we assume a 70% overall efficiency (30% thermal losses), going into Fig. 11.1 with a 30 degree temperature increase we find an estimated fuel oil requirement of about 3.8 gal/day per gpm or about 38 gal/day.
case of a s i m p l e counter-flow heat e x c h a n g e r , a s t r a i g h t f o r w a r d a p p r o x i m a t i o n is avai l abl e for e x c h a n g e r sizing (see E x a m p l e 11.2). A n e x a m p l e for h e a t i n g in an o p e n tank is given in E x a m p l e 11.3. T h e s e a w a t e r input and d e s i r e d output t e m p e r a t u r e s are usual l y k n o w n , as is the input t e m p e r a t u r e of the w o r k i n g fluid. Q can be c a l c u l a t e d f r o m the s e a w a t e r flow rate and r e q u i r e d t e m p e r a t u r e c h a n g e , l e a v i n g only the u n k n o w n h e a t - e x c h a n g e r h e a t - t r a n s f e r area. If n e e d e d , the w o r k i n g fluid output t e m p e r a t u r e can be c a l c u l a t e d f r o m fluid properties, a s s u m i n g that there are no heat losses out of the heat exchanger. C o r r e c t i o n factors for m u l t i - p a s s shell and tube units can be o b t a i n e d f r o m e n g i n e e r i n g texts such as M a r k ' s H a n d b o o k ( B a u m e i s t e r et al., 1978) or e q u i p m e n t m a n u f a c t u r e r s . A m o r e c o m p l e x shell and tube e x a m p l e , w i t h o u t s o m e of the s i m p l i f y i n g a s s u m p t i o n s p r e v i o u s l y m a d e , is p r e s e n t e d in E x a m p l e 11.4. W h i l e heat e x c h a n g e r s are not u s u a l l y cheap, they are not n o r m a l l y the d o m i n a n t cost c o m p o n e n t in a h e a t i n g or c o o l i n g system. B e c a u s e of m a n y uncertainties in r e q u i r e d c a p a c i t y or transfer coefficients, it is g o o d practice to c o n s e r v a t i v e l y size the heat exchanger. In this
157 way, the system's performance will be limited by the capacity of the heat source or sink, rather than by the heat exchanger. When money is limited, a number of inexpensive heat exchangers can be built, especially for temporary or small-scale applications. If a little rust is acceptable, epoxy painted cast iron radiators can be successfully used. Interestingly, a thin epoxy coat is better than a thicker one, because it is less likely to develop small cracks in the coating due to thermal stresses and will have higher heat-transfer rates. Even materials with poor thermal conductivity can be used, if the area is sufficiently large. Large coils of polyethylene pipe have been used as a heat exchanger in outdoor algae tanks. In this case, the heat source was brackish ground water at about 55~ (13~ It was sufficient to keep the algae ponds from freezing and in dramatically improving winter algal production for a commercial shellfish hatchery. 11.3 P r o b l e m areas
Cross-contamination between the working fluid and the process water can cause serious damage to both systems. Probably the biggest single risk is leakage through the heat exchanger or some of its associated fittings. Glass and carbon heat exchangers in particular are brittle and can crack due to miss handling during shipment or installation. Leakage of seawater into the working fluid can cause corrosion problems in the heating or chilling unit and dramatically alter the performance of the system. Since the working fluid is likely to be at a higher pressure than the process water, leakage into the culture system is more likely than the other way around. Such leakage of working fluid into the culture system can be serious, since many of the possible working fluids have significant toxicity to culture organisms. Even if the working fluid is fresh water, it may contain significant quantities of undesirable contaminates (rust inhibitors, metals, hydrocarbons, etc.). The heat source or sink may be the heating or cooling system of a building or industrial plant. This can work, but there are some potential problems in addition to cross-contamination. The building systems were probably not designed with culturing uses in mind. The loads on these systems from the culturing requirements and their consequences, as well as the mechanics of 'tapping-in', have to be investigated. From the culturing standpoint, it may be critical to understand the workings and timings of other demands and functions upon the system. The heat source/sink may be sufficiently variable in its temperature and flow as to make control of the culture unit's heating/cooling system very difficult. Resulting temperature variations could be lethal. The probability of total or partial failure also has to be considered. In addition, the plant being relied upon may have inconveniently planned and unplanned shutdowns over which the culturist has little control or influence. Some of these shutdowns may be for a number of possible reasons, such as maintenance, overhaul, or during holidays. Powering down during periods of school holidays is common at some universities. It may be cheaper and less risky to have a completely dedicated system to support culturing operations. There are a number of ways in which heating and cooling systems can fail. The heat source or sink can be cut off. Even momentary events may shutdown the system. Even when the power source is an on-site fuel, electric power losses to the controls can shutdown the system. On return of electric power, the system will often have to be manually restarted. Pressure losses, flame losses, insufficient fluid levels, etc. can also cause shutdown. If culture water continues to flow, the culture organisms may see a sudden and dramatic temperature change and the consequences could be lethal. It is usually better to immediately stop the flow, even though
158
Example 11.3. Open tank seawater heating A thin-walled titanium heat transfer panel is required to heat 5 gpm of 30 g/kg seawater from 30~ 70~ in a large open tank (8 ft long, 4 ft wide and 1 ft water depth) as shown in Fig. 16.1. This is the most demanding heating requirement that the system will experience. Hot water (200~ in large quantities is supplied from the building's hot water heating system. (A) Calculate the minimum panel heat transfer area required (being optimistic) and state the actual size that you would specify for this application. Table 11.1 gives a U-value of 100-175 BTU h -1 ft -2 ~ for this type of heat exchanger and states that the value is highly dependent on circulation. From Fig. 16.1 it is clear that circulation in the tank is artificially aided by a pump, greatly increasing convective heat transfer. The desired temperature increase is 70 ~ - 30 ~ = 40~ From Fig. 11.1, this requires an input of 345 BTU/min per gpm and 26 gal of fuel oil/gpm per day at 100% efficiency. If there is little opportunity for heat loss except between the two fluids across the heat exchanger, the efficiency will be high. At 100% and a flow of 5 gpm, the total requirement is for 1725 BTU/min or 130 gal of fuel oil per day or equivalent at the exchanger. Remember that this is a small research system and that the heating requirement is continuous seven days a week. The total fuel requirement would have to be adjusted for boiler inefficiencies and heat losses in the building's distribution system. Even though the panel is not completely a counter flow situation, the forced convection makes it a fair assumption and Eq. 11.1 can be used. The flow rate from the building's hot water system is not known but is stated to be high. The temperature drop of the hot water across the heat exchanger can therefore be assumed to be negligible. DTA = 2 0 0 - - 3 0 = 170~ DTB = 2 0 0 - 7 0 - - 130~ LMTD--(DTA--DTB)/ln(DTA/DTB) = (170 ~ 130~176 ~ = 150 U -- 175 BTU h-l ft-2 OF-1 (good circulation and being optimistic) Q = 1735 BTU/min = 103,500 BTU/h Q - - ( U ) ( A ) ( L M T D ) = 103,500 = 175 • a • 150 A -- 4
ft 2
(minimum panel heat transfer area, no losses, optimistic U)
A safety and contingency factor of 2 would not be inappropriate, making the area requirement 8 ft 2. The same operating conditions but using the lowest U (100), calculates out to 7 ft 2. The tank's water depth is 1 ft. If the panel is oriented in the preferred vertical plane (why would a horizontal orientation be less efficient?), this makes its maximum width to be 1 ft. For a required heat transfer area of 8 f t 2 and considering that the panel has two sides, results in a 1 x 4 ft heat transfer panel as shown in Fig. 16.1. (B) What are the potential problems inherent with this type of system? Discuss and quantify these problems and your approaches to reduce the risks and potential adverse consequences. 9
If the cold incoming seawater is saturated with dissolved gases, heating it will result in substantial levels of supersaturation. From Table 2.7, it can be seen that the saturation concentration of oxygen at 0~ is 11.9 mg/1 but only 7.5 mg/1 at 21~ producing a 158% supersaturation of oxygen. Similarly for the other dissolved gasses, especially for the more critical inert gases such as nitrogen. Fortunately this supersaturation takes place in a large open tank. The flow of 5 gpm (0.67 ft 3/min) and the tank's water volume of 32 ft 2, results in a residence time of 48 rain. The amount of gas removed in the tank will be limited, and some type of degassing system may be needed. For sensitive larval stages, degassing will be needed if hot and cold water are mixed even if both waters are saturated with dissolved gases. This is a result of the non-linearity of gas solubility with temperature.
159
E x a m p l e 11.3. (continued) 9
Another potential problem is loss of heat input, possibly due to heating plant shutdown or failure. It is vital to not expose the culture organisms to a rapid 40~ temperature drop. Even if the thermal shock is tempered by the water mass in the tank, it is almost certain to still be fatal. Prompt action is essential. Note in Fig. 16.1 that an alarm system is provided. If prompt restart of heating can not be accomplished, the cold water inflow must be stopped and water conservation measures (priority allocations and aeration) implemented. Auxiliary heating, such as immersion heaters, might be used to allow some cold water inflow. Electric heaters may not help much if the original failure is a loss of electric power to the building heating system's controls. In this case, a backup power source, even if limited, might prove critical. If the tank runs dry, the headbox pump must be turned off to prevent pump damage.
9
Another potential disaster is loss of seawater inflow. Since this heat exchanger is manually controlled, the water in the tank would be heated to levels well beyond those desired before being depleted. This type of failure would expose the culture organisms to a high temperature thermal shock followed by loss of flow. Prompt action is also required in this case. If water inflow can not be quickly restored or otherwise acquired, the heating must be turned off and the same water conservation measures previously mentioned imposed.
loss o f w a t e r m a y r e s u l t in d e c r e a s i n g d i s s o l v e d o x y g e n c o n c e n t r a t i o n or b u i l d u p o f a m m o n i a a n d o t h e r m e t a b o l i c s . T h e loss o f w a t e r c a n b e p a r t i a l l y c o m p e n s a t e d b y i n c r e a s i n g a e r a t i o n . H o w e v e r , this c a n b e d o n e o n l y if the f a i l u r e is i m m e d i a t e l y d e t e c t e d a n d a c t i o n taken. A s a c o n s e q u e n c e , h e a t - e x c h a n g e r c u l t u r e - w a t e r o u t p u t t e m p e r a t u r e s are a c o m m o n m o n i t o r i n g
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-
I ~
~ Drain
Fig. 11.2. Typical temperature control configuration using hot water heat source. Expanding and reducing fittings, circulator pump and line valves are not shown.
160
Example 11.4. Performance of given shell and tube heat exchanger You acquire use of a single pass counter-flow shell and tube titanium heat exchanger with 10 ft 2 of heat transfer area in a hot water loop as shown in Fig. 11.2. The dedicated hot water circulator provides 10 gpm at a temperature of 200~ at the heat exchanger. You have to maintain marine animals at 70~ during the winter. The coldest winter seawater temperature is 30~ the salinity is 30 ppt and you can assume negligible heat losses in and around the heat exchanger. (A) What is the maximum seawater flow rate you can count upon during the coldest part of the winter? The heat gained by the seawater must equal the heat lost by the hot water. The seawater inlet and outlet temperatures and hot water inlet temperature and flow are known. The seawater flow rate, hot water outlet temperature and the heat transfer (Q in BTU/h) are not known. The specific heat of seawater and freshwater are both about 1. The average specific weight of the seawater is estimated at 63.8 lb/fl 3 and 61.0 for the hot freshwater. F is the unknown seawater flow rate, T the hot water exit outlet temperature and 0.1337 the conversion factor (Table A-l) for gal to ft 3. The most pessimistic U of 225 from Table 11.1 for this type of heat exchanger is used in Eq. 11.1. Heat gained by seawater (BTU/h) = heat loss by hot water (BTU/h) A TswgswFsw = A THW)/I-IWFHW Q -- (60)(0.1337)(40)(61)F -- (60)(0.1337)(200 ~ - T)(63.8)(10) = 225(10)LMTD Q = 19,574F = 1,023,607-5,118T = 2,250(LMTD) Three equations and three unknowns, assuming a seawater flow rate, solving for T and LMTD and comparing to Q from Eq. 11.1, see below: Seawater flow, F (gpm)
Hot water outlet temperature, T
DTB, T - 7 0 (~
(~ 12 13 14 15
154.1 150.3 146.5 142.6
LMTD, 1 7 0 - DTB
Water balance, Q (BTU/h)
Eq. 11.1, Q (BTU/h)
234,888 254,462 274,036 293,610
274,651 269,090 263,475 257,625
In ( 170 \ 84.1 80.3 76.5 72.6
122.1 119.6 117.1 114.5
From inspection of the above and interpolation, the maximum seawater flow rate is about 13.6 gpm. (B) Assume that the boiler is relatively inefficient and the hot water distribution system is poorly insulated so that only half of the fuel oil's energy reaches the heat exchanger. If your seawater flow rate is the maximum, what is your worst case daily fuel consumption? From Fig. 11.1, temperature change is 40~ 50%, read 6.8 gal/day per gpm 6.8(13.6) = 92.5 gal/day
p o i n t o n a l a r m s y s t e m s . A f t e r the p r o b l e m c a u s i n g the s h u t d o w n is r e s o l v e d , the s y s t e m m u s t be c a r e f u l l y r e s t a r t e d to p r e v e n t t h e r m a l t r a n s i e n t s that m a y stress or kill the c u l t u r e o r g a n i s m s . Another major source of problems with heating and cooling systems involves problems w i t h the c o n t r o l s y s t e m . M u c h o f the a v a i l a b l e l i t e r a t u r e d e a l s w i t h t e m p e r a t u r e - c o n t r o l p r o b l e m s , e s p e c i a l l y for r e s e a r c h a p p l i c a t i o n s ( A p p e n d i x H). A t y p i c a l h e a t i n g a r r a n g e m e n t u s i n g h o t w a t e r f r o m a b o i l e r is s h o w n in Fig. 11.2. I f the p r o c e s s ( c u l t u r e ) w a t e r flow rate a n d
161 the ambient seawater temperature stay constant or change only slowly over a long period of time, this control system may be unnecessary (see Fig. 16.1). In this case the control system in the boiler, which maintained the hot water output from the boiler at a constant set temperature, and a heat-exchanger hot-water supply valve could be relied upon to accurately control the heat-exchanger culture-water output temperature. If there are tidal or daily ambient-seawater flow rate or temperature variations, a more active control system as shown in Fig. 11.2 will be required. This system monitors the process-water output temperature from the heat exchanger and controls a three way valve to bypass or use hot water from the boiler. If the 'dead band' on the heat-exchanger controller is set too wide, the resulting temperature fluctuations may stress the animals. This effect can be greatly reduced with higher water residence time in the culture units to dilute the temperature-controlled influent. If the band is too tight, the control system will be continuously operating, may overshoot and may become unstable, producing temperature pulses. If the power or air to the controller fails, the heating system will usually fail closed, which is the same as a heat loss. If it is configured to fail open, the culture organisms may be unintentionally cooked.