Complete condensation in a vertical tube passive condenser

International Communications in Heat and Mass Transfer 32 (2005) 593 – 602 www.elsevier.com/locate/ichmt

Complete condensation in a vertical tube passive condenserB Seungmin Oh, Shripad T. RevankarT School of Nuclear Engineering, Purdue University, West Lafayette, IN 47907, USA Available online 31 March 2005

Abstract An experimental study is performed for the steam condensation in a vertical tube where steam is completely condensed. A condenser tube is submerged in a water pool where the heat from the condenser tube is removed through boiling heat transfer. The experiment data showed that the operating pressure is uniquely determined by inlet steam flow rate for the complete condensation. The condensation heat transfer rate increases and the condensation heat transfer coefficient decreases with the system pressure. For the condenser submerged in a saturated water pool, strong primary pressure dependency was observed on the condensation heat transfer. D 2005 Elsevier Ltd. All rights reserved. Keywords: Passive condenser; Complete condensation; Condensation heat transfer coefficient

1. Introduction Condensation phenomenon plays an important role in the heat transfer process in the chemical and power industry, including nuclear power plants. This mode of heat transfer is often used in engineering because high heat transfer coefficients can be achieved. In the General Electric’s simplified boiling water reactor (SBWR) the passive containment cooling system (PCCS) is a passive heat exchanger that allows the transfer of heat via steam condensation to the water pool [1]. The PCCS condenser must be able to remove sufficient energy from the reactor containment to prevent containment from exceeding its design pressure following a design basis accident. The rate of heat transfer in the PCCS condenser is strongly coupled to the hydrodynamic characteristics of the PCCS. B

Communicated by J.P. Hartnett and W.J. Minkowycz. T Corresponding author. E-mail address: [email protected] (S.T. Revankar).

0735-1933/$ - see front matter D 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.icheatmasstransfer.2004.10.017

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Hence a detailed knowledge of the variation of the heat transfer coefficient is necessary in order to predict the performance of the PCCS and for design optimization. Recently, the relevant separate effects experiments on PCCS condensation were conducted by Vierow [2], Ogg [3], Siddique [4,5], Kuhn [6,7] and Park and No [8] with the secondary jacket cooling method. In these experiments highly subcooled water supplied at the bottom of the cooling jacket is heated along the condenser tube, and exits from the top of the jacket. For the PCCS condenser application, the condensation data obtained from the secondary jacket cooling method has two shortcomings; one is difference in boundary condition and the other is difficulty in accurate measurement of secondary heat removal rate. Actual PCCS condenser tubes are submerged in water pool. During a postulated accident, the secondary pool quickly reaches saturated condition. So, the boundary condition at the actual PCCS condenser is a constant water temperature and heat transfer mechanism at the secondary side is the boiling heat transfer. But for the secondary cooling jacket method the boundary condition is a varying coolant temperature with forced convective heat transfer. Forced convective heat transfer mode is strongly dependent on the cooling water flow conditions, which are affected by the gap size of the cooling jacket annulus, coolant flow rate and temperature, the location of inlet and outlet nozzles for cooling jacket, the entrance and exit effects, and degree of turbulence mixing. The steam flow condition in the PCCS condenser tube is not always forced convection. Three different operational modes are possible in the PCCS depending on the non-condensable (NC) gas concentration and the pressure difference between the dry well (DW) and the suppression pool (SP). A schematic flow diagram of the PCCS for the SBWR is shown in Fig. 1. The PCCS condensers are immersed in a large interconnected pool of water. The pool is located outside and above the containment. The driving force of the PCCS is provided by the pressure difference between the DW and the SP. Condensed water produced in the PCCS condensers returns to the gravity drain cooling system (GDCS) pool and then to the reactor pressure vessel (RPV). The NC gas and the uncondensed steam from the PCCS are vented to the SP.

Non-Condensable Gas and Steam

PCCS

Condensed Water

Non-Condensable Gas and Uncondensed Steam

GDCS Steam SP

RPV Condensed Water DW

Fig. 1. Flow diagram in SBWR during a loss of coolant accident.

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The PCCS will be in Bypass Mode when the pressure difference between the DW and the SP is relatively high compared with the head due to the submergence of the vent line in the SP. In this mode, uncondensed steam and NC gas pass through the PCCS condensers with condensation. This mode of operation corresponds to forced convection and realized at the initial stage of the accident. When the pressure difference between the DW and the SP is comparable with the head due to the submergence of the vent line in the SP, the vent path from the PCCS to the SP is closed. Since the vent path is closed, the uncondensed steam and the NC gas are accumulated in the PCCS condensers. As the NC gas mass fraction increases the condensation performance is degraded, which results in the pressure increase in the DW. When the pressure difference between the DW and the SP is high enough to overcome the head due to submergence of the vent line in the SP, the vent path is opened and the NC gas is vented to the SP until the pressure difference is less than the head due to submergence of the vent line in the SP. The condensation performance is recovered after clearing of NC gas from the PCCS. This cycle repeats. In this mode, the DW pressure, the condensation rate and the NC gas mass fraction in the condensers are periodically changed. The PCCS will be in Complete Condensation Mode when the NC gas concentration is very low. This condition is realized at the later stage of an accidental transient after most of NC gas is vented to the SP. In this mode, the pressure difference between the DW and the SP is maintained below the head due to the submergence of the vent line in the SP and all the steam entering the PCCS condensers are condensed in the tubes. The previous separate effect tests were performed only for the forced convection condition, i.e., Bypass Mode. Present work provides a new database on the complete condensation mode of the PCCS condenser submerged in water pool.

2. Experiment program 2.1. Experimental loop The schematic of the experimental loop and test section is shown in Fig. 2. The test loop is comprised of steam generator (SG), instrumented condenser test section with secondary pool boiling section, condensate tank, suppression pool, storage tank, and associated piping and instrumentation. An immersion type sheathed electrical heater of 10 kW capacity is mounted at the lower flange of the SG. The specific design of the PCCS condenser tube test section was based on the scaling analysis. A half diameter and height scaling was taken in the present design. For the pool boiling heat removal at the secondary side, test section is designed with two subassemblies, primary condensing tube and secondary boiling tube. The primary condensing tube is made of 26.6 mm ID, 3.38 mm thickness, 2.4 m long type 304 stainless steel pipe and 108 mm ID, 114 mm OD stainless steel pipe with a top flange. The middle plate welded between 26.6 mm ID and 108 mm ID pipes act as a border of active condenser. 26.6 mm pipe below this plate is an actual condenser. The gap between 108 mm and 26.6 mm pipe above this plate is filled with thermal insulation material. The top flange welded to 108 mm ID pipe is connected to the top flange of the secondary boiling tube. The secondary boiling tube is made of 161 mm ID type 304 stainless steel pipe with a top flange. Three 38.1 mm diameter steam exit nozzles are located at the almost top level of pipe with 1208 each other. The secondary pool is maintained at saturation condition during experiment. Active length of condenser is 0.978 m. At

596

T

F

F

F

PG

Air supply

steam exit

Insulation

PT

TP1

FT

DP

Sight Glass

test section

1016

T

TP2 26.6

24

blowdown PG

TP3

LT

T

steam generator

blowdown & secondary steam purge

condensate tank

S/G

T

storage tank

TW2

TS2

228

LT

LT

TS1

TW3

TS3

TW4 TW5

PG

Fig. 2. Schematic diagram of test loop and test section.

TP4

TP5

228

suppression pool

TW1

30 228

vent valve

978 active condenser

LT PG

228 36

T

unit : mm

TS4 64 TP6 TS5

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steam

Flowmeters

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different axial and circumferential locations of the active tube, penetrations for inside steam bulk temperature measurement are located. At the opposite side of these penetrations, thermocouple junctions are attached to measure the tube outside wall temperature at five axial locations. To measure the secondary side temperature, six thermocouples are installed at the secondary pool at different axial locations. The evaporation rate during the experiment was calculated using the measurement of the secondary pool water level. A vortex flow meter is used to measure steam flow rate. The flow conditions at vortex flow meter are measured with pressure transducer and thermocouple installed at the downstream of the vortex flow meter. The condensate tank collects the condensate. It is mounted vertically and equipped with three thermocouples, a pressure gauge and a DP cell for level measurement. Condensation rate is calculated by the measurement of the condensate water level. For the venting of the NC gas and uncondensed steam, a vent valve is mounted on the vent line connected between steam space of the condensate tank and water space of the suppression pool. During the complete condensation experiment, this vent valve is closed. The storage tank serves as heat and mass sink and de-ionized water storage. The steam generated in the test section secondary side is discharged to this tank through three independent 38.1 mm copper tubing. A steady state steam flow rate can be obtained by pressurizing the SG up to 1.15 MPa. This pressure is maintained during the experiment. Since the condensation experiments are performed below 0.5 MPa at the test section, steam flow is choked at the flow control valve located at downstream of the SG. So, steam mass flow rate is independent of the pressure condition at the test section. When the SG pressure reach to about 1.15 MPa, the flow control valve is opened and steam condensation begins in the test section that heats up the secondary water pool. During this heat-up process, existing NC gas in the piping, test section and condensate tank is vented to the suppression pool. Tests for the complete condensation mode were performed for the pure steam condition varying the inlet steam flow rate. To obtain the complete condensation condition, the vent line valve to the SP is closed during the experiment. When the saturation condition is achieved at the secondary pool and the system pressure is stabilized, the data is acquired. 2.2. Data reduction The mass balance can be checked by the comparison between the supplied steam flow rate measured by vortex flow meter output and the condensation mass flow rate measured by the condensate tank water level difference during the complete condensation mode test. In this mode, all the steam supplied from the SG must be condensed in the condenser tube and collected in the condensate tank. Energy balance can be checked by the comparison between the condensation heat transfer rate and the secondary side heat removal rate, which is sum of the evaporative heat transfer rate and the heat loss from the secondary tube surface. The condensation heat transfer rate, Q c is calculated as Qc ¼ mc hfg ðPSAT Þ:

ð1Þ

The condensation mass flow rate, m c is calculated from the condensate tank water level difference during the test. The secondary heat transfer rate, h fg(T P) is calculated as Qsec ¼ meva hfg ðTP Þ þ QHL :

ð2Þ

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The evaporation mass flow rate, m eva is calculated from the secondary pool water level difference during the test. Latent heat, h fg(T P) is based on the secondary water pool temperature, T P. During the experiments, the measured steam bulk temperature was slightly higher than the saturation temperature at the system pressure. This implies that the state of the steam is at superheat. However, the amount of sensible heat transfer is much less than that of condensation heat transfer. So, the sensible heat transfer is ignored in data reduction. For the subsequent data reduction, the condensation heat transfer rate was used for the reference heat transfer rate instead of the secondary heat transfer rate. The overall heat transfer coefficient (HTC) in condenser tube is determined from U¼

Qc : Ai ðTSAT  TP Þ

ð3Þ

The overall HTC can be decomposed into the following equation using heat transfer at the condenser tube inside, across tube wall and at the tube outside.  1 1 1nðdo =di Þdi di U¼ þ þ : ð4Þ hc 2kw hsec do The first term of the right hand side of above equation corresponds to the tube side condensation heat transfer, the second term corresponds to the tube wall conduction heat transfer, and the third term corresponds to the secondary side pool boiling heat transfer. The secondary side pool boiling HTC is determined from Qc : ð5Þ hsec ¼ Ao ðTwo  TP Þ Neglecting the axial heat transfer along condenser tube and sensible heat, the condensation HTC is defined as Qc : ð6Þ hc ¼ Ai ðTSAT  Twi Þ From Eq. (4), the condensation HTC can be expressed as follows: 1 1 1nðdo =di Þdi di ¼  :  hc U 2kw hsec do

ð7Þ

By combining Eqs. (6) and (7), the average inside wall temperature can be calculated. The experimental error associated with the average condensation HTC was conservatively estimated to be F 20%. 2.3. Test results Fig. 3 shows the condensation heat transfer rate of the complete condensation mode with the system pressure. For a given inlet steam flow rate, the system pressure is uniquely determined by the heat removal capacity of the condenser. If the inlet steam flow rate is large, the system pressure increases to condense all the steam incoming to the condenser. Fig. 4 shows the overall, secondary, and condensation HTC for the complete condensation mode with system pressure and Fig. 5 shows the measured secondary pool temperature (T P), the condenser outside wall temperature (Two), steam saturation

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Condensation Heat Transfer, kW

12

8

4

0 0. 1

0.2

0. 3

0.4

0. 5

System Pressure, MPa

Fig. 3. Condensation heat transfer rate.

temperature (T SAT) and the calculated tube inside wall temperature (Twi) with system pressure. The overall HTC remains almost constant since the temperature difference (T SAT  T P) increases with the same rate of heat removal. The tube outside wall temperature increases slightly with system pressure. So the secondary HTC increases with the same rate of heat removal. The calculated inside wall temperature also increase and the temperature difference (dT SAT = T SAT  Twi) increases more rapidly than the increase of heat removal rate. So, condensation HTC decreases with increase of system pressure, i.e., the condensation HTC decreases with increase of the temperature difference. This result is consistent with 20 Heat Transfer Coefficient, kW/m2-K

Overall HTC Secondary HTC 16

Condensation HTC

12

8

4

0 0.1

0.2

0.3

0.4

System Pressure, MPa

Fig. 4. Heat transfer coefficients.

0.5

600

S. Oh, S.T. Revankar / International Communications in Heat and Mass Transfer 32 (2005) 593–602 150 Average Tp Average Two

Temperature, C

140

Average Tsat Calculated Twi

130

120

110

100 0.1

0.2 0.3 0.4 System Pressure, MPa

0.5

Fig. 5. Temperatures.

the classical Nusselt [9] solution for the vertical flat plate in which the average condensation HTC is 0.25 given as; proportional to DT SAT hNu

  4 4dlL dLd ðTSAT  Twi Þ 0:25 ¼ d 3 : 3 kL dqL dðqL  qv Þdgdhfg

ð8Þ

Fig. 6 shows the comparison of the average condensation HTC between the data and the Nusselt solution given in Eq. (8) with the temperature difference dT SAT. The trend of the average condensation 16

Condensation HTC, kW/m2-K

Data Nusselt Solution 12

8

4 0

5

10 dTsat, C

15

Fig. 6. Comparison of condensation HTC.

20

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HTC data is very similar to the Nusselt solution and the experiment data are about 15% greater than the Nusselt solution.

3. Conclusions Single tube complete condensation experiments were performed for the passive condenser system of the SBWR. A condenser tube is submerged in the secondary water pool where the condensation heat transferred from the tube is removed by the boiling process. PCCS condenser tube with half length and half diameter scale is used to obtain the condensation heat transfer coefficient. At the late stage of a postulated loss of coolant accident of the SBWR, the reactor containment is cooled by the PCCS condenser where steam is completely condensed. The experiment data showed that the operating pressure is uniquely determined by inlet steam flow rate for the complete condensation. The condensation heat transfer rate increases and the condensation heat transfer coefficient decreases with the system pressure. For the PCCS condenser design which has the fixed secondary side temperature, the strong primary pressure dependency is observed. The tube average condensation heat transfer coefficient data are about 15% greater than the Nusselt solution. The present experimental data provide a new database for the in-tube steam condensation submerged in water pool for the complete condensation mode of the PCCS.

Nomenclature A area d diameter g gravitational constant h heat transfer coefficient (HTC) latent heat of vaporization h fg k thermal conductivity L tube length m mass flow rate P pressure Q heat transfer rate T temperature U overall heat transfer coefficient Greek symbols l dynamic viscosity q density Subscripts avg average c condensation eva evaporation HL heat loss

602

i L Nu o P SAT TOT v w

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inside liquid Nusselt solution outside pool saturation total vapor wall

Acknowledgement This work was supported by US Department of Energy under Nuclear Energy Education Research (NEER) research grant with award number DE-00ID13928 for a project entitled: Analytical and Experimental Study of the Effects of Non-Condensable in a Passive Condenser System for the Advanced Boiling Water Reactor.

References [1] GE Nuclear Energy, SBWR Standard Safety Analysis Report, Report No. 25A5113 Rev. A (1992). [2] K.M. Vierow, Behavior of Steam–Air Systems Condensing in Cocurrent Vertical Downflow, MS Thesis, University of California at Berkeley (1990). [3] D.G. Ogg, Vertical Downflow Condensation Heat Transfer in Gas–Steam Mixture, MS Thesis, University of California at Berkeley (1991). [4] M. Siddique, The Effects of Noncondensable Gases on Steam Condensation under Forced Convection Conditions, PhD thesis, Massachusetts Institute of Technology (1992). [5] M. Siddique, M.W. Golay, M.S. Kazimi, Nuclear Technology 102 (1993) 386. [6] S.Z. Kuhn, Investigation of Heat Transfer from Condensing Steam–Gas Mixtures and Turbulent Films Flowing Downward inside a Vertical Tube, PhD thesis, University of California at Berkeley (1995). [7] S.Z. Kuhn, V.E. Schrock, P.F. Peterson, Nuclear Engineering and Design 177 (1997) 53. [8] H.S. Park, H.C. No, Nuclear Technology 127 (1999) 160. [9] W. Nusselt, Die oberflachenkondensation des wasserdampfles, Zeitschrift des Vereines Deutscher Ingenieure 60 (1916) 541.