microchannel heat exchanger for two-phase mixed …...printed circuit heat exchanger (pche) is one...

Microchannel heat exchanger for two-phase Mixed Refrigerant Joule Thomson processSeungwhan Baek, Jisung Lee, Cheonkyu Lee, and Sangkwon Jeong

Citation: AIP Conference Proceedings 1573, 612 (2014); doi: 10.1063/1.4860758 View online: http://dx.doi.org/10.1063/1.4860758 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1573?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Mixed refrigerant Joule-Thomson sorption cryocoolers AIP Conf. Proc. 1573, 1049 (2014); 10.1063/1.4860821 Investigations on two-phase heat exchanger for mixed refrigerant Joule-Thomson cryocooler AIP Conf. Proc. 1434, 706 (2012); 10.1063/1.4706982 EXPERIMENTAL INVESTIGATION ON MIXEDREFRIGERANT FOR CLOSEDCYCLE JOULETHOMSONCRYOCOOLERS AIP Conf. Proc. 1218, 1121 (2010); 10.1063/1.3422274 Properties of Gas Mixtures and Their Use in MixedRefrigerant JouleThomson Refrigerators AIP Conf. Proc. 710, 1677 (2004); 10.1063/1.1774866 Research on the change of mixture compositions in mixed-refrigerant Joule-Thomson cryocoolers AIP Conf. Proc. 613, 881 (2002); 10.1063/1.1472107

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Microchannel Heat Exchanger for Two-phase Mixed Refrigerant Joule Thomson Process

Seungwhan Baek, Jisung Lee, Cheonkyu Lee and Sangkwon Jeong

Cryogenic Engineering Laboratory, Dept. of Mech. Eng. KAIST Yuseong-gu, Daejeon, 305-701, Republic of Korea

Abstract. Mixed Refrigerant Joule Thomson (MR-JT) refrigerators are widely used in various kinds of cryogenic systems these days. Printed Circuit Heat Exchanger (PCHE) is one of the promising cryogenic compact recuperators for MR-JT refrigerators due to its compactness, high NTU and robustness. However, PCHE composed with microchannel bundles can cause flow mal-distribution, and it can cause the degradation of thermal performance of the system. To mitigate the flow mal-distribution problem, the cross link (or intra-layer bypass) can be adapted to parallel microchannels. Two heat exchangers are fabricated in this study; one has straight channels, and the other one has intra-layer bypass structure between channels to enhance the flow distribution. The MR-JT refrigerators are operated with these two heat exchanger and the no-load temperatures are compared. The lower no load temperature achieved with the intra-layer bypass structured heat exchanger. The results indicate that the flow mal-distribution in the microchannel heat exchanger can be mitigated with intra-layer bypass structure, and relaxation of flow mal-distribution in the heat exchanger guarantee the MR-JT refrigerator’s performance.

Keywords: flow mal-distribution, microchannel, mixed refrigerant, Joule Thomson, heat exchanger PACS: 51.30.+i, 44.35.+c, 07.20.Mc ;

INTRODUCTION

The cryogenic mixed refrigerant Joule Thomson (MR-JT) refrigerator requires a high performance heat exchanger for the successful operation. The most frequently reported configuration of a high performance heat exchanger in the mixed refrigerant Joule Thomson refrigerator is the tubes in tube configuration 1. The hydraulic diameter of the tubes in tubes heat exchanger is fairly large (Dh > 1 mm), therefore the size of the system becomes larger. To reduce the size of the system, the microchannel (Dh < 1 mm) heat exchanger can be applied to the MR-JT refrigerator.

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FIGURE 1. (a) The simple mixed refrigerant Joule Thomson refrigerator (b) effectiveness of heat exchanger with cold end temperature of mixed refrigerant Joule Thomson refrigerator (Ar:R14:R23:R218:R134a=12%:19%:20%:25%:24%, operating pressure = 1500 kPa ~ 400 kPa.)

Advances in Cryogenic EngineeringAIP Conf. Proc. 1573, 612-617 (2014); doi: 10.1063/1.4860758

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The microchannel heat exchanger can be manufactured by the technology of Printed Circuit Heat Exchanger (PCHE). The PCHE is a plate type compact heat exchanger with microchannels. PCHE was invented in 1980, in Australia, and subsequently incorporated into refrigerators in 1985, by Heatric (UK) 2. The development of PCHE is achieved with simple innovative manufacturing technologies, such as chemical etching and diffusion bonding processes. The flow channels of the PCHE are chemically etched into thin metal plates in various forms and plates are then stacked and diffusion bonded together. It is possible to construct a PCHE with a flow channel diameter that is much smaller than that of a commercial plate type heat exchanger by employing these techniques.

The major disadvantage of the PCHE is the flow mal-distribution in parallel microchannels. It is well known that the flow mal-distribution in heat exchangers deteriorates the effectiveness of heat exchangers 3, 4.

In this study, the impact of flow mal-distribution in a microchannel heat exchanger is experimentally observed through the MR-JT refrigerator operation. The method to mitigate flow mal-distribution in parallel microchannel heat exchanger is proposed, and applied to the fabrication of the heat exchanger to confirm the performance improvement. The feasibility of a microchannel heat exchanger for MR-JT refrigerator is also investigated in this paper.

HEAT EXCHANGER IN MR JT REFRIGERATOR

The effectiveness of the heat exchanger is the fundamental design parameter for a successful operation of a MR-JT refrigerator, because the no-load temperature of a MR-JT refrigerator is deeply related to the effectiveness. The effectiveness of the heat exchanger in MR-JT refrigerator can be calculated with the energy balance between the hot and the cold fluids (Figure 1 (a)). When the hot fluid inlet/outlet temperatures and the operating high/low pressures are assumed, the cold fluid inlet temperature can be calculated with the hot fluid outlet enthalpy value, because the JT expansion process is the isenthalpic process. The energy balance is taken account between the hot and the cold fluids; thus, the cold fluid outlet temperature can be calculated. The effectiveness is calculated with the four inlet and outlet temperatures (or enthalpy values) by equation (1).

max

q

q� � (1)

Figure 1 (b) displays the heat exchanger effectiveness according to the cold end temperature of the MR-JT

refrigerator, where the mixture composition is Ar:R14:R23:R218:R134a=12:19:20:25:24 (mol %), and the operating pressure is 1500 kPa and 400 kPa. The REFPROP9 5 is used for property calculations. The higher effectiveness of the heat exchanger is required to attain the lower cold-end temperature in the MR-JT refrigerator

When the heat exchanger’s specifications, such as the heat transfer area and the hydraulic diameter, are given, one can calculate the NTU value of the heat exchanger. The effectiveness of the heat exchanger can be identified with the NTU� � relation 6. Lastly, the no-load temperature of a MR-JT refrigerator can be easily estimated from Figure 1 (b).

MICROCHANNEL HEAT EXCHANGER FOR MR JT REFRIGERATOR

The previously fabricated microchannel printed circuit heat exchanger (PCHE-1) is shown in Figure 2 (a). The

specific geometry and the fabrication procedure is precisely described in the literature 7. The PCHE-1’s NTU value can be calculated with equation (2).

min

NTU=( )

HT

p

UA

mc(2)

The overall heat transfer coefficient (U) is defined with reciprocal sum of local heat transfer coefficients as

equation (3).


1 1 1

hot coldU h h� ��

�

(3)

When the Reynolds number is very low in microchannel, it can be assumed as fully developed laminar flow 8.

Therefore, the Nusselt number can be assumed as constant of 4.36 (equation (4)).

4.36h

MR

hDNu

k� � (4)

To calculate the NTU value, the mass flow rate is measured from several preliminary experiments. The

temperature averaged heat capacity of mixed refrigerant is used for the NTU calculation (equation (5)).

in outp

in out

i ic

T T

��

�(5)

The temperature averaged values of thermal conductivity and viscosity are utilized to calculate NTU. The

calculation result shows the NTU value of 29, and the effectiveness regarding to the NTU value indicates 0.97. The estimated no-load temperature of mixed refrigerant Joule Thomson refrigerator with the PCHE-1 is around 180 K.

However, the PCHE-1 is composed with multi-parallel microchannels, the flow mal-distribution in the header is expected. To mitigate the flow mal-distribution effect, the flow re-distribution structure inside the heat exchanger is considered. Megahed and Dang 9, 10 have reported that the cross link (or intra-layer bypass)structure is effective for the flow re-distribution in parallel microchannels. The PCHE-2 is fabricated with the cross link (or intra-layer bypass) structure (Figure 2 (b)). The channel shape and the number of layers of the PCHE-2 is identical with those of the PCHE-1. If the operating conditions are equivalent, then two heat exchangers will show same effectiveness value, because the PCHE-2 has same heat transfer area with the PCHE-1. However, the degree of flow mal-distribution is different due to the flow re-distribution structure in the PCHE-2. Therefore, the PCHE-2 is expected to show the higher effectiveness than the PCHE-1. The MR-JT refrigerator is operated with these two heat exchangers to compare the no load temperature.

FIGURE 2. (a) PCHE-1: composed with straight parallel channels (b) PCHE-2: intra-layer bypass installed to enhance flow distribution


EXPERIMENTAL APPRATUS

The MR-JT refrigerator with a microchannel heat exchanger is operated using the test apparatus depicted in Figure 3 (a). The MR is charged from each separate component bottles to the compressor. The MR is circulated by a helium compressor (Helix, CTI-8200-air cooled), and passes through the heat exchanger. When the mixed refrigerant passes through a JT (Joule Thomson) expansion part, it generates cold temperature. The JT part is made of 70 cm thin stainless tube with 1.0 mm inner diameter. The MR flows to the heat exchanger after the expansion through a JT part. The MR from the heat exchanger goes back to the compressor to be pressurized to constant high pressure.

The mass flow rate of MR is measured by a mass flow meter (Micromotion, CMF025 with 1700 transmitter) which is located between the compressor and the cold out flow of heat exchanger. Silicon diode thermometers (Lakeshore DT-670SD) are attached to the surface of inlet and outlet tubes of the heat exchanger to measure the flow temperatures with respect to various mass flow rates. Pressure transducers (SENSYS, PSHD 30 bar) are attached to the inlets and outlets of the heat exchanger. The experimental setup is vacuum insulated to eliminate background losses due to conduction and convection. All the pipes inside vacuum chamber are soldered to eliminate the leakage of mixed refrigerant at cryogenic temperature.

Temperature data are collected by monitoring device (Lakeshore, Temperature Monitor 218). Mass flow rate and pressures are collected by data acquisition system (NI-USB6210). All collected data are recorded by software from personal computer (National Instrument, Labview 8.2). The composition of the mixed refrigerant is measured by gas chromatography instruments (Younglin, GC6000) during the operation. The Figure 3 (b) shows the picture of installed PCHE to MR-JT system.

FIGURE 3. (a) Schematic of experimental setup (b) The PCHE installed to the MR JT refrigerator

EXPERIMENTAL RESULTS

The Figure 4 (a) and (b) show the experimental results of MR-JT test with the PCHE-1 and the PCHE-2. The

lowest temperature recorded with the PCHE-1 is 185.6 K after 3 hours of operation (Figure 4 (a)). The pressure of hot stream is kept as 1500 kPa, and cold stream’s pressure is kept as 400 kPa. In the meantime, the expected minimum low temperature is around 180 K, however the experimental result with PCHE-1 shows quite higher value than expected value. The cause of the higher no load temperature is the flow mal-distribution in the heat exchanger.

The MR-JT refrigerator experiment is continued with the PCHE-2. Figure 4 (b) shows the cool down characteristics of MR-JT refrigerator with the PCHE-2. The lowest temperature recorded with the PCHE-2 is 180.6 K.


To ensure that two experiments were in identical condition, the experimental temperatures before and after JT expansion are compared. Furthermore, the predicted temperatures before and after JT expansion with REFPROP 9 are also compared with experimental results. Figure 5 (a) shows the comparison results. The experimental temperatures before and after JT expansion from two experimental results show identical tendency. From this comparison results, we can confirm that the mixed refrigerant composition and the pressure ratio are kept identical within two experiments. The only difference is the lowest temperature achieved.

It is sure that the flow mal-distribution is mitigated in PCHE-2, therefore the lower no-load temperature is achieved in the MR-JT refrigerator. The experimental effectiveness values of two heat exchangers are compared in Figure 5 (b). The PCHE-2 shows the lower no load temperature and the higher effectiveness than the PCHE-1. These results indicate that the flow mal-distribution significantly affects the performance of the heat exchanger and the refrigerator, simultaneously. The cross link (or intra-layer bypass) structure is effective to mitigate the flow mal-distribution in the parallel microchannel heat exchanger.

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FIGURE 4. Experimental MR-JT refrigerator results; temperature and pressure with time (a) PCHE-1, (b) PCHE-2

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FIGURE 5. (a) Comparison of temperature before/after JT expansion from two experiment results (b) effectiveness comparison for two experiment results

SUMMARY

The microchannel heat exchanger is a candidate for the high effectiveness heat exchanger for MR-JT applications.

The two microchannel heat exchangers are fabricated with PCHE technology; one has straight channels, and the other one has the cross link (or intra-layer bypass) structure between channels to enhance the flow distribution. These two heat exchangers are installed to the MR-JT refrigerator to compare the heat exchanger’s performances. The MR-JT refrigerator equipped with the better flow distribution heat exchanger has achieved the lower no load temperature and indicated higher effectiveness. From these experimental results, the cross link (or intra-layer


bypass) structure is effectively mitigated the flow mal-distribution inside the heat exchanger and guarantees the MR-JT refrigerator’s performance.

ACKNOWLEDGMENTS

This work was supported by the Power Generation & Electricity Delivery of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 2011101050002B).

REFERENCES

1. B. Maytal, J. M. Pfotenhauer, Miniature Joule-Thomson cryocooling. (Springer, 2013). 2. D. A. Reay, Heat Recovery Systems and CHP 14 (5), 459-474 (1994). 3. B. Prabhakara Rao, P. Krishna Kumar and S. K. Das, Chemical Engineering and Processing: Process

Intensification 41 (1), 49-58 (2002). 4. J. Jung and S. Jeong, Cryogenics 47 (4), 232-242 (2007). 5. E. W. Lemmon, M. L. Huber and M. O. McLinden, (National Institute of Standards and Technology, Standard

Reference Data Program, Gaithersburg, 2010). 6. G. Nellis and S. Klein, Heat transfer. Cambridge University Press, 2009. 7. S. Baek, J. Kim, G. Hwang and S. Jeong, AIP Conference Proceedings 1434 (1), pp. 631-638 (2012). 8. B. S. Haynes and A. M. Johnston, (2002). 9. A. Megahed, International Journal of Multiphase Flow 37 (4), 380-393 (2011). 10. M. Dang, I. Hassan and R. Muwanga, Exp Fluids 43 (6), 873-885 (2007).


microchannel heat exchanger for two-phase mixed …...printed circuit heat exchanger (pche) is one...

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