The details of a simple single-stage refrigeration system, a two-stage refrigeration system employing one flash tank economizer, and with heat exchanger economizer system are given in Chapter 15 of Gas Conditioning and Processing, Volume 2 [1].

In the January 2008 Tip of the Month (TOTM) [2], we compared the performance of a simple refrigeration system with another employing a flash economizer. Specifically, we evaluated compressor power saving, the effects of compressor suction–line pressure drop and the interstage pressure drop on compressor power requirement and condenser duty.

A second type of economizer configuration is the heat exchanger economizer shown in Figure 1, which is the same as Figure 15.9 of reference [1]. Cold, low-pressure chiller vapor is used to subcool the saturated liquid refrigerant. This decreases the refrigerant circulation rate, and may reduce compressor power. In this TOTM we will evaluate quantitatively the performance of a case study comparing a simple refrigeration system with another one containing heat exchanger economizer.

The process flow diagrams for the simple and with heat exchanger (HEX) economizer refrigeration systems are shown in Figure 2. Note that provisions have been made to consider pressure drop in different segments of the loops.

”]Let’s consider removing 1.0×107kJ/h which is equal to 2778 kW (9.479 MMbtu/hr) from a process gas at -35°C (-31°F) and rejecting it to the environment by the condenser at a condensing  temperature of 35°C (95°F). Assuming 5 kPa (0.7 psi) pressure drop in the chiller and 5 kPa in the suction line pressure drop, the compressor suction pressure is 132.4 kPa (19.1 psi). The condenser pressure drop plus the pressure drop in the line from the compressor discharge to the condenser was assumed to be 50 kPa (7.3 psi); therefore, compressor discharge pressure is 1270 kPaa (184.2 psia). The compressor discharge temperature is 66°C (150.8°F). At these conditions, the condenser duty is 4434 kW (15.13 MMbtu/hr). Pure propane is used as the working fluid. In this study, all of the simulations were performed by UNISIM software [3].

Figure 2. Process flow diagrams for a simple refrigeration system and with a heat exchanger economizer

In order to study the effect of HEX economizer, we considered the following scenarios:

  1. The condensed liquid at temperature of 35°C (95°F) was cooled starting from 33 to 24 °C (91.4 to 75.2°F) with a step change of -1°C (-1.8°F).
  2. Step 1 was repeated three times for pressure drops of  20, 25, and  30 kPa ( 2.9, 3.63,  or 4.4 psi) on both sides of HEX economizer.
  3. For the above four cases the following variables were calculated:
  4. The required compressor power
  5. Compressor suction temperature
  6. Compressor discharge temperature
  7. Refrigerant (propane) circulation rate
  8. Condenser heat duty
  9. Chiller inlet temperature

Figures 3, 4, and 5 present the required compressor power, condenser duty, and HEX duty as a function of liquid propane subcooled temperature (at the outlet of HEX economizer), respectively. Figures 3 and 4 indicate that as the propane subcooled temperature decreases the compressor power and condenser duty decrease, too. However as the pressure drop in the cold vapor (low pressure) side increases, the compressor power and condenser duty increase. The pressure drop significantly increases the compressor power. Figure 5 indicates as the propane subcooled temperature decreases, the HEX duty increases independent of  HEX pressure drop.

Figure 3. Compressor power as a function of refrigerant subcooled temperature and HEX economizer pressure drop

 

Figure 4. Condenser duty as a function of refrigerant subcooled temperature and HEX economizer pressure drop

 

Figure 5. HEX duty as a function of refrigerant subcooled temperature and HEX economizer pressure drop

The refrigerant mass circulation rate, compressor suction temperature, and compressor discharge temperature as a function of propane subcooled temperature and HEX pressure drop are presented in Figures 6, 7, and 8, respectively. Figure 6 indicates as the propane subcooled temperature decreases, the mass circulation rate decreases independent of HEX pressure drop. Figures 7 and 8 indicate that compressor suction and discharge temperatures increase with decrease in propane subcooled temperature. However, the effect of HEX pressure drop on discharge temperature is more pronounced.

Contrary to a refrigeration system with a flash thank economizer in which the compressor power is reduced [2], by employing HEX economizer the power requirement increased. With regard to the compressor power, two factors offset the reduced circulation rate. The first is HEX pressure drop. The pressure drop on the low pressure side significantly increased compressor power, since the suction pressure was near atmospheric. Secondly, the refrigerant vapor entering the compressor is now super-heated. Although this reduces the likelihood of liquid carryover into the compressor, it resulted in higher power consumption due to the higher suction temperature.

To learn more, we suggest attending our G40 (Process/Facility Fundamentals), G4 (Gas Conditioning and Processing), G5 (Gas Conditioning and Processing-Special), and PF81 (CO2 Surface Facilities), PF4 (Oil Production and Processing Facilities), courses.

John M. Campbell Consulting (JMCC) offers consulting expertise on this subject and many others. For more information about the services JMCC provides, visit our website at www.jmcampbellconsulting.com, or email us at consulting@jmcampbell.com.

By: Dr. Mahmood Moshfeghian

Figure 6. Refrigerant mass circulation rate as a function of refrigerant subcooled temperature and HEX economizer pressure drop

Figure 7. Compressor suction temperature as a function of refrigerant subcooled temperature and HEX economizer pressure drop

Figure 8. Compressor discharge temperature as a function of refrigerant subcooled temperature and HEX economizer pressure drop

Reference:

  1. Campbell, J.M., “Gas conditioning and Processing, Vol 2: The Equipment Modules”, 9th Edition, Edited by R.A. Hubbard K.S. McGregor, John M. Campbell & Company, Norman, USA, 2014.
  2. Moshfeghian, M., “Refrigeration with Flash Economizer vs Simple Refrigeration System,” http://www.jmcampbell.com/tip-of-the-month/2008/01/refrigeration-with-flash-economizer-vs-simple-refrigeration-system/ , 2008
  3. UniSim Design, Version 410 Build 17061, Honeywell International, Inc., Calgary, Canada, 2013.
Did you enjoy this post? Do you have a question?
Leave us a Comment below!

Want to read more articles like this?
Subscribe to our RSS Feed or visit the Tip of the Month Archives for past articles.