In this Tip of the Month (TOTM), we will continue our discussion on the performance of mechanical refrigeration systems employing propane as the working fluid. Specifically, we will study the effect of the flash tank and heat exchanger (HEX) economizers on the compressor power, the refrigerant circulation rate, and the condenser duty.

The details of a simple single-stage refrigeration system and a two-stage refrigeration system employing one flash tank economizer and HEX economizer are given in Chapter 16 of Gas Conditioning and Processing, Volume 2 [1]. The process flow diagrams for a simple system and a system with HEX economizer refrigeration are shown in Figure 1. The two stage compression with the flash tank and HEX economizers are presented in Figure 2. Note that pressure drop in different segments of the loops have been considered.

Figure 1 

Figure 2

In this TOTM, we will revisit the previous TOTM on mechanical refrigeration in which the chiller outlet temperature was kept at -35°C. Let’s consider a case in which the objective is to remove 2778 kW from the process gas at -35°C and reject it to the environment by the condenser at 35°C. The pressure drop assumptions were: in the line from the compressor discharge to the condenser and in the condenser 50 kPa, in the chiller 5 kPa and in the compressor suction line 10 kPa, between the two stages of flash economizer 20 kPa and between the flash tank and second stage of compressor 20 kPa; in the tube side and shell side of HEX economizer 20 kPa. Pure propane was used as the working fluid. An isentropic efficiency of 75 % was used in all cases. For the flash tank and HEX economizer, the optimized interstage pressure (530 kPa) which minimized the total compressor power was used. In this study, all of the simulations were performed by HYSYS software [2].

We considered the single stage (simple), single stage HEX (economizer) and the two stage (economizer) refrigeration systems. First we studied a simple refrigeration system and a single stage HEX economizer (Figure 1). The summary of simulation results is shown in Table 1. As can be seen in this table, no compressor power saving is achieved unless the HEX outlet temperature is dropped below 10 ºC due to pressure drop in the HEX. Practically, no appreciable improvement is achieved using a single stage HEX economizer. Note that the total pressure drop from chiller to the suction of compressor was assumed to be 10 kPa for the simple system and 30 kPa for the single stage HEX economizer (20 kPa pressure drop in HEX), respectively.

Table 1

Next, we studied the two stage flash tank and HEX economizer. The summary of simulation results is presented in Table 2. Total compressor power requirement is compared with the power requirement for simple system. As can be seen in this table, flash tank economizer is the most efficient which results in 14.1 % power saving (compared to simple system). For the two stage HEX economizer, the best results are obtained when the cold side approach temperature is 2 ºC.  This corresponds to 13.2 % compressor power saving. Note, this is a study in power reduction only and that CAPEX comparisons together with life cycle analysis may point towards different conclusions. The approach temperatures chosen on a HEX will have a profound effect on the area required for heat exchange (refer to Chap 13 of Gas conditioning and Processing, Vol 2).

Table 2

On reviewing Tables 1 and 2, the following observations can also be made:

  1. Single stage HEX economizer does not reduce the compressor power consumption appreciably. Its only advantage is superheating the feed to the suction of the compressor. Superheating the compressor suction will increase power requirements but decrease the likelihood of liquid carryover into the compressor.
  2. Both flash tank and HEX economizers with two-stage compression reduce the compressor power requirement by at least 12 percent.
  3. For the case of two-stage compression, the flash tank economizer is more efficient than the HEX economizer.
  4. Pressure drop in HEX economizer is an important design parameter and increases the compressor power.
  5. For the case of two-stage compression, the approach temperature in the cold side of HEX economizer affects the compressor power requirement.

To learn more about similar cases and how to minimize operational problems, we suggest attending our G4 (Gas Conditioning and Processing) and G5 (Gas Conditioning and Processing – Special) courses.

By Dr. M. Moshfeghian

Reference:

  1. Campbell, J.M., “Gas Conditioning and Processing, Vol 2: The Equipment Modules”, 8th Edition, Edited by R.A. Hubbard, John M. Campbell & Company, Norman, USA, 2000.
  2. ASPENone, Engineering Suite, HYSYS Version 2006, Aspen Technology, Inc., Cambridge, Massachusetts U.S.A., 2006.
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