 1 Dec, 2017

# Optimizing Performance of Refrigeration System with Flash Tank Economizer

Continuing the January 2008  Tip of The Month (TOTM), this tip demonstrates two methods to optimize the performance of a refrigeration system employing a flash tank economizer and two stages of compression. Specifically, we will minimize the compressor total power and condenser duty by optimizing the interstage pressure.

The details of a simple single-stage refrigeration system and a refrigeration system employing one flash tank economizer and two stages of compression are given in Chapter 15 of Gas Conditioning and Processing, Volume 2 . The process flow diagram for a flash tank economizer refrigeration system with two stages of compression is shown in Figure 1. Note that provisions have been made to consider pressure drop in the suction line of the first stage compressor. Figure 1. Process flow diagram for a refrigeration system with a flash tank economizer and two stages of compression

Let’s consider removing 10.391x106 kJ/h (2886 kW) from a process gas at -35°C and rejecting it to the environment by the condenser at 35°C. Pure propane is used as the working fluid. In this study, all the simulations were performed by UniSim Design software . Assuming 5 kPa pressure drop in the chiller, the pressure of saturated vapor leaving the chiller at -35°C is 137.4 kPa.  Also, assuming 30 kPa pressure drop in the suction line, the first stage compressor suction pressure is 107.4 kPa. The condensing propane pressure at 35°C is 1220 kPa. The condenser pressure drop plus the pressure drop in the line from the compressor discharge to the condenser was assumed to be 50 kPa; therefore, compressor discharge pressure is 1270 kPa. In addition, an adiabatic efficiency of 75% was assumed for both stages of compression.

Assuming no pressure drop between the two stages, Figure 2 presents the variation of the compressor stages 1, 2, and the total power as a function of the interstage pressure.

Method 1:

The “Databook” option from “Tools” menu of the UniSim was used to generate powers (dependent variables) as a function of interstage pressure (Independent variable). The interstage pressure was varied from 200 kPa to 1000 kPa with an increment of 10 kPa.

As can be seen in this figure, the optimum interstage pressure is about 470 kPa. This pressure corresponds to the minimum total power and also the equality of stages 1 and 2 power. Figure 2. Impact of interstage pressure on compressor power.

Similarly, Figure 3 presents the compressor total power, stages 1 and 2 compression ratios. Figure 3 clearly shows that the minimum total compressor power does not occur at equal stage compression ratios of  3.44. Yet Chapter 14 (Compressors) of Gas Conditioning and Processing, Volume 2  states “The total power is typically minimized when the ratio in each stage is the same.”  Why is that not the case here?

The ideal optimum interstage pressure based on equal compression ratios can be found by the following equation: The equal compression ratio for each stage is R= 369.3/107.4 = 3.44 and  R= 1270/369.3 = 3.44. The above equation is valid if the mass flow rates through both stages were the same and the suction temperatures for both stages were equal. In a refrigeration system with flash tank economizers and multiple stages of compression, usually neither of these conditions are met.  In this case, the mass flow rates through stages 1 and 2 are 3.106 x 104 and 4.171 x 10 4 kg/h, respectively. The suction temperatures are -35.8°C and 21.1°C, respectively. Figure 3. Impact of interstage pressure on the total compressor power and stages compression ratio.

Method 2:

An alternative and easier method to determine the optimum interstage pressure is the “Adjust” tool in the simulation software. As shown in Figure 1, ADJ-2 was used to vary interstage pressure (stream R-12) so that first stage “R-Comp-LP Power” power becomes equal the second stage “R-Comp-HP Power” power. The setup for ADJ-2 is shown in Figure 4 and the detail of iterations and final results are shown in Figure 5. As shown in Figure 5, the optimum interstage pressure is 471.3 kPa and each stage compression power is 793 kW which adds up to a minimum total power of 1586 kW.

Summary:

Because the mass flow rates and suction temperatures were different in each stage of compression, the minimum total compressor power does not occur at equal compression ratios in each stage.

Two methods of “Databook” and “Adjust” were used to minimize the total compression power and condenser duty by selecting the optimum interstage pressure.

In the first method “Databook”, the optimum interstage was determined by minimizing the total compressor power. In the second method “Adjust”, the interstate pressure was determined by equalizing stages 1 and 2 powers. Both methods gave the same interstage pressure and total compressor power. Figure 4. Detail of “Adjust” set up Figure 5. Iteration and final results of “Adjust”

For the same chiller duty, chiller and condenser temperatures, and pressure drops, the results of the flash tank economizer system are compared with the results of a simple refrigeration system in Table 1. This table indicates that the compressor power and condenser duty saving are 17.4 % and 6.97 %, respectively. The interstage pressure drop is unique to flash tank economizer and its effect is the reduction of the power saving when compared to the simple refrigeration system and increases the condenser duty.

Table 1. Refrigeration specifications and calculated results To learn more about similar cases and how to minimize operational problems, we suggest attending our G4 (Gas Conditioning and Processing), G5 (Practical Computer Simulation Applications in Gas Processing), and G6 (Gas Treating and Sulfur Recovery) courses.

By: Dr. Mahmood Moshfeghian 