Oct-2010

Estimating compressor power and condenser duty in a refrigerant system

A simple-to-use predictive tool calculates compressor power and condenser duty per refrigeration duty in a three-stage propane refrigerant system

Alireza Bahadori and Hari Vuthaluru
Curtin University

Article Summary

Refrigeration systems are common in the natural gas processing industry and in processes relating to the petroleum refining, petrochemical and chemical industries. Several applications for refrigeration include natural gas liquids (NGL) recovery, liquefied petroleum gas (LPG) recovery, hydrocarbon dew point control, reflux condensation for light hydrocarbon fractionators, and liquefied natural gas (LNG) plants. A predictive tool, which is less complicated than existing approaches, has been developed with fewer computations to minimise the number of complex and time-consuming calculation steps. It is formulated to arrive at an appropriate estimation of compressor power and condenser duty per refrigeration duty in three-stage propane refrigerant systems as a function of evaporator temperature and refrigerant condensing temperature — important parameters that should be considered when designing any refrigeration system. The new correlation is suitable for a range of evaporator temperatures between -40°C and 60°C and a refrigerant condensing temperature range of 10–70°C. 

Results show that the proposed predictive tool is in very good agreement with the reported data, with the average absolute deviation around 1.38%. The tool could be of great practical value to engineers and scientists, allowing them to perform a quick check on the compressor power and condenser duty in propane refrigerant systems under various conditions without opting for any experimental measurements or pilot plant trials. In particular, gas and process engineers will find that the proposed tool features transparent calculations that involve no complex expressions.

Propane refrigeration systems
Propane refrigeration systems are often required in gas processing to provide chilling when condensing the heavy components of a rich gas.¹ In this process, the natural gas stream is chilled with an external propane refrigeration system, then the condensed liquids are separated in a low-temperature separator and stabilised in a column.² Figure 1 shows a schematic flow diagram of a three-stage propane refrigeration system.

Propane has zero ozone depletion potential and negligible global warming potential.³ It also has excellent thermodynamic properties, quite similar to those of ammonia. Its molar mass of 44 is ideal for turbocompressors and is only about one-third of its halocarbon competitors.⁴ Propane is cheaply and universally available.³ The major advantage of selecting propane as the refrigerant over ammonia is that propane is non-toxic.³ Its flammability is a serious concern, though, so safe design and operating practice are of paramount importance. However, this disadvantage can be overcome by using it as a refrigerant for the LT cycle.

It is important to note that propane can be used for very low-temperature applications in refrigeration (-30 to -60°C) compared to ammonia due to its lower normal boiling point.³ There has been a surge in the installation of CO2-based systems, and a large 
number of research studies have highlighted its extremely favourable thermodynamic and environmental properties.⁵

Propane is not corrosive to many materials such as aluminium, brass, bronze, copper, stainless steel and silver. Therefore, it is fully compatible with existing components such as heat exchangers, expansion valves, compressors, lubricants and copper tubing. Refrigeration systems utilising one, two, three or four stages of compression have been successfully operated in various services. The number of levels of refrigeration generally depends upon the number of compression stages required, interstage heat loads, economics and the type of compression.¹

In addition, environmental concerns have increased interest in using natural refrigerants such as hydrocarbons (for instance, propane, isobutane and mixtures) as alternatives to synthetic fluorocarbon refrigerants in a wide range of applications.^6,7,8,9  Generally, the research studies report significant performance and economic benefits for hydrocarbons compared with fluorocarbons.

In view of these issues, it is necessary to develop an accurate and simple method for predicting the compressor power and condenser duty per refrigeration duty in three-stage propane refrigerant systems.

Methodology to develop a predictive tool
A predictive tool should be developed to aid a modular approach to designing refrigeration systems. In order to apply this proposed tool to most commercially available compressors, a polytropic efficiency of 0.77 was assumed.¹ The polytropic efficiency was converted into an isentropic efficiency to include the effects of compression ratio and specific heat ratio (k = Cp/Cv) for a given refrigerant.¹ For well-balanced and efficient operation of the compressor, an equal compression ratio between stages was employed.1 The refrigeration level is defined as the temperature of the dew point vapour leaving the evaporator. The pressure at the compressor suction and side load inlet nozzles was adjusted by 10 kPa to allow for any pressure drop. This tool also includes a 70 kPa pressure drop across the refrigerant condenser for propane. The relevant coefficients can also be retuned quickly for various cases.

The data required to develop this predictive tool include the compressor power and condenser duty per refrigeration duty in a three-stage propane refrigerant system, the evaporator temperature and the refrigerant condensing temperature. In this work, the compressor power and condenser duty per refrigeration duty in a three-stage propane refrigerant system is predicted rapidly. First, these are correlated as a function of the evaporator temperature (oK) for different refrigerant condensing temperatures (also oK). Next, the calculated coefficients for these polynomials are correlated as a function of the refrigerant condensing temperature. The derived polynomials are applied to calculate new coefficients for equations 1 and 2, to predict the compressor powers and condenser duties per refrigeration duty in a three-stage propane refrigerant system. Table 1 shows the tuned coefficients for equations 3 to 6 for the percentage of blowdown that is flashed to steam in the design of boilers with blowdown systems according to the reported data.¹

In brief, the following steps are repeated to tune the correlation’s coefficients:
• Correlate the compressor powers and condenser duties per refrigeration duty as a function of evaporator temperature (oK) for a given refrigerant condensing temperature (oK)