Further Discussions

Describe the major differences between an industrial refrigerator and an commercial refrigerator.

There are mainly 2 versions of industrial refrigerators in the market.

1) Cascade Vapor Refrigeration Cycles

Cascade Vapor Refrigeration Cycles are used industrial applications to boost the performance of vapor compression cycles.

The use of two or more refrigeration cycles in series is called Cascade Cycles.

Cascade cycles allow for the use of two separate types of refrigerants to suit the varying operating conditions of each individual cycle, and are seen in application where relatively low temperatures are desired.

Fig 3.1 - Schematic Diagram of Cascade Cycle

F ig 3.2 – T-s Diagram of a Cascade Cycle

From Fig 3.2, we can see that compressor work decreases and the amount of heat absorbed from the refrigerated space TL increases as a result of cascading. Therefore, industrial refrigerators that make use of the Cascade cycle enjoy a higher coefficient of performance (COP) compared to normal commercial refrigerators.

If a certain temperature is desired, the number of cascading stages can be increased but the increase in processes also mean a higher cost.

If the refrigerants used in the cascade refrigeration cycle are the same, the heat exchanger between the stages can be replaced by a mixing chamber (known as a flash chamber) since it has improved heat characteristics, then the Cascade Refrigeration Cycle will be know as a Multistage Compression Refrigeration System


2) Absorption Refrigeration Systems

Another form of refrigeration that becomes economically attractive when there is a source of inexpensive heat energy at a temperature of 100 to 200oC is absorption refrigeration, where the refrigerant is absorbed by a transport medium and compressed in liquid form. The most widely used absorption refrigeration system is the ammonia-water system, where ammonia serves as the refrigerant and water as the transport medium. The work input to the pump is usually very small.

Since in liquid form, work input into the system is minimal because liquid requires significantly less energy to pump compared to gases. It is also environmentally friendly, as it makes use of generally non-CFC or HCFC refrigerants.

Though it has a relatively lower COP compared to other industrial refrigerators, it can be improved by multi-staging.

Fig 3.3 – Schematic Diagram of an Absorption Refrigeration Cycle

Improvements to the vapor compression refrigeration cycle:

The performance of vapor compression cycle can be improved by the following methods:
-- Cascade Vapor Refrigeration Cycle
-- Multipurpose Compression with intercooling
-- Multistage Compression Refrigeration cycle
-- Gas liquefaction

Cascade Vapor Refrigeration Cycle
Cascade cycles are two or more refrigeration cycle arranged in series. In figure 1 below, the cascade cycle is shown in which two vapor-compression refrigeration cycles, labeled A and B are arranged in series with a counterflow heat exchanger linking them. In the intermedium heat exchanger, the energy rejected during condensation of the refrigerant in the lower temperature cycle A is used to evaporate the refrigerant in the higher-temperature cycle B.

Figure 1: Cascading Cycle

The desired refrigeration effect occurs in the low-temperature evaporator and heat rejection from the overall cycle occurs in the higher temperature condenser. In the cascading cycles, the coefficient of performance is the ratio of the refrigeration effect to the total work input.

A significant feature of the cascade system is that the refrigerants in the two or more stages can be selected to have reasonable evaporator and condenser pressures in the two or more temperature ranges. In double cascade system, a refrigerant would be selected for cycle A which has a saturation pressure-temperature relationship that allows refrigeration at a relatively low temperature without extensively low evaporator pressures. In turn, the refrigerant for cycle B would have saturation characteristics that permit condensation at the required temperature without excessively high condenser pressure.

Multistage Compression with intercooling
Net work output can be increase by decreasing the compressor work consumed. This can be achieved by multistage compression with intercooling the gas between stages. Intercooling is achieved in gas power systems by heat transfer to the lower-temperature surroundings. In refrigeration systems, the refrigerant temperature is below that of the surroundings for much of the cycle. Thus, other means must be employed to accomplish intercooling so as to achieve the attendant savings in the required compressor work input.

Figure 2: Multistage Compression with intercooling

Multipurpose Compression Refrigeration cycle
Refrigeration at several temperatures can be achieved in a refrigerator with a single compressor by throttling the refrigerant in stages. One type of multistage compression cycle is the house-hold refrigerator. In the refrigerator, the flow of cool air to the multipurpose storage chamber is selectively controlled in accordance with a variety of operational modes. In order to accomplish such a cool air flow control, a refrigerating compartment fan and a damper are provided at positions above a refrigerating compartment evaporator.

Figure 3: Multipurpose Compression Refrigeration cycle

Gas liquefaction
Refrigeration by liquefaction of gases is the process of refrigerating a gas to a temperature below its critical temperature so that liquid can be formed at some suitable pressure, also below the critical pressure. Gas liquefaction is a special case of gas refrigeration. Firstly, The gas is compressed to a higher pressure in an ambient-temperature compressor. This high-pressure gas is passed through a countercurrent heat exchanger to a throttling valve or expansion engine. Upon expanding to the lower pressure, cooling may take place, and some liquid may be formed. The cool, low-pressure gas returns to the compressor inlet to repeat the cycle. The purpose of the countercurrent heat exchanger is to warm the low-pressure gas prior to recompression, and simultaneously to cool the high-pressure gas to the lowest temperature possible prior to expansion. Both refrigerators and liquefiers operate on this same basic principle.

Figure 4: Gas liquefaction