Refrigeration Principles of Cold Room: A Complete Guide

Refrigeration Principles of Cold Room

It is a fact that heat can only transfer from high – temperature objects to low – temperature objects but not the other way around just the same as how water only flows from high to low but not in the reverse direction.

In a cold storage facility, products like food will undergo cooling, freezing, and refrigeration in an environment with low temperature while constantly releasing heat. To maintain the low temperature, it requires continuous transfer of heat from within the cold storage or warehouse to the external environment – a process in need of artificial mechanical refrigeration.

The goal of refrigeration is met when a vapor compression refrigeration device takes advantage of the changes in the substance’s state which is to transfer heat from the item or product being cooled to the high-temperature object by doing an amount of external work.

Cold Storage

 

 

I. Single-stage compression refrigeration cycle

 

Principle of single-stage compression refrigeration cycle system

A vapor compression refrigeration system is comprised of compressors and refrigeration equipment that are interconnected by various pipes. In the system, the refrigerant is continuously being cycled to make sure that the refrigeration process remains uninterrupted.

Figure 2 illustrates the operational concept of a single-stage refrigeration system. The system is composed of a refrigeration compressor, a condenser, and expansion valve, and an evaporator which are linked by pipelines resulting to an enclosed refrigeration system.

Refrigeration System

As indicated in Figure 2, the compressor will compress the refrigerant in gas form to transform the low-pressure gas into a high-temperature and high-pressure gas. The condenser will then cool and condense the high-pressure and temperature gas from the compressor to release heat and at the same time change the state of the gas into liquid.

The high – pressure and temperature liquid will then be decompressed by the throttle valve to expand it into a low-pressure and temperature refrigerant still in liquid form.

The evaporator’s task is to absorb heat from the surroundings and then vaporize the low-pressure and temperature liquid from the throttle valve to turn it into gas to reduce the temperature of the surroundings.

Basically, a refrigeration system is comprised of four essential components: the refrigeration compressor, the condenser, the throttle valve, and the evaporator. To improve the efficiency and effectivity of the refrigeration operation, various equipment are also added such as oil separators, intercoolers, liquid receivers and auxiliary liquid receivers, low – pressure circulation liquid receivers, ammonia liquid separators, and control valves to name a few.

These additional components aid the operation by ensuring that the refrigeration process is continuous and effective while enhancing the reliability of the system. Systems operating at -15℃ such as food cooling, ice production, ice storage, and fruit and vegetable cold storage usually use the single – stage compression refrigeration cycle.

 

 

II. Various Basic Cycle Forms of Single-Stage Compression Refrigeration Cycle

The single-stage compression refrigeration cycle is the most basic among all the refrigeration systems. That is why it serves as the basis and foundation of other refrigeration cycles. It has different cycle forms based on conditions before the refrigerant enters the compressor and throttle valve.

Refrigeration system of Cold Storage

 

Saturated Compression Refrigeration Cycle

A saturated compression refrigeration cycle is a cycle where the refrigerant enters the compressor cylinder in the form of a dry saturated steam then goes to the throttle valve as saturated liquid.

Figure 3 illustrates the saturated compression refrigeration cycle as well as the isothermal and isobaric evaporation and vaporization of the liquid within the evaporator.

  • 1-2 indicates the isentropic adiabatic compression process within the compressor for the refrigerant.
  • 2-3-4 indicates the isotropic adiabatic compression process in the condenser, which involves pressure cooling and heat release through condensation.
  • 4-5 indicates the adiabatic throttling expansion undergone by the refrigerant in the throttle valve.
  • Lastly, 5-1 indicates the refrigerant’s vaporization and refrigeration process within the evaporator.

 

Vapor Superheat Compression Refrigeration Cycle

 

Figure 4 illustrates a vapor superheat compression refrigeration cycle. This term indicates a cycle wherein a gas drawn into a compressor is in a superheated steam state.

Figure 4 illustrates that in the compression refrigeration cycle, the compressor’s suction temperature shifts from point 1 to point 1′, moving from the saturated vapor line to the superheated vapor region, indicating the suction of superheated gas by the compressor.

While the use of a superheated compression refrigeration cycle has disadvantages for ammonia refrigeration systems, it prevents the liquid ammonia in the wet vapor zone from entering the compressor and causing liquid shock incidents. Consequently, during the actual refrigeration system operation, compressors often employ varying degrees of superheated compression.

Excessive superheating also has its disadvantages. It can degrade the compressor and lessen its refrigeration coefficient that is why excessive overheating should be avoided especially by the operators of the system.

 

Liquid Subcooling Compression Refrigeration Cycle

 

 

 

 

 

A refrigeration compression cycle that lowers the temperature of the liquid within the refrigeration system below its saturation temperature is termed a liquid subcooling compression refrigeration cycle, illustrated in Figure 5.

Illustrated in Figure 5 is a liquid subcooling compression refrigeration cycle. This is a refrigeration compression cycle that lowers the temperature of the liquid within the system below its saturation temperature.

Figure 5 describes the compressor’s increased cooling capacity as Δq0, keeping the specific volume of the compressor suction vapor and the power consumption W1 constant. The subcooling compression cycle of the refrigerant liquid improves the refrigeration coefficient but they are not commonly used.

 

Gas Superheating and Liquid Subcooling Compression Refrigeration Cycle

When the compressor suction gas is superheated, and the liquid is subcooled it is termed as gas superheating and liquid subcooling compression refrigeration cycle as illustrated in Figure 6.

Liquid subcooling leads to an increase in the cooling capacity Δq0 while the gas superheat compression cycle also increases the cooling capacity Δq0, it also, at the same time, elevates the work consumed ΔWL, therefore an increase in the cooling coefficient is not guaranteed. The gas superheat and liquid subcooling cycle is more advantageous for Freon refrigeration systems.

Heat exchangers are commonly integrated into Freon refrigeration systems to convert gas superheat into liquid subcooling, increasing the cooling capacity but, in ammonia systems, excessive superheating can deteriorate the operational conditions of the refrigeration compressor and decrease the refrigeration coefficient therefore, controlling the gas superheating becomes crucial.

 

Wet Compression Refrigeration Cycle

Figure 7 illustrates a wet compression refrigeration cycle. It is when a refrigerant gas that is entering the compressor cylinder is in a wet saturated state or in other words, the gas still contains liquid particles.

This refrigeration cycle both increases the temperature of the refrigerant in the compressor and amplifies the transfer of heat. This means that it reduces the cooling capacity, decreases the refrigeration efficiency of the compressor while increasing the power consumption.

Additionally, it may result in significant risks that may lead to damage due to inadvertent operation, such as liquid shock (frost) incidents and cylinder flushing accidents.

Due to these disadvantages, in practical refrigeration systems, an ammonia-liquid separator is typically incorporated to segregate gas and liquid, preventing the occurrence of wet compression refrigeration cycles within the system.

 

Actual Compression Refrigeration Cycle

The previously described compression refrigeration cycle represents the ideal scenario, differing from the actual compression refrigeration cycle due to several factors:

1) In the ideal compression refrigeration cycle, the refrigerant’s evaporation and condensation processes assume constant pressure and temperature. However, both the pressure and temperature decrease during condensation while they increase during refrigerant evaporation.

2) The ideal compression refrigeration cycle assumes no losses within the refrigeration system’s pipelines, and the pressure drop is because of the throttle valve. There are resistances in pipelines, valves, and other components that contribute to a certain degree of pressure drop.

3) In the ideal compression refrigeration cycle, the compressor is assumed to undergo adiabatic compression without any loss. However, the compressor cylinder performs heat exchange with the surroundings which lead to losses such as friction and valve throttling within the cylinder, alongside factors like clearance volume, all of which increase the compressor’s energy consumption.

Two-Stage Compression Refrigeration Cycle

The two-stage compression refrigeration cycle has evolved from the single-stage compression refrigeration cycle, which involves a compression process divided into two stages.

The refrigerant vapor coming from the evaporator initially enters the low-pressure cylinder (or low-pressure stage compressor), where it undergoes compression to an intermediate pressure.

Following this stage, it will be cooled by the intercooler and subsequently directed into the high-pressure cylinder (or high-pressure stage compressor), where it will reach the final compression stage leading into the condenser. This method constitutes the two-stage compression refrigeration cycle.

 

Two-Stage Compression Refrigeration Cycle with Single-Stage Throttling and Full Middle Cooling

In an ammonia-based two-stage refrigeration setup, liquid refrigerant at condensing pressure acts as the throttle for the intercooler. This part of the medium-pressure liquid refrigerant cools the superheated steam released from the low-pressure stage, reducing its temperature to the saturation level corresponding to the intermediate pressure. this intermediate cooling method called “intermediate complete cooling is illustrated in the Figure 8.

 

Two-Stage Compression Refrigeration Cycle with Partial Middle Cooling

Within the refrigeration system, the distinction between the two-stage compression refrigeration cycles lies in incomplete versus complete intermediate cooling.

A two – stage compression refrigeration cycle has two types namely incomplete and complete cooling. In the incomplete intermediate cooling, the discharge gas coming from the low-pressure stage compressor directly combines with the steam from the intercooler while bypassing it then enters the high-pressure stage compressor. This type of system is solely used in Freon-based two-stage compression refrigeration systems. Figure 9 illustrates the two-stage compression refrigeration cycle with partial middle cooling.

Cascade Compression Refrigeration Cycle

Usually, only one refrigerant is used in either single-stage or two-stage compression. It becomes a cascade refrigeration compression cycle if two or more different refrigerants are used separately for cyclic refrigeration.

Figures 10 and 11 illustrate the cascade compression refrigeration cycle system. Usually, R22 is used as the refrigerant in the high-temperature section while R13 is used in the low-temperature part. This allows for an evaporation temperature as low as -90°C to -80°C.

NH3-CO2 cascade screw refrigeration system has been introduced because ammonia refrigerants have flammable, toxic, and explosive characteristics. This is one of the solutions made to reduce the quantity of ammonia in refrigeration systems to eliminate the risks of hazards.

Figure 11 illustrates a system divided into two segments: the high-temperature cycle and the low-temperature cycle.

In the high-temperature cycle, NH3 is being compressed in the compressor, followed by oil and gas separation. It will then proceed into the NH3 condenser for condensation, entering the condensing evaporator after throttling from the liquid reservoir. In the reservoir, it absorbs CO2, resulting to evaporation of the condensed heat. Afterwards, a gas-liquid separation occurs in the gas-liquid separator. The gas then re-enters the compressor to facilitate circulation within the high-temperature system.

In the low-temperature cycle, CO2 will undergo compression by the compressor, followed by oil and gas separation. It will then proceed to the condensing evaporator, where it mixes with the high-temperature NH3. After heat exchange, it condenses into a liquid, then enters the CO2 liquid reservoir.

CO2

After entering the reservoir, it undergoes drying via a drying filter before being throttled and depressurized to enter the CO2 gas-liquid separator. This separator supplies liquid to the evaporator through pump circulation.

The gas-liquid mixture from the evaporator then re-enters the gas-liquid separator for separation, afterwards, the gas returns to the compressor, enabling circulation within the low-temperature refrigeration system.

 

 

II. Refrigeration System Schematic Diagram

The schematic diagram of the cold storage refrigeration system (also known as the refrigeration system diagram) illustrates the interconnections among all the equipment, containers, pipes, valves, and other components of the refrigeration apparatus. Its purpose is to give a holistic view of the entire refrigeration system.

The schematic diagram encompasses the following key details:

1) Scale and characteristics of the refrigeration system.

2) Specifications, names, models, and quantities of all equipment.

3) System advancements, comprehensiveness, and identification of any existing issues or challenges.

It is imperative that construction unit personnel and refrigeration system installers review the refrigeration system schematic diagram. It enables them to understand the refrigeration apparatus and become acquainted with the installation project.

Moreover, operators in the refrigeration machine room should read the refrigeration system schematic diagram, equipment lists, annotations, among others to familiarize themselves. The first step in understanding how a diagram works is through the identified legends. Common examples of pipes and valves frequently found in cold storage refrigeration schematic diagrams are detailed in Table 1-8.

refrigeration system installers

Once you can determine what the legends indicate, you will now be able to understand the refrigeration system’s overall process flow within the entire project. When interpreting the diagram, start by understanding the functions of each compressor and the available refrigeration solutions that can be switched between them such as identifying various auxiliary processes like air release, oil release, frost flushing, and refrigerant charging.

By having the knowledge about these refrigeration cycles and processes, you will be able to develop an understanding of the entire process system. If you familiarize yourself with the different valves and instruments in the system and the diagram, you will be able to identify the model, specifications, quantity, and other pertinent information for each piece of equipment.

 

 

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