Refrigerant and Brine System of Cold Room: A Quick Guide

Refrigerant and Brine System of Cold Room

 

 

Pressure-Enthalpy Diagram of Refrigerant

The pressure-enthalpy diagram of the refrigerant shows the saturated liquid line and dry saturated vapor line. The figure is divided into three areas, as shown in the figure below.

Pressure-Enthalpy Diagram of Refrigerant

The figure above illustrates three zones: to the left of the saturated liquid line is the subcooled liquid zone; the area between the saturated liquid line and the dry saturated vapor line represents the wet saturated vapor zone; and to the right of the dry saturated vapor line is the superheated vapor zone.

Figure 1 shows eight types of lines and six parameters:

  • Saturated liquid line (X = 0)
  • Dry saturated steam line (X = 1)
  • Iso-dryness line, parameter X (with fixed values; in the figure, lines between X = 0 and X = 1 are not displayed)
  • Isobars, parameter p (fixed values)
  • Isotherms, parameter t (fixed values)
  • Isotropic enthalpy line, parameter h (fixed values)
  • Isoentropy line, parameter S (fixed values)
  • Isometric volume line, parameter v (fixed values), or isopycnal line, parameter p (values).

Among the mentioned parameters, the saturation pressure and temperature are both independent state parameters. When either the pressure or temperature is known, the other value can be found using the table of saturated thermodynamic properties for the refrigerant.

Additionally, knowing any two of these parameters allows for locating a point in the lgp-h diagram that represents this state where the values of other relevant parameters can be read. It is necessary to understand and familiarize yourself with the different component curves shown in Figure 1 together with the application of the pressure-enthalpy diagram and refrigeration cycle represented in it.

 

Let us consider Figure 2 where the theoretical cycle of a single-stage refrigerator is depicted in the pressure-enthalpy diagram.

As shown in Figure 2, the middle section shows the following processes: 1—2 represents the isentropic compression process of the refrigerant in the compressor, starting from the evaporation pressure (PO) to the condensing pressure (Pk).

The next process, 2—4, involves both isobaric cooling and condensation occurring in the condenser where the superheated steam is cooled to become dry saturated steam then the condensation process will occur to turn dry saturated steam into a liquid state. The last process 4—5 illustrates the throttling of the liquid refrigerant where it partially vaporizes and at the same time, its pressure decreases.

The valve’s throttling process (where the enthalpy value remains unchanged) is illustrated by the condensing pressure Pk, descending to the evaporation pressure PO.

Following this, the process 5—1 shows the isobaric evaporation process in the evaporator. In this process, the refrigerant liquid absorbs heat within the evaporator and then fully evaporates into dry saturated vapor.

 

 

Refrigerants and Brine of Cold Room

 

I.  Refrigerant System

The medium used in the refrigeration cycle is called the refrigerant. The refrigerant is essential for the refrigeration cycle to occur within a system. It is the one responsible for the continuous exchange of energy with the refrigeration system and the external environment that results in its constant change of thermal state.

Since the refrigerant’s objective is to cool, it operates by utilizing the compressor in the refrigeration system to continuously transfer heat from the products that need cooling to the surrounding environment.

The refrigerant will undergo the vaporization process at both low pressure and temperature within the evaporator and it will condense in the condenser under high pressure at normal temperatures, changing its state from gas to liquid.

Not all substances can be used as refrigerants, only those that are capable of vaporization and condensation within the operational range are allowed. The refrigerant’s change of state during the operation is mainly a physical transformation without any chemical changes.

When a refrigerant within the refrigeration system remains sealed, it can be recycled and used for an extended period. Sometimes minor leakages may occur in valves which is normal, but it is advisable to immediately repair or replace them to decrease or minimize refrigerant loss.

Currently, there are a variety of refrigerants available in the market, each with different and varying characteristics and properties but none exactly aligns with the ideal requirements.

A comprehensive analysis of different refrigeration systems and operating temperatures should be done by manufacturers before choosing a refrigerant. Since we are now in an environment focused on safety, informed decisions through comparison are a necessity after the assessment and evaluation of different refrigerants.

refrigeration system

 

1. Ammonia Refrigerant

Ammonia (NH3), first discovered in 1774 was initially known for its solubility in water which is why it was initially used in absorption refrigerators in 1859. The first ammonia refrigeration compressor emerged in 1873, signifying its early application as a refrigerant.

With its long history as a natural refrigerant, it is used extensively in most medium and large-sized cold storage facilities because of its excellent thermal properties and low energy consumption when compared with other refrigerant alternatives. Additionally, it is more affordable, friendly to the environment, and does not deplete the ozone layer. In the United States alone. Approximately 85% of public cold storage facilities use ammonia as a refrigerant, indicating its widespread popularity even in more developed countries.

Ammonia

 

a.  Ammonia is a Colorless Gas

One of the characteristics of ammonia is its strong pungent odor at normal temperature and pressure. Under normal conditions, ammonia’s evaporation pressure in refrigeration systems ranges between 0.098 to 0.491 MPa, reducing the possibility of air infiltration into the system.

Ammonia’s condensation pressure generally ranges from 0.981 to 1.57 MPa, indicating moderate pressure. Its refrigeration range spans from -70℃ to +5℃, which is the range that is commonly used in various refrigeration systems not lower than -60℃.

Ammonia gas dissolves easily in water and at normal temperature, a unit volume of water can dissolve 900 times the volume of ammonia gas. In refrigeration systems, ammonia with a purity of 99.8% (volume fraction) or higher is generally required; otherwise, it may affect the ammonia’s cold production characteristics.

Ammonia exhibits minimal flow resistance in refrigeration systems and possesses high thermal conductivity. It is readily available in the market at low prices.

 

b. Ammonia Safety Precautions

At normal pressure and temperature, ammonia does not burn but when it reaches a specific concentration and temperature, it becomes flammable which may cause fires or explosions. This happens because, at high temperatures, ammonia decomposes into hydrogen and oxygen. Ammonia can ignite when mixed with air in concentrations between 11% to 14%, while concentrations between 16% to 25% create a risk of explosion if exposed to an open flame.

flammable

To prevent accidents, it is necessary to ensure that the exhaust temperature and pressure of the compressor and pressure remain within safe limits during operation. You should also purge non-condensable gases regularly from the refrigeration system to reduce the risk of explosions.

Although ammonia is classified as a mildly hazardous Class 4 chemical, it is highly toxic to the human body. It also possesses a strong pungent odor and can irritate the eyes, skin, and respiratory organs. Contact with the skin can result in swelling or in worse situations – frostbite. Prolonged exposure to ammonia in the air (around 0.5% – 0.8%) will result in dizziness, decreased blood pressure, and weak pulses due to poisoning.

In severe cases, it causes damage to the respiratory tract leading to bronchitis, pneumonia, pulmonary edema, and breathing difficulties. In cases of an ammonia gas leak in a warehouse, it is necessary to promptly transfer the goods out of the area to prevent contamination.

One good thing about ammonia is it does not harm or deplete the ozone layer, nor does it contribute to the greenhouse effect, making it an environmentally friendly and sustainable refrigerant.

 

c. Anhydrous Pure Ammonia

Ammonia does not corrode steel, but it becomes destructive and corrosive to zinc, copper, and bronze alloys except for tin-phosphorus bronze when moisture is present. This is the reason why components and parts located within the refrigeration system should not be made from the materials stated above except for parts that require lubrication like bushings, bearings, and rings that may be made from tin-phosphorus bronze.

 

d.  Pure Ammonia 

The important thing to remember when dealing with pure ammonia is it does not chemically react with lubricating oil but if moisture is present in the refrigeration system, emulsions can form resulting in a decrease in the performance of the lubricating oil. It is therefore crucial that an oil separator is installed in the refrigeration system to isolate the lubricating oil from the exhaust gas since ammonia is not soluble in it. The lubricating oil can contaminate the pipes, surfaces, and heat exchangers in the system resulting in a decreased performance in heat transfer.

lubricating oil

 

e. Ammonia is a Highly Irritating Gas

Ammonia, when it leaks even in small amounts can be easily detected because of its smell. You can also use phenolphthalein test papers and reagents to identify if there are leaks. The paper will change its color to red if a leak is detected.

leaks

 

2. Freon

Freon, whether in vapor or liquid form, is colorless, transparent, odorless, mostly non-toxic to the human body, and difficult to ignite or explode. It evaporates at -40.8℃ and solidifies at -160℃. It is also non-corrosive to metals and non-metals which is why is one of the widely used refrigerants used in the refrigeration and air conditioning industry.

When compared with ammonia, Freon has a smaller refrigerating capacity per unit volume, but it has higher density and greater flow resistance. Even though it has these characteristics, freon contributes to the greenhouse effect and ozone layer depletion problems which is why environmental protection agencies advocated for its prohibition.

When Freon content in the air reaches between 20% – 30% volume fraction, it becomes detectable and poses a risk of suffocation. As a result, alternatives such as hydrocarbons and chlorofluorocarbons are sought after as replacements.

Freon Refrigerant

When Freon is used as a refrigerant, proper ventilation, and air circulation is necessary because when there is moisture within the system, there is a possibility that it may hydrolyze especially if it is in contact with metal. It can react because of the acidic hydrogen chloride and hydrogen fluoride produced that leads to corrosion specifically in magnesium and its alloys so avoid materials like magnesium and zinc.

The presence of moisture in the refrigeration system can lead to other problems such as an ‘ice plug’ forming at the throttle valve. You will then need to install a filter drier in the Freon refrigeration system. You also must choose specific materials such as Chloroethanol rubber or CH-1-3 as a sealing material when working with freon because it dissolves organic plastic and rubber which may result in leakage of the refrigerant.

One of the characteristics of Freon is its lack of odor, which is why detection is difficult. Halogen detectors are commonly used to check for Freon leaks. leak lamps, electronic leak detectors, or concentrated soapy liquid can also be used for detection.

 

Freon, as a chemical substance, can be categorized according into three classes:

  • Chlorofluorocarbon products, abbreviated as CFC, such as R11, R12, R113, and others.
  • Hydrochlorofluorocarbon products, abbreviated as HCFC, like R22 and R123.
  • Hydrofluorocarbon products, abbreviated as HFC, such as R32, R152, R404A, and R507A.

CFCs, like R12, are categorized as Class I controlled substances because they have the most detrimental impact on the ozone layer which is why they are prohibited to use. HCFCs, such as R22, because of the Montreal Protocol will stop being used in developed countries around 2030 while developing countries are required to stop using them by 2040.

Germany, Italy, Switzerland, and some developed nations have already ceased using it as early as 2000 while in the United States, R22 has been prohibited since 2010. It is expected that China will accelerate the discontinuation of its use.

R404A is a ternary azeotropic mixed refrigerant that is a colorless, odorless, and non-flammable substance. It is composed of R125 (44% volume fraction), R143a (52% volume fraction), and R134a (4% volume fraction).

R404A Refrigerant

R404 also has good thermal stability and low toxicity and has a higher cooling capacity which ranges between 4% to 11% when compared to R22. That is why it is a good choice in various applications such as air conditioning, medium to low-temperature refrigeration, and industrial cold-water systems. It’s considered one of the best and primary replacements for R22 because it aligns with environmental regulations.

One downside of R404A is its relatively high cost, resulting in counterfeit products being offered in the market. It is advisable that you need to exercise caution and verify the authenticity of the product before purchase and use.

R507A is an azeotropic mixed refrigerant that is colorless, odorless, non-flammable, has good thermal stability, and exhibits low toxicity. It consists of R125 (50% volume fraction) and R143a (50% volume fraction).

It has a standard boiling point of -47.1°C and offers a higher cooling capacity ratio ranging from 7% to 13% when compared to R22. It is also slightly better than R404A in terms of performance which is why it is used in the same applications as R404A.

The best choice when switching refrigerants is to shift to environmentally friendly products like R404A and R507A. Even if the cost is high at present, with the cessation of R22, future prices may be lowered. Moreover, since manufacturers are constantly upgrading their technology, most refrigeration systems can accommodate various types of refrigerants such as R22, R404A, and R507A which is why it is not necessary to change the refrigeration equipment when switching refrigerants.

3. Carbon Dioxide

Carbon dioxide (CO2) is a natural refrigerant, known for its safety, non-flammability, and environmental friendliness. It is both colorless and odorless, denser than air, chemically stable, and poses no harm to people or food.

In the case of a fire, carbon dioxide can also function as a fire-extinguishing agent. With a volume fraction of nearly 99.9% in the refrigeration system, carbon dioxide is almost non-corrosive to all materials.

In China, a secondary cooling system named NH3-CO2 cascade, which utilizes both carbon dioxide and ammonia, is gaining popularity and practical application.

In comparison to ammonia refrigeration systems, carbon dioxide exhibits a lower gaseous volume expansion rate post-evaporation.

Due to carbon dioxide’s significant cooling capacity per unit volume, which is five times that of ammonia, a small amount of liquid evaporates into a gas state within the original heat preservation conditions.

The evaporation process results in a small volume increase when compared with the ammonia refrigeration system, thus contributing to external heat absorption. Pressure changes after evaporation in the carbon dioxide system are notably smaller and exhibit a stable, gradual increase compared to the rapid rise in ammonia systems. As such, there is no immediate need for pressure maintenance upon system shutdown.

Carbon dioxide Refrigerant

 

In the event of carbon dioxide leakage within a cascade refrigeration system, two distinctive characteristics are observed:

  • firstly, in case of liquid pipeline leaks, carbon dioxide tends to form dry ice on the outer wall of the pipeline, effectively blocking small leakage points.
  • secondly, when the pressure surpasses 3.5 MPa (Gauge pressure), a significant amount of carbon dioxide liquid leakage results in the formation of substantial dry ice.

This occurrence is not directly threatening to staff or personnel but in the unlikely event that a pipeline bursts because of mishandling, most leaked carbon dioxide transforms into dry ice, with a small portion in a gaseous state. This gives the personnel enough time to evacuate, preventing suffocation, unconsciousness, or fatal outcomes. It is important to note that during the sublimation process of dry ice, direct skin contact may result in localized frostbite.

 

4. Comparison of Ammonia Refrigeration System and Freon Refrigeration System

Refrigeration systems are mainly divided into ammonia refrigeration systems and Freon refrigeration systems.

Comparison of Ammonia Refrigeration System and Freon Refrigeration System

The comparison is as follows.

  1. One-time investment: Medium to large-sized ammonia refrigeration systems need less investment than Freon systems because of the high prices of the equipment, pipelines, refrigerants, oils, and other components needed by the Freon refrigeration systems.
  2. Operating costs: Ammonia as a refrigerant is cheaper, has a large cooling capacity per unit, consumes less power, and is easy to operate. While the price of Freon refrigerant is relatively high, the unit cooling capacity is small, and the power consumption and operating costs are relatively high.
  3. Environmentally friendly characteristics: Ammonia refrigerant is a natural refrigerant and has a low ozone depletion coefficient, thus, it does not harm the environment while Freon refrigerants will be phased out due to their destructive properties in the atmosphere.
  4. Energy saving characteristics: Ammonia refrigeration systems have larger refrigeration coefficient and better energy-saving effects compared to Freon which has a smaller coefficient and poor energy-saving effect.
  5. Safety: Even though ammonia is a flammable, explosive, and toxic refrigerant, its pungent smell makes it easier to detect while Freon is a non-toxic, non-flammable refrigerant that is safe to use.
  6. Degree of automation: Ammonia refrigeration systems are mostly manually operated and managed, and the degree of automation is relatively low while Freon systems have a higher degree of automation and usually do not require manual operations.

Medium and large-sized cold storage facilities commonly use ammonia because of their stability, lower investment and operational costs, and better energy-saving effects. But for safety concerns, specifically in smaller facilities, they opt for Freon refrigeration systems.

 

5. Classification of Media Hazard Levels

Occupation exposure to toxicants refer to different substances that are present in raw materials, intermediates, and wastes that can enter the human body through the nose, skin, or mouth during operation or production which causes harm to the person. The “Classification of hazards from Occupational Exposure to Toxic Substances” classified the substances into four namely: extremely hazardous, highly hazardous, moderately hazardous, and mildly hazardous.

hazardous Chemicals

 

 

II.  Secondary Refrigerant 

A substance that indirectly transfers heat in a cooling system is called a secondary refrigerant. Its purpose is to accept the cooling capacity of the primary refrigerant to cool other components.

Refrigerants are classified into gas, liquid, and solid. Gas refrigerants include air while liquid refrigerants include water and brine. Both ice and dry ice are categorized as solid refrigerants. Take note that during sublimation, dry ice directly turns to gas without passing through the liquid state.

The requirements for the secondary refrigerant are as follows:

  1. Non-discoloring, non-corrosive, and non-toxic to the human body. It should not deal damage to other substances such as corrosion on metals.
  2. Its specific heating capacity should be high to help reduce the difference in temperature between the inlet and the outlet to keep the cooling capacity within standards.
  3. It should possess both low viscosity and density to reduce resistance loss. Highly viscous refrigerants reduce heat transfer performance.
  4. Its freezing point should be low to remain in a liquid state during the operation.
  5. It has good chemical stability.
  6. It should be cheap and can easily be located and bought.

In refrigeration systems, the most used secondary refrigerants are air and brine. Water’s freezing point is O℃, but when salt is added, this freezing temperature is lowered.

When the temperature of the brine drops to its freezing point, part of the water in the solution freezes, which then increases the concentration of the remaining solution. This then further lowers the freezing point of the remaining solution to its lowest point.

The amount of salt contained in a unit mass or unit volume of a brine solution is called the concentration of the brine. At a certain temperature, the maximum amount of salt that can be dissolved per unit volume of a solution is called solubility. The saltwater solution is called a saturated solution.

If the concentration of a solution is less than its solubility, it becomes an unsaturated solution. The rate at which salt dissolves in water varies with temperature.

 

Figure 3 illustrates the curve of a sodium chloride aqueous solution

In Figures 3 and 4, a solubility curve is shown, with any point on this curve representing a saturated solution. There is a boundary between the saturated and unsaturated solution and points above the curve indicate an unsaturated state.

KB signifies an unsaturated state, where points above its curve also denote an unsaturated solution.

OK designates the freezing point, representing frozen water below the curve.

The O point signifies pure water, where temperature and salt concentration are zero. Upon adding salt to the water, the freezing temperature initially drops along the OK curve.

When reaching the K point, additional salt ceases to lower the freezing temperature, shifting along the KB curve. The intersection of OK and KB indicates the eutectic point, the lowest temperature for the brine solution and at this point, brine exists as a mixture of solid salt and ice.

The eutectic point for sodium chloride aqueous solution is -21.2℃, while the eutectic point for calcium chloride aqueous solution is -55.5℃. During operation, the saltwater concentration decreases due to moisture absorption from the air. In open systems like saltwater ice-making pools, moisture from melting ice also reduces the brine concentration over time.

To prevent decreased concentration and increased solidification temperature, regular density measurements using a densimeter are needed. If the concentration drops, replenishing with salt will maintain the brine at an appropriate concentration in the ice-making tank.

Aside from sodium chloride and calcium chloride, refrigerants like ethylene glycol, propylene glycol, ethanol, and methanol are also used.  Ethylene glycol and propylene glycol are colorless, odorless, non-electrolytic, and non-flammable. Propylene glycol is non-toxic and safe for direct food contact while methanol and ethanol are flammable so safety precautions are a must.

Ethylene glycol may cause slight corrosion, often requiring corrosion inhibitors to minimize its corrosiveness. To reduce ammonia storage in cold storage, some facilities employ R404A or R507A together with ethylene glycol in a cascade refrigeration system.

A dedicated pump sends low-temperature ethylene glycol to pipe heat exchangers in each warehouse, ensuring efficient liquid refrigeration.

 

 

III. Production of Ammonia Leak Test Paper

 

1. High-sensitivity test paper

a. Prepare 0.25 grams of Phenolphthalein powder, 250 milliliters of pure alcohol, and 50 milliliters of pure glycerin. Place all three in a container and stir using a stick until the powder is completely dissolved. Soak a clean general white tissue paper in the solution and air dry it. Then, cut the dried test paper into small strips (usually cut into 10cm × 5cm) and store it in a plastic bag for later use.

b. Put 0.1 grams of Phenolphthalein powder in a glass and add 100 milliliters of 95% alcohol, stirring evenly to dissolve the powder completely. Soak the white paper in the solution, remove and dry it after wetting, then cut the test paper into strips for later use.

plain white paper

2. Medium sensitivity test paper

Combine 0.3 grams of Phenolphthalein powder and 250 milliliters of alcohol in a container then soak the test paper in the solution to create a medium sensitivity test paper. This process is actually used in production, but the amounts are just approximations

When conducting leak tests, soak the test paper in tap water, it will turn red if it detects ammonia.

 

3. Plain white paper

Ammonia leaks in valves, pipes, and equipment can be detected using a test paper. First you must make a solution by mixing appropriate amounts of Phenolphthalein powder and water, then soak a strip of plain white paper in the solution. You can use the soaked paper to identify leaks on-site.

Even though there are instruments readily available for detecting ammonia leakage, test strips are still used widely because of their suitability in checking if the evaporator is leaking.

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