Basic Refrigeration Cycle

Basic Refrigeration Cycle

Principles of Refrigeration

1. Liquids absorb heat when changed from liquid to gas2. Gases give off heat when changed from gas to liquid.

For an air conditioning system to operate with economy, the refrigerant must be used repeatedly. For this reason, all air conditioners use the same cycle of compression, condensation, expansion, and evaporation in a closed circuit. The same refrigerant is used to move the heat from one area, to cool this area, and to expel this heat in another area.

1. The refrigerant comes into the compressor as a low-pressure gas, it is compressed and then moves out of the compressor as a high-pressure gas.

2. The gas then flows to the condenser. Here the gas condenses to a liquid, and gives off its heat to the outside air.

3. The liquid then moves to the expansion valve under high pressure. This valve restricts the flow of the fluid, and lowers its pressure as it leaves the expansion valve.

4. The low-pressure liquid then moves to the evaporator, where heat from the inside air is absorbed and changes it from a liquid to a gas.

5. As a hot low-pressure gas, the refrigerant moves to the compressor where the entire cycle is repeated.

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All air conditioner units must have the Four basic components to work:

1. The compressor

2. The condenser

3. The expansion device

4. The evaporator

A thorough understanding of the role of a refrigeration compressor cannot exist without a discussion of the refrigeration cycle, which essentially consists of the transformation of a liquid to a gas and back again.

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The Refrigeration Cycle.

1) Compression : To maintain the necessary lower pressures and lower temperatures, a compressor is needed to remove the vapor. Because the refrigeration circuit is closed, equilibrium is maintained. That means that if the compressor removes vapor faster than it can be formed, the pressure will fall and with it the temperature in the evaporator. Alternately, if the load on the evaporator rises and the refrigerant evaporates quicker, the temperature and pressure in the evaporator will rise. The energy that a compressor requires is called compression input and is transferred to the refrigeration vapor.

2) Condensing : After leaving the compressor, the refrigerant moves to the condenser, which gives off heat that is transferred to either air or water having a lower temperature. The amount of heat given off is the heat absorbed by the refrigerant in the evaporator plus the heat created by compression input. The byproduct of this is that the vapor changes to a liquid, which is then sent to the receiver.

3) Evaporation : Liquid refrigerant enters the evaporator. It absorbs heat when it evaporates, which produces cooling. The refrigerant from the evaporator is fed to a tank as a weak or saturated superheated gas. The pressure in the tank rises until it equals the pressure in the evaporator. Refrigerant flow stops and the temperature in both tank and evaporator both rise to ambient.

4) Receiving : The pressure in the receiver is higher than the pressure in the evaporator because of compression, and thus must be lowered to match the evaporative pressure. This is achieved through the use of an expansion valve

5) Expansion : Before the liquid enters the expansion valve, the temperature will be just under the boiling point. Suddenly reducing the pressure in the expansion valve causes the liquid to boil and evaporate. This evaporation takes place in the evaporator and the circuit is complete.

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Mollier Chart.a graph showing the enthalpy of a substance as a function of its entropy when some physical property of the substance, as temperature or pressure, is kept at a specified constant value

Read more: Mollier diagram: meaning and definitions | Infoplease.com http://dictionary.infoplease.com/mollier-diagram#ixzz3NlOObt1e

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Accessories in refrigeration

1.High Pressure Liquid Receivers

High pressure liquid receivers are designed to hold excess refrigerant between the condenser and expansion device. Our high pressure receivers are designed with liquid capacities in accordance with ASHRAE standards and have ODS connections only. Receivers can also be modified to meet your specific requirements.

Features

• Welded design for higher strength

• Nitrogen tested for cleanliness

• Powder paint finish

• 675 psig maximum working pressure

• UL version includes fusible plug

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2.High Pressure Accumulators

designed to act as a temporary holding vessel between the outlet of the evaporator and the inlet of the compressor. During flood back conditions, the accumulator traps the liquid charge and allows it to be evaporated and fed to the compressor at a controlled rate. The internal U-tube includes an orifice, which ensures proper metering of the oil back to the compressor.

The accumulator should not be sized for less than 50% of the total system charge. Size the accumulator for minimum pressure drop. Accumulator capacities below are rated at a 1 psig drop, which facilitates proper oil return. The minimum capacity is the lowest amount that will ensure proper oil return.

Features

• Welded design for higher strength

• Nitrogen tested for cleanliness

• Powder paint finish

• 675 psig maximum working pressure

• UL version includes fusible plug

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3.Oil Filters

Oil filters should always be used in conjunction with any oil control system. The filter ensures refrigeration oil is clean of any foreign matter that may obstruct float valves in oil regulators and oil separators. Our filters are designed to be a low pressure drop filter, while remaining effective at removing particulate matter. The A 18337 model is designed using XH-9 desiccant for removal of moisture in POE oils and contaminants.

Features

• Welded and brazed design for higher strength

• Nitrogen tested for cleanliness

• Good for all oil types

• Powder paint finish

• 450 psig maximum working pressure

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4.ASME Receivers

ASME receivers have been designed to conform to the many requirements of the system designer. All receivers are ASME certified and are either “U" or “UM" stamped in accordance with ASME Section VIII code. Catalog models are provided in both vertical and horizontal designs. Inlet and outlet connections may be modified to other connection styles such as rotolock spuds or pipe threads. A receiver should be selected based on the operating charge of the entire system, including all liquid lines. Pump-down capacities shown are calculated based on 90% at 90oF for R-22. All receivers are powder painted to provide corrosion protection.

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5.Liquid Receivers

Liquid receivers are designed to hold excess refrigerant between the condenser and expansion device. Our receivers are designed with liquid capacities in accordance with ASHRAE standards. All catalog low pressure receivers (450 psig) include a swivel valve. Receivers can also be modified to meet your specific requirements.

Features

• Welded and brazed design for higher strength

• Nitrogen tested for cleanliness

• Removable swivel valves

• Powder paint finish

• Removable 362oF fusible plug— part number 110-001

• 450 psig maximum working pressure

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6.ASME Accumulators

ASME accumulators are used to prevent liquid slugging of the compressor during a flood-back condition. If liquid enters the compressor cylinder, costly damage can occur. Our accumulators capture excess liquid leaving the evaporator, allowing the system to vaporize it during normal operation, returning only refrigerant vapor to the compressor. A weep hole is included in the internal tube to facilitate proper oil return from the accumulator to the operating system.

The accumulator must be placed between the evaporator and compressor to intercept the excess liquid. To size the accumulator, you must select the model with the tonnage nearest to the tonnage of the operating system. Care must also be taken to evaluate the amount of liquid holding capacity needed.

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7.crankcase pressure regulator

A crankcase pressure regulator (CPR) is a common accessory added to many low-temperature refrigeration applications, such as walk-in and reach-in freezers. They are designed to prevent the compressor's motor from overloading when its crankcase pressure rises above its designed working pressure. On many low-temperature applications, this can occur during or after a defrost cycle, or after a normal shutdown period. The CPR is an outlet pressure regulator and will not allow the crankcase pressure to rise above a predetermined level.

Typically the CPR is installed in the suction line between the compressor and the evaporator. Normally, there are no other components installed downstream, between the outlet of the CPR and the compressor. This is to ensure that the outlet of the CPR senses the true crankcase pressure of the compressor.

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