Open Type Compressor
Open type compressors are externally driven, this could be either driven by external engine or electric motor. Compressor used in car air conditioning system is a good example of open type compressor where in a clutch is used to engage and disengage the compressor while engine is running. Open type compressor is purely composed of mechanical parts without motor winding inside.
This is completely serviceable in the field and could be overhauled. This uses shaft seal to prevent from refrigerant and oil leak from its housing as the shaft extends outside. If compressor is driven by an electric motor, belt and pully is used, if it is driven by an engine, fly wheel is connected to the shafting. There are also cases in which a flange or coupling is used to connect the compressor shaft to the source of force to drive the compressor. Open type compressors are most commonly used in transport refrigeration manufactured by Thermo king Corporation.
This compressor compresses refrigerant by a rotating motion. This is commonly used in some domestic and commercial air conditioning units. Most of the small rotary compressors are hermetic and could no longer be repaired when internal parts such as electric motor and some moving parts become defective. Electric motor is installed inside along with the rotating vane. As the electric motor runs, the vane rotates and draws vapor from the suction line. The drawn vapor will be carried by the vane to the exit point and shall be pushed away going to the discharge line. Rotary compressors are using high speed electric motors to rotate and drive the vane to be able to compress refrigerant into high pressure vapor.
This type of compressor is using an impeller to draw and compress vapor refrigerant in a rotating motion. As the impeller rotates it sucks vapor refrigerant along with its axis and squeezes it into perpendicular direction. The direction of flow of vapor using centrifugal compressor resembles the direction of flow of cooled air in window type air conditioning units that are mostly using radial fan. This type of compressor is also driven by high speed electric motor.
Screw Type Compressor
According to Design
There are two types of evaporator according to design. These are direct expansion evaporators or those are not using secondary cooling medium to cool a certain confined space. These evaporators are used in domestic refrigeration and air conditioning system or in some commercial and industrial units using capillary tube or thermostatic expansion valve as refrigerant metering device.
Another design of evaporator used in large application is called indirect expansion evaporator which need secondary refrigerant to cool a certain confined room. Secondary refrigerant are sometimes water that could be a source of cooled air to be distributed to the refrigerated place. This design of evaporator is using float valve to control the flow of liquid refrigerant.
Evaporator According to Application
Dry Expansion Evaporator
Direct expansion evaporators are also called dry expansion, widely used on domestic and commercial units using TEV as refrigerant flow control. This type of evaporators only needs the exact amount of liquid refrigerant that must enter to the evaporator inlet based on the amount of heat that should be absorbed to let such liquid refrigerant to evaporate completely before it leaves evaporator coil.
Flooded Type Evaporator
Flooded type evaporators are indirect expansion evaporators. This uses water as secondary refrigerant to cool the desired space. This type of evaporator requires a liquid refrigerant to fill the lower portion of the shell or the evaporator tank to submerge the water pipe.
As the liquid refrigerant gets in contact with the water pipe inside the shell, the heat from water will be absorbed by the liquid refrigerant causing it to stay at the upper part of the shell in a vapor form and shall be drawn by compressor suction pressure. As the water leaves the evaporator coil, it becomes chilled water and ready to be used for cooling. This type of evaporator is widely used in large application for industrial and commercial purposes. Package type units are using flooded type evaporator.
Condenser is used to separate the heat from the body of refrigerant. In order for the heat to leave from refrigerant, there must be a considerable amount of temperature difference between the vapor refrigerant and the surrounding where condenser coil is placed since heat will always travel from a warmer body to a colder body, and the greater the temperature difference, the greater would be the rate of heat transfer. In order to attain this condition the use of external cooling medium such as air and water is applied.
Types of Condenser
Air Cooled Condenser
Air cold condensers are designed to let vapor refrigerant give off heat using air. This could be either natural draft or using no external force to move the air to facilitate heat transfer, or forced draft condensers which are using fan motor to blow up air into condenser coil to remove heat. Domestic refrigerator condenser is a perfect example of natural draft, while domestic air conditioning units are using force draft condensers. If the direction of flow of heated air is inward, it is called induced draft condenser.
Water Cooled Condenser
Water cooled condensers are using water to separate heat from the refrigerant inside the condenser coil. This is most common on large units using cooling tower to cool heated water from the condenser. There are two types of water cooled condenser according to its design. These are shell and tube, and tube within a tube.
Shell and Tube
This condenser is made of shell where vapor refrigerant with high temperature and pressure is contained, and a pipe where water circulates driven by an external pump going to the cooling tower. When the superheated vapor gets in contact with water pipe, heat transfer from the vapor refrigerant to the water. As the vapor releases heat, it turns into liquid and falls at the bottom of the shell where refrigerant line going to the liquid receiver and to the metering device is located.
Tube within a Tube
This is composed of two fluid circuits inside, the outer is for vapor refrigerant and the inner pipe is for water to flow in opposite direction. Heat transfer from refrigerant to water via conduction. In this design condenser coil is directly exposed to open air that also allows some amount of heat to transfer to the air aside from the heat that absorbed by the water inside the pipe.
Air and Water
Some large units are using both air and water to cool the condenser coil or to remove heat from the vapor refrigerant inside. This uses a blower and fan motor to form a forced draft condenser plus the water sprayed from the top to hasten heat dissipation.
Refrigerant Flow Control Device
Metering device controls the flow of liquid refrigerant into the evaporator. Large units are using different metering device according to the design of evaporator.
Thermostatic Expansion Valve
The opening action of this kind of refrigerant control depends on the temperature of vapor refrigerant leaving the evaporator coil. Thermostatic expansion valve has a sensing element mounted at the suction line to sense the temperature of the leaving vapor. Sensing element or the sensor has a volatile fluid inside that evaporates at low temperature similar with the refrigerant used in the system. Sensor is connected to the bellows by a capillary tube. As the temperature of the vapor leaving the evaporator coil increases, This means more heat load at the evaporator , the pressure of the volatile fluid also increases expanding the bellows and opening the refrigerant line to allow more liquid to enter to the evaporator.
Thermostatic Expansion Valve
When there is no more heat load at the evaporator, or when the desired set point temperature of the system has been attained, the temperature of the vapor leaving the evaporator reduces. This causes the volatile fluid to liquefy and reduce the pressure within the bellows causing the bellows to contract. As the bellows contracts, the spring pushes the valve element to close the refrigerant line. Thermostatic expansion valve is widely used in commercial and industrial units using dry expansion evaporators.
Capillary tube is just a copper tube in small diameter. This is used as refrigerant flow control in most domestic refrigeration and air conditioning unit. The principle of operation is no less than reducing the liquid refrigerant passage from the condenser to create more high pressure and suddenly enlarging the tube diameter at the inlet of the evaporator. The sudden change of tube diameter from very small to large creates a considerable low pressure for the refrigerant creating more space to expand and absorb heat.
Float valve could be either high side float or low side float valve. High side float valves is located at the liquid line inside the liquid receiver and maintains the level of liquid refrigerant coming from the condenser. In units using float valve as metering device, the rate of evaporation of liquid refrigerant at the evaporator must be equal to the rate of condensation of vapor refrigerant at the condenser. If the heat load at the evaporator coil rises, more amount of vapor refrigerant should be condensed at the condenser to increase the liquid level in order for the float valve to rise opening the liquid line to the evaporator. If poor condensation happens due to some factor the evaporator will be starved increasing the temperature of the refrigerated space.
Low side float valve is used on flooded type evaporators as refrigerant flow control to maintain the level of liquid refrigerant inside the shell where water pipe is submerged. As the liquid refrigerant absorbs heat from the water, it evaporates and rises on the upper part of the shell and then goes to compressor in complete vapor form. As the liquid refrigerant evaporates, its level decreases, as the liquid level decreases, low side float valve sinks pulling the needle that closes the refrigerant line. When liquid refrigerant again reaches its desired level as it rises, float also rises closing the liquid refrigerant entrance.
Automatic Expansion Valve
Some old units are using automatic expansion valve as refrigerant flow control but it is used today very rarely. This resembles to the operation of the thermostatic expansion valve but it senses pressure of vapor refrigerant leaving the evaporator instead of temperature. There are also some types of metering device using magnetism to open and closed refrigerant line which are called thermal electric expansion valve. Even manually operated metering device are also used before on some units.
Compressor Service Valve
Service valve is used to isolate the compressor from the system during servicing. When removing the compressor it is required to keep the refrigerant in the system at its constant amount, although few vapor may escape to the air during servicing. This is also used to access the system during evacuation, refrigerant charging where hoses from manifold gauge are being connected.
Service valves are mostly used from small split type air conditioning system up to large commercial and industrial units. It is installed at both inlet and outlet of the compressor. Service valve at the inlet of the compressor that connects compressor and suction line is called suction service valve, while the other installed at the outlet of the compressor connecting the discharge line is called discharge service valve.
Service valves are installed at both sides of the system to identify its low side and high side pressure. There are also service valves installed after the condenser coil of some units which are used to close the liquid refrigerant line during manual pump down and when charging liquid refrigerant to the system. There are 3 positions of the valve spindle in the system according to the direction of the flow of refrigerant.
Service valve must be in front seat position when isolating the compressor from the system, closing the refrigerant line from suction line or discharge line. This is done by turning the valve in full clockwise direction. Front seating the valve leaves the refrigerant charging port open allowing pressure from the compressor to escape.
This position of the valve spindle allows the refrigerant to flow in all direction, from the refrigerant lines to the compressor and to the charging port. This should be done when identifying the system pressure with manifold gauge hoses fitted, or when charging port cap is not removed to avoid refrigerant leak. Mid seating the valve could be done by turning the valve spindle half of its full travel inside the valve body.
Turning the spindle in full counter clockwise direction does back seat position. This closes the way of refrigerant going to the charging port allowing no pressure to come out from the system to the manifold gauge. This should be the normal position when the leaving the unit operational under good condition. Fully back seat position allows the refrigerant to circulate to the whole system from the compressor suction line going to the high side of the system and complete the whole cycle.
This is widely used in transport refrigeration to protect the system from too much vibration during unit operation when in travel. This is installed directly after the service valves in both suction and discharge line, where copper tube in both lines are being brazed.
As the unit operates, small amount of oil goes together with vapor refrigerant coming out from the compressor going to the condenser. When condensation happen inside the condenser, vapor refrigerant transform into liquid. Too much oil content in the system may lessen its performance and may cause clog up in the liquid receiver and expansion valve. To separate oil before refrigerant will turn into liquid, oil separator is installed before the condenser coil. This is to collect back the oil that goes together with vapor refrigerant as compressor operates. It is quite easy to separate oil from refrigerant at this point since oil which is in liquid form precipitates at the bottom of the oil separator tank while vapor refrigerant remains on the upper part of the tank where line going to the condenser is connected. The compressor will then draw the collected oil back to its crankcase.
Sight glass is used to visually identify the condition and amount of refrigerant circulating the system. It is usually installed at the liquid line where refrigerant is in liquid state. In some cases the use of sight glass in other point of the system may be applicable. When bubbles appear in the sight glass as the system operates, it indicates low refrigerant charge. It has also moisture indicator that allows technician to diagnose the system.
Liquid receiver is used to collect liquid refrigerant before it travels to the refrigerant flow control. There are times which the need for liquid refrigerant in the evaporator is lesser than the amount of liquid that should flow, when the heat load is lesser or when the system reaches its desired temperature, metering device is partially closed. In this condition, some amounts of liquid refrigerant are temporarily stored at the liquid receiver until the system again needs it when heat load increases. Liquid receiver is widely used on system with automatic pump down cycle in which during off cycle, all refrigerant must be stored at the high side of the system.
High Pressure Relief Valve
This is a protective device against excessive build up of pressure at the high side of the system. During high ambient temperature, high side pressure may rise and exceeds to its maximum capacity. In this critical condition, the element of the relief valve melts in respond to the amount of pressure build up allowing some pressure to burst into air. Relief valve closes again when pressure normalizes.
Check valve is used to maintain the refrigerant to flow in one direction. It is installed at the liquid line before the refrigerant control device where the flow of refrigerant is suddenly hindered as a result of metering action of metering device. Its operation is based on the direction of pressure, when back pressure arises or some of the refrigerant tends to return, a covering element inside closes the passage push by the pressure itself.
Filter drier is also located at the liquid line before the refrigerant control device. Its function is to trap all foreign particles such as solid impurities and moisture that may harm the system. When unit is not properly evacuated, some amount of air that contains moisture may mix with refrigerant. When refrigerant containing air releases heat at the condenser, droplets of water will be formed as a result of condensation, this moisture freezes and blocks the way of refrigerant when it undergoes low temperature when it reaches the refrigerant control where change in pressure and temperature occurs.
To avoid this circumstances, a drier or moisture absorbing element is placed inside the filter plus the strainer that traps all solid particles that may also cause clog up in some point of the system where refrigerant lines are in smaller diameter.
Heat exchanger is used in some unit to aid the condensation process of the refrigerant in the condenser at the same time avoiding liquid refrigerant to flow into compressor since liquid is non compressible fluid. This is done by sealing part of the discharge line that is in high temperature together with the suction line, which is in low temperature to let some amount of heat to flow into colder body. Heat from the discharge line helps to completely vaporize the refrigerant in the suction line before it reaches compressor, at the same time, as heat is taken from the vapor refrigerant inside the discharge line, it aids the condensation process.
Accumulator is widely used in some units that are not using liquid receiver and heat exchanger, but large units are using both to collect liquid refrigerant. Unlike the liquid receiver, accumulator is installed at the low side of the system between the evaporator and compressor. Its main purpose is to collect liquid refrigerant that has not turned into vapor. Accumulator separates liquid refrigerant from vapor in a normal process. As refrigerant flows into accumulator, liquid particles will be collected at the bottom of its tank while vapor rises and shall be drawn by the suction pressure of the compressor. I some point of time liquid refrigerant evaporates inside the accumulator tank when it senses heat.
This is an electrically operated device use to close and open refrigerant lines. Solenoid valves are used in any part of the system depending on the operation of the unit. In some units with automatic pump down cycle, solenoid valve is installed at the liquid line before the metering device that stops the refrigerant flow from high side to low side during pump down.
This is also used in units with hot gas defrost system where vapor refrigerant in the discharge line is directed to the evaporator to melt build up of ice during defrost cycle. Reverse cycle air conditioning units are also using solenoid valves to reverse the flow of refrigerant during heating.
Muffler is used to eliminate noise as a result of reciprocating action of some reciprocating compressors. This is commonly used on hermetic compressors for domestic units where noise during operation must be eliminated.
The function of any strainer as used in filter drier is to trap solid particles that may alter the refrigerating capacity of the unit. Compressors are using strainer installed at its inlet to prevent solid particles from getting in. Strainer is also used in lubricating system of the compressor where it is submerge with oil at the compressor crankcase.
Role Of RAC Technician
Refrigeration and Air Conditioning technicians are responsible for maintaining the operating efficiency of Refrigeration and Air Conditioning units. This means ensuring the maximum heat dissipating capability of the condenser and heat absorbing capability of refrigerant when it reaches the evaporator. Failure of the unit to do so is also a failure of the technician who handled that unit to perform his role and responsibility. To avoid such failures, there are many things to consider technically and the basic knowledge in refrigeration principles is a great foundation to become an effective technician and worthy to handle great responsibilities.
Factors that Affects Refrigerating Capacity
This refers to the amount of heat within a certain enclosed space that is subjected for refrigeration or where cooling must be accomplished. Heat load changes depending on some factors such as, insulation and the kind of foodstuff being refrigerated. The greater the heat load, the greater heat absorption and extraction should be done by the unit.
Functionality of Components
Each component involved in the whole operation affects the whole system. It must always be ensured that all parts installed both electrical and mechanical are in good condition and able to perform their individual duty in the system as part of the whole.
This refers to the temperature of the surrounding where the unit is installed. Ambient temperature may vary based on factors like location of installation of the condensing unit. When condensing unit is directly exposed to the sun at noontime, when there is no open air ventilation due to the scarcity of space, when there is possible foreign source of heat near the condensing unit installation, the condensing capacity of that unit is surely poor.
This refers to the task that should be done by the technician regularly regardless of the unit condition. This involves monitoring of operating capacity of every component and operational time. Some parts are designed to last only for a certain period of time and should be replaced right away even there are no signs of deterioration.
Knowledge and Skills of Technician
Some refrigeration and air conditioning technicians are quietly knowledgeable and has acquired more skills than the other. This may vary mostly based on work experience, educational background and interest towards this area of technology. Incapacity of any technician to perform his job skillfully and knowledgeably, may harm the performance or lessen the capacity of that unit to do heat absorption and extraction. It may eventually lead to damage of life and properties and may harm the environment.