1 HVACR317 - Refrigeration Condensers and Subcooling

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HVACR317 - Refrigeration

Condensers and Subcooling

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Condensers

• The part of the refrigeration system which receives high-temperature, high-pressure vapor from the compressor, and removes heat from the refrigerant until it returns to a liquid state.

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Condensers

• Must be larger than the evaporator.• Must be able to hold 100% of the

charge without a receiver.

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Condensers

• Removes heat via:– Heat of evaporation

• The heat absorbed from the evaporator.

– Heat of compression• The heat produced by the compression

process.

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Condenser Capacity

• Factors that affect capacity:– Surface area

• The physical contact area of the condenser.

– Temperature difference between the• Cooling medium (air or water)• Refrigerant gas

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Condenser Capacity

• Factors that affect capacity, cont’d:– Velocity of the gas

The greater the velocity (the faster the gas flows through the condenser) the

• better the heat transfer.• greater the capacity.

– Material the condenser is made of– The cleanliness of the surface area

• Dirty condensers do not transfer heat well.

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Condenser Capacity

• Large Condensers– Good temperature difference between the

refrigerant and the cooling medium.– The result is:

• Low discharge pressure• Good subcooling

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Condenser Capacity

• Small Condensers– Little temperature difference– The result is:

• High discharge pressure• Very little subcooling

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Condenser Capacity

• Dirty Condensers– Little temperature difference– The result is:

• High discharge pressure• Very little subcooling

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Condenser Capacity

• The end result of a condenser that is to small or is dirty:– Overheating of compressor– Boiling of oil– Compressor failure

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Condenser Types

• Two Types of Condensers: Air Cooled and Water Cooled−Air Cooled. Use air to cool the

condenser as found in most residential or light commercial units.

−Water Cooled. Use water to cool the condenser, found in larger commercial and some ice machines, and some geothermal applications.

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Air-cooled Condensers

• Natural Convection– Transfers heat to the surrounding air by

means of natural air flow.

• Wire Static (Static)– Constructed out of steel tubing interlaced

with wire.– Mounted to the back of the unit.– Cooled by ambient air.

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• Plate-Static (Warm Wall Condenser)– Tubing is in contact with outer wall. They

use the same principle as radiant heat to cool.

– The advantage of this type is that it allows the unit to go flush to the wall, and no side-wall heaters are needed.

– The disadvantage is that leaks are hard to find and costly to repair.

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• Forced Draft Condensers– Use a fan to move air through the coils.– Fins are connected to condenser tubes to

provide more surface area for heat transfer.

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• The normal temperature difference between the condensing point of the refrigerant and the ambient air is between 20-30° F.

• The condensing point of the refrigerant is found by converting the pressure in the condenser to the temperature.

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• Subcooling takes place in the last few passes of the condenser.– To measure subcooling, use the

condensing point and subtract the temperature of the refrigerant, leaving the condenser (what).

– The temperature of the refrigerant is found by placing a thermometer on the outlet of the condenser.

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• If the condenser has a fan that enhances air flow, it is a forced draft condenser.– Window A/C– Refrigeration systems– Split A/C systems– All our shop condensers

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Water-cooled condensers

• Water cooled condensers were developed before air cooled condensers.

• Water cooled condensers are more efficient than air cooled condensers.

• Same principle as air cooled but water is used to extract the heat instead of air.

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Water-cooled condensers

• Water-cooled Condensers come in different types, like:– Tube in tube– Shell and coil– Shell and tube

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Water-cooled Condensers

• Tube in Tube Characteristics:− Made by putting one pipe inside another,

larger diameter tube. − The tubes are wrapped into a circle to

save space.− The refrigerant is in the outer tube with

water in the inner tube.

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• Tube in Tube, Cont’d:− The heat is transferred from the

refrigerant to the water through the walls of the inner tube.

− This style is frequently used in ice machines and geothermal heat pumps.

− Mineral deposits will eventually build up in these condensers and will need to be treated or cleaned.

− As minerals build up it will decrease water circulation.

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• Shell and Tube Condenser Characteristics:−There is a shell, or a container that

contains the refrigerant; the tube is running through the refrigerant with a high rate of water.

−The heat exchanges from the refrigerant to the water and condenses to a liquid, and comes out of the bottom in the liquid line.

−Also known as ‘water boxes.’

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• Shell and Tube Condensers, Cont’d.:− The end caps can be removed so that

the tubes can be brushed out and cleaned to prevent mineral build up.

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• Shell and Coil Condenser Characteristics: −Same idea as shell and tube, but a little

less expensive.−The water line is coiled down the center.−Hard to clean if minerals build up.

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Water-Cooled Condensers, General

• What happens to the water?– Water-cooled condensers must have a

way for the water to reject the heat.• Some are called wastewater systems – water

is used once then wasted.• Some are called re-circulated – water is used

to cool the condenser, then the water is cooled and re-circulated.

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• Cooling Towers– Use is limited by the outdoor wet-bulb

temperature.– Require yearly cleaning, consistent water

treatment and air movement.

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• The refrigerant will normally condense at 10° F higher than the leaving water temperature.

• A re-circulating water system will circulate about 3 gallons of water per minute per ton of refrigeration.

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Water Regulators

• The water regulating valve will regulate the flow of water through the condenser.

• The water regulating valve is controlled by the head (discharge) pressure of the compressor.

• As the load increases so does the head pressure and so does the water flow.

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Water Regulators

• Always adjust a water regulator to the manufacturer’s recommended flow rate in gallons per minute.

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Additional Notes on Condensers

• A properly functioning condenser will de-superheat, condense and sub-cool the refrigerant.

• When a condenser is placed in a low temperature environment, a low ambient (fan cycling) control can be used to cycle the fan on and off, thus maintaining head (high side) pressures.

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Additional Notes on Condensers

• Low ambient temperatures seriously affect the efficiency of air-cooled condensers.

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Subcooling

• What is subcooling?– A temperature of a liquid when it is cooled

below its condensing temperature.– Subcooling is the sensible heat removed

from the liquid after the change of state has taken place.

– Subcooling is used to determine if the condenser has the proper level of refrigerant.

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Subcooling

• The complete condensing of refrigerant should occur in the bottom quarter of the condenser.– Any heat that is removed after this point

is subcooling.

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Subcooling

• Subcooling is needed to maintain proper system balance.

• Normal subcooling is between 15-20° F.– Depending on how efficient the condenser

is, subcooling may be a little higher or lower.

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Subcooling

– If the subcooling is low and you have a high head pressure• Possible dirty condenser• Check condenser fan operation• Look for overcharged condition • Could have non condensibles in it• Possible restriction before evap coil

– If the subcooling is high • Low charge (could be overcharged) • Check comp amps, if lower than RLA

rating than low charge

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Subcooling

• Subcooling can be accomplished by placing the liquid line and the suction line in direct contact with each other.

• This is usually done in small or low-temperature units.

• This higher subcooling results in a more efficient unit.

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Subcooling

• The liquid in the liquid line will be cooled below its condensing temperature.– Also the suction line may be warmed

slightly to boil any refrigerant that may be present before entering the compressor.

• The lower the temperature in the liquid line, the greater the heat removal capacity.

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Subcooling

• This greater subcooling can also be achieved by one system cooling another so that the system can more efficiently reach lower temperatures.– This is known as a cascade system.

• Cascade systems are usually required for ultra-low temperature operations.

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Measuring Subcooling

• Determine the condensing temperate.

– Use your gauges and convert the high side.

– This should be the ambient temperature plus 30-35 degrees for air cooled systems.

• Measure the condenser outlet or the liquid line temperature.

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Measuring Subcooling

• Subtract the liquid line temperature from the condensing temperature.

• The result is subcooling.

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Subcooling Example

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1 HVACR317 - Refrigeration Condensers and Subcooling