Cooling Systems 030203c Flashcards
What are the main parts of a cooling system?
Compressor.
Condenser.
Evaporator.
Metering Device.
Refrigerant.
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What is the purpose of the compressor in cooling systems?
Draws in suction vapour in a cyclinder. Compresses the vapour with reduces the volume and increases the refrigerant pressure and temperature, then moves the refrigerant through the cooling system.
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What is the difference between a hermetic and semi-hermetic compressor?
Hermetic - Consists of a motor and compressor sealed in a welded shell. Not intended to be repaired.
Semi-hermetic - Consists of a motor and compressor that is mechanically sealed. Designed to be disassembled and repaired.
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What is the main purpose of the condenser?
Transfer heat energy from refrigerant to the ambient air where it is located, typically outside.
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What are some characteristics of most air-cooled condenser coils?
Slab-type construction.
Several rows of horizontal copper tubing covered in tightly spaced aluminum fins.
Refrigerant circuits with only one or two passes.
Depth of less than 6 inches (7.5 cm)
Large face area to depth ratio.
Formed to fit in specific cabinets.
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What is the purpose of the evaporator?
A finned heat exchanger to transfer heat from the air being cooled to the refrigerant.
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What is the purpose of the fins on an evaporator?
Increase the efficiency of the coil by increasing the heat transfer area of the evaporator.
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What is the purpose of the metering device and what is it also known as?
Also known as the refrigerant control valve.
Measures refrigerant levels and delivers it to the evaporator at specific rates of flow, pressure and temperature.
Reduces the pressure of the liquid refrigerant coming from the condenser.
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What are the three most common types of metering devices?
Capillary tube. - uses a fixed length and inside diameter.
Fixed orifice. - uses a predetermined opening or hole entering the evaporator. Simplest device.
Thermal expansion valve. - variable metering device. Changes size of valve opening based on temperature of the refrigerant leaving the evaporator.
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What is a refrigerant?
Chemical substance in a cooling system that absorbs heat from a conditioned space and releases heat outside of the conditioned space.
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What are some commonly used refrigerants used for air-conditioning?
R22, R134a, and R410.
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What should never be done with refrigerants?
Never be released to the atmosphere.
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What is the purpose of the condenser fan?
Draws air through the condenser coil.
Mechanically moving air through the coil helps with the heat transfer from the refrigerant to the ambient air being used to cool the coil.
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What is the purpose of the evaporator fan?
Draws air through the evaporator coil. The air being moved through the coil is the air to be conditioned / cooled.
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What is the purpose of the high limit pressure switch?
N/C pressure switch safety device. Shuts down the compressor if the pressures in the cooling system rise to a level that could damage equipment.
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What is the purpose of the low limit pressure switch?
N/O pressure switch that is held closed by the pressure in the system. Protects the compressor from overheating.
Detects if the pressure in the system drops below a set pressure. Could indicate a refrigerant leak.
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What is a definite purpose contactor?
A contactor labelled for a specific use. For example a contactor labelled for use with HVAC equipment or compressors.
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What is the purpose of the low ambient temperature limit controller?
Interrupts the control circuit to the compressor if the outside air temperature drops below a safe operating temperature. Usually below 10 degrees Celsius.
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What is the purpose for a compressor crankcase heater?
Resistance heater that prevents refrigerant migration by keeping the compressor crankcase oil warmer than the ambient temperature.
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What is the purpose of a high-temperature sensor?
Shuts down the system if the compressor operates at a higher temperature than is intended.
Located inside the compressor.
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What is a freeze thermal sensor?
Shuts down the compressor if a temperature below the freezing point of water is detected.
Located on the evaporator.
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What is the purpose of a filter dryer?
Removes small foreign particles from the refrigerant and the dryer removes moisture from the refrigeration system.
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What is the purpose of a sight glass?
Typically installed right before the metering device for visual verification that 100% liquid is arriving to the meter.
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What is short-cycling protection?
Prevents a refrigeration system from operating when it tries to start and stop too frequently.
Commonly programmed into the cooling thermostat.
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What is a receiver?
Pressure tank located after the condenser that collects liquid refrigerant. Dip tube draws from the bottom to ensure 100% liquid is delivered to the meter device.
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What is the difference between latent heat and sensible heat?
Latent heat causes a change in state. Sensible heat causes a change in temperature.
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When does cooling occur?
When the vapour of a liquid that evaporates at atmospheric pressure is released.
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What cools the remaining liquid as it passes through the metering device?
Flash gas.
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What can heat pumps do that a standard cooling unit cannot?
Reverse the direction of refrigerant flow.
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What are the two levels of commissioning confirmation?
Start-up - quick informal confirmation of the sequence of operation.
Formal commissioning process/report - includes the measurable and operational data. Legal document.
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Which device at the compressor ensures the refrigerant remains a gas during cold conditions?
Crankcase heater.
At what component does the refrigerant pressure drop?
Metering device.
Where does the refrigerant turn from a gas into a liquid during cooling?
The condenser.
Which device increases the pressure of the refrigerant during the cooling cycle?
Compressor
