HVAC / Refrigeration Flashcards
Define latent heat (enthalpy)
The heat energy required to change the state of a refrigerant from a solid to liquid or from a liquid to a vapour or gas at a constant temperature and pressure.
Define vapour pressure
The pressure of a vapour in contact with its solid or liquid form
Define saturation
The point at which the greatest possible amount of substance has been absorbed by a solution or vapour at a given temperature. Air is said to be saturated when it will absorb no more water vapour.
Define superheat
The heat energy added to raise the temperature of a vapour or gas above the saturation temperature at which it changes from a liquid to a vapour or gas.
Define sub cooling
The amount of heat energy removed below saturation temperature of the liquid refrigerant.
Define zeotropic blend
A zeotrope is a blend of two, or more, gases which when mixed compose a blend that has two, or more, boiling temperatures (depending upon the number of substances). When boiling the more volatile substance evaporates first, modifying the blend composition locally.
Define azeotropic
An azeotrope is a blend of two, or more, gases which when mixed compose a blend that has a common boiling temperature (independent of the boiling temperature of each component).
Define temperature glide
The temperature difference of boiling temperatures of a blended substance in a zeotropic blend
Conditions allowing legionella to proliferate
Air-conditioning systems have the potential for supporting colonies of bacteria particularly if they have the conditions needed to proliferate. Bacteria need the following;
Relatively warm temperatures – ideal growth is in temperatures between 35–50° C
Presence of stagnant water – contaminated water is an ideal breeding ground for bacteria
Source of nutrients – need a food supply such as rust, sludge, scale, algae, and other bacteria to survive.
Transmission of legionella
Transmission can be by inhalation of infected water droplets or by ingestion of contaminated water by some means, such as water droplets settling on food or hand to mouth.
Symptoms of legionella
Flu like symptoms including;
High temperature, feverishness and chills
Cough
Muscle pains
Headache
M notice regarding legionella
MGN 38
Two causes of excessive discharge pressure in a domestic refrigeration system
Air in system – rectified by pumping the gas into the condenser and bleeding off air
Over charge of gas – reduce charge by bleeding off excess gas into recovery bottle until correct pressure is obtained
Two causes of low pressure at evaporator suction in a domestic refrigeration system
Expansion valve shut in – rectified by adjusting the expansion valve manually until suction pressure is correct
Suction filter of expansion valve choked – clean filter
Two causes of compressor cycling in a domestic refrigeration system
Low pressure cut out set too high – check and reset
Expansion valve iced up due to water in the system – defrost, and replace drier
Two causes why compressor is running for long periods but not lowering the temperature in the cold rooms
Cold room door not shut – shut door
Under charge of gas – check and recharge
Two causes why the compressor is operating noisily in a domestic refrigeration system
Liquid refrigerant returning to compressor – adjust expansion valve to correct amount of super heat
One compressor – start standby machine and overhaul compressor
With reference to a vapour compression refrigeration plant explain the purpose of an expansion valve
The expansion valve controls the flow of refrigerant to the evaporator. It uses the effect of a convergent nozzle, lowering the pressure and thus the saturation temperature of the refrigerant. As the pressure drops, the liquid is above the saturation temperature for that pressure and so the liquid starts to change to vapour. Because this change takes heat, and the source of energy is the refrigerant, the temperature of the refrigerant must fall until it reaches its saturation temperature when it will be a mixture of gas and liquid.
Symptoms and remedy of air in a refrigeration system
Air will cause the compressor to overheat and a temperature and pressure in the condenser will be too high. Air being lighter than refrigerant will collect at the top of the condenser, reducing condensing capacity.
Pump the charge into the condenser with the condenser liquid stop valve shut and after waiting a few minutes remove the air and a small amount of refrigerant from the top of the condenser by pumping into a recovery bottle. Do not vent to atmosphere.
Symptoms and remedy for water in a refrigeration system
Water or moisture will cause internal choking of the expansion valve with ice.
Replace drier cartridges. Warm expansion valve with hot water soaked rag.
Symptoms and remedy for oil carryover in a refrigeration system
Cold room temperature not reaching desired value. Compressor cycling on low pressure switch. Apparent loss of oil from compressor.
The plant must be pumped out and shut down. The evaporator must be removed for cleaning internally, the oil may have congealed on the heat transfer surfaces. Oil separating unit must be replaced. Plant must be recommissioned.
Symptoms and remedy for overcharge in a refrigeration system
High compressor discharge pressure. Full sight glass at compressor. Compressor icing on such a side.
Extract refrigerant into recovery cylinder using vacuum pump. Do not vent to atmosphere.
Symptoms and remedy for under charge in a refrigeration system
Lower suction pressures, higher evaporator outlet temperature. Compressor cycling, bubbles inside glass.
Possibility of leak, investigate and rectify. Recharge system
What is meant by secondary refrigerant
A primary refrigerant is one that circulates through the compressor, condenser and evaporator of a refrigeration circuit. In order to limit the amount of the primary refrigerant in circulation through large cases requiring cooling a secondary refrigerant is employed. This is cooled in the evaporator and then circulated.
