Define The Term Refrigeration Flashcards
Define the term refrigeration
process of removing heat from a substance thus lowering its temperature, then transferring this heat to another substance at a higher temperature
State the basic principles of refrigeration
- heat will only flow from high to low temperature.
- rate of heat flow is directly proportional to difference in temperature
- boiling point of a liquid can be lowered or raised by increasing/ decreasing pressure on it.
- vapour can be liquified by increasing/ decreasing its temperature at constant pressure
- to change the state of a substance, latent heat must be added or removed.
- if vapour is compressed without loss of heat it will become superheated
- with liquid and Vapor in a sealed container, liquid will be at boiling point, Vapor at saturation point
State the function of the following components of a refrigeration system: compressor, evaporator, liquid receiver, sight glass, TEV
Compressor: compresses Vapor from the evaporator so that it’s heat is concentrated and temperature is higher than cooling medium. Pumps refrigerant around system.
Evaporator: heat exchanger to convert refrigerant into Vapor absorbing heat from room or surroundings. Imparts superheat to refrigerant
Liquid receiver: acts as a liquid seal between the expansion valve and the vapour in the condenser. act as a reservoir for the liquid refrigerant under varying loads.Can accommodate the whole refrigerant charge during maintenance.
Sight glass: for visual inspection of refrigerant level, moisture content. Green is dry, yellow is moisture
TEV: keep evaporator charged with liquid refrigerant. Prevent liquid carryover to compressor. Controlling amount of superheat at evaporator outlet.
List the types of expansion device used in refrigeration systems
- capillary tube
- orifice plate
- TEV/ TXV
- constant PRV
- electronic/ electrical
State the purpose of the following safety devices normally fitted to refrigeration systems: HP trip, LP trip, LO pressure differential trip, bursting trip
HP trip:
fitted to discharge side of compressor to prevent over pressurization of the refrigeration circuit
LP trip:
fitted on suction side of the compressor to prevent damage to sys components due to low refrigerant flow
LO pressure differential trip:
Fitted to force lubrication to compressors to prevent damage in the event of a LO pressure failure
Bursting disc and relief valves :
Fitted on discharge side of compressor on some system to prevent over pressurization of refrigerant circuit.
Backup in case the HP trip does not work
State the desirable properties for lubricating oils selected for use in a refrigeration system
- must have low freezing point
- must mix well (miscible) with refrigerant
- wide working range of pressures and temps.
- flow well at low temp
List the methods of adding oil to a refrigeration system
- by hand - small plants
- stirrup pumps - smaller fridge compressors
- hand pump - fitted to some larger forced lubrication systems
- dosing pot - some larger systems mounted on delivery oil separator
List the precautions to be taken when adding oil to a refrigeration system
- never use damaged or seeping cans
- never use open can- correctly dispose unused oil
- do not use empty containers for other use
- do not decant oil into another container
Explain why it is necessary to defrost refrigeration plant evaporators
- occurs below temps of 0 degrees
- frost has insulating effect on evaporating coils
- causes drop in evaporator efficiency and saturation pressure
- can cause prolonged running of plant
State the factors that determine the suitability of a refrigerant for a particular duty
- moderate condensing pressure - don’t need heavy compressors, condensers or piping
- high critical temp - impossible to condense at temp above critical
- low specific volume - reducing size of plant required
- non corrosive - refrigerant should be non corrosive to all parts in system
- safe - non explosive, non flammable, non toxic
State the points of responsibility the MEO has with regard to the Montreal & Kyoto protocols
- qualification of personnel
- monitoring usage
- liability for negligible
- ensure proper disposal of refrigerant
- maintenance of equipment needed to prevent damage
- recovery, reclamation, recycling
State the precautions to be observed when entering a refrigeration cool room
- test atmosphere for presence of refrigerant
- if dizzy, exit immediately
- ensure breathing apparatus available
- ensure safety is available
- beware of CO2 buildup
- no smoking strictly enforced
State the precautions to be taken before carrying out maintenance on refrigeration machinery
- ensure compartment ventilation is running
- pump down refrigerant charge
- continuously monitor for refrigerant gases
- isolate plant electrically
- never work alone
- place warning signs / notices
List the safety precautions relating to the handling and storage of refrigerant cylinders
- stow spare cylinders correctly in storage racks
- never heat cylinder with naked flame
- valve protection caps to be fitted
- handle carefully (slinging arrangements)
- never fill recovery cylinder to more than 75%
Give three methods of capacity control used in fridge systems
Max flow ratio of TEV. Is 5:1. Therefore need additonal flow control:
- starting and stopping of compressors
- mechanical unloading of a compressor
- flow modulation of the liquid or gas bypass
