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ITI/NCVT RAC Mechanic (AITT Theory) study materials
ITI/NCVT RAC Mechanic (AITT Theory) study resources

ITI/NCVT RAC Mechanic (AITT Theory): Domestic and commercial refrigeration appliances: refrigerators, deep freezers, water coolers, bottle coolers, visi coolers

ITI/NCVT RAC Mechanic (AITT Theory) · Work through your exam, one topic at a time.

All guide sections

Domestic and commercial refrigeration appliances: refrigerators, deep freezers, water coolers, bottle coolers, visi coolers

What this domain tests

Circuit and control layout of domestic and frost-free refrigerators, chest and upright deep freezers, water coolers, bottle coolers and visi coolers, plus typical faults and their symptoms.

Core rules

  • Domestic refrigerator: hermetic compressor (commonly R600a, charge 150 g or less), capillary tube, wire-and-tube or roll-bond evaporator, thermostat, PTC or current relay, run capacitor, door gasket, anti-sweat pipe in the frame.
  • Frost-free refrigerator: timer, bimetal defrost thermostat, defrost heater, evaporator fan, drain heater and drain pan. No cooling with the freezer warm and the heater permanently on points to welded timer contacts or a stuck bimetal.
  • Deep freezer: low-back-pressure compressor, low-temperature thermostat, larger condenser, thick insulation. Chest models retain cold air when opened; uprights are convenient but spill cold air.
  • Water cooler: storage tank with bubbler, cooling coil, thermostat, agitator or circulation pump; tank sanitising is part of maintenance.
  • Bottle cooler and visi cooler: forced-air evaporator, glass door with anti-sweat or mullion heater to prevent fogging.
  • Diagnostic rule: low suction pressure with normal or high head and no cooling means restriction or undercharge; high suction with low head points to faulty compressor valves or a leak.

Worked example

A water cooler must cool 40 litres per hour from 30 °C to 10 °C. Heat removed = m cp delta T = 40 x 4.187 x 20 / 3600 = 0.93 kW, about 0.27 TR. Adding ambient heat gain through the tank, say 0.1 kW, and pulldown allowance, a nominal 0.5 TR unit (about 1.75 kW) is selected so the compressor cycles rather than runs continuously.

Traps

Forgetting the 3600 s per hour in litres-per-hour sums; blaming the thermostat when the defrost heater is stuck on; fitting a high-back-pressure compressor to a freezer; overlooking the anti-sweat heater as the cause of a warm visi cooler; ignoring a door gasket leak.