Refrigeration Plant — Most Asked Questions
19 questions asked about refrigeration plant across the General and Fn 6 and Fn 4b papers, heaviest first.
Learn the system first
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1 Thermostat and thermistor
Also asked: “Thermistor vs thermostat”
Answer
Thermostats switch, thermistors sense: bimetallic snap for control duty, semiconductor curve for measurement duty.
- Thermostat is an on-off switch — bimetallic strip bending with temperature to make or break the circuit.
- Thermistor is a semiconductor resistor whose resistance falls as temperature rises — nonlinear but highly sensitive.
- Thermistors top out near 100–200 degrees, so they serve refer boxes and coolers, never exhausts.
- Refer plant uses both: thermostat cycling the compressor, thermistor feeding the controller its temperature.
Surveyor follow-up: box warm with the compressor short-cycling — thermostat, thermistor or charge, and what reading proves it?
Full theory: Expansion Valves and Liquid Level Control -
2 Oil charging in reefer
Answer
Charge refer oil without air or moisture: purge the line, feed above suction pressure or pull vacuum first.
- Purge the charging connection until oil, not air, shows at the joint before coupling to the crankcase.
- Pressure method: raise oil above suction pressure so flow runs into the crankcase with the compressor stopped.
- Vacuum method: pump the crankcase down first so vacuum draws the charge in cleanly.
- Prove on the sight glass at the book level with the system leak-free and pressures normal.
Surveyor follow-up: oil level high with foaming sight glass — overcharge, refrigerant flood-back or water, and what proves it?
Full theory: The Refrigeration Compressor -
3 Refrigerant charging procedure
Also asked: “refrigeration oil charging procedure”
Answer
Charge refrigerant leak-free and weighed: prove tight, purge hoses, weigh the bottle, watch the sight glass.
- Pressure-test and confirm no leaks with a steady gauge before a gram goes in.
- Weigh the bottle for the exact charge size, connect to the liquid-line charging valve with kink-free hose.
- Crack the bottle to purge air from the line, then charge watching pressures and sight glass.
- Log weight in against system size with leak retest after — overcharge hunts and trips like undercharge starves.
Surveyor follow-up: sight glass flashing after the weighed charge — leak, moisture or restriction, and what do you check first?
Full theory: The Refrigeration Compressor -
4 How to purge air from refrigeration system?
Also asked: “How do you release the air trapped in Refrigeration system?”
Answer
Purge air at the condenser after proving sub-cooling: pump down, compare pressure to saturation, vent the top.
- Air shows as high condenser and discharge pressure with bubbles in the glass, hot running and swinging gauges.
- Prove by liquid pressure-temperature against the saturation chart with cooling water trimmed to near-saturated.
- Pump down to the condenser with the outlet shut, then crack the purge valve until pressures settle.
- Find the entry — shaft seals, hoses or long vacuum-side standstill — or the air returns.
Surveyor follow-up: pressures normal a week after purging — leak, overcharge or moisture, and what proves it?
Full theory: The Refrigeration Compressor -
5 Q1.) How the leaks in refrigeration system be detected and procedure for rectification of pipe leaks? Explain how a copper can be renewed without hotwork? (temporary repair
Also asked: “Q1. How the leaks in refrigerating system can be detected and procedure of rectification of pipe leaks. Explain how a copper pipe can be renewed without hot work?”
Answer
Find leaks by pressure, senses and instruments: standing drop, soap, halide and electronic nose, then clamp or braze.
- Standing nitrogen pressure drop proves the leak exists; section isolation finds which leg.
- Soap bubbles, halide flame colour and electronic detectors pinpoint joints, flares and valve stems.
- Permanent fix is cut, re-flare or braze with nitrogen purge and filter-drier renewed.
- Temporary copper renewal without hot work clamps a compression coupling or epoxy sleeve until the next stop.
Surveyor follow-up: leak at a flared joint with charge aboard — tighten, remake or clamp, and what do you recover first?
Full theory: Expansion Valves and Liquid Level Control -
6 Safeties in Refrigeration system
Also asked: “refrigerator system safety”
Answer
Refer safeties guard pressure, oil and flow: HP and LP switches, oil-differential cutout, reliefs, solenoids and fusible links.
- High-pressure switch trips the compressor on condenser overpressure with hand reset.
- Low-pressure switch guards loss of charge near 1 bar with hand reset behind it.
- Oil-differential cutout near 1.2 bar guards bearings with start-up time delay built in.
- Reliefs on discharge and condenser, solenoids sequencing flow, fusible plugs on heat complete the set.
Surveyor follow-up: HP trips with the condenser clean — overcharge, air or water flow, and what gauge proves it?
Full theory: The Refrigeration Compressor -
7 For a minute leak from refrigerant piping or condensing unit, use a gas detector
Answer
Pinhole leaks need instruments, not eyes: halide torch for chlorinated gas, electronic sniffer for the rest.
- Bubble solution finds weeps you can see; a minute leak needs a detector drawn slowly along every joint.
- Halide torches burn green in chlorine-bearing refrigerant — useless on chlorine-free gases and risky in closed attics.
- Electronic sniffers read corona-suppression or heated-diode current rising with gas concentration to an audio alarm.
- Heated-diode types run more accurate with fewer false alarms — pinpoint every hit, then prove with pressure hold.
Surveyor follow-up: torch stays blue but the charge keeps falling — wrong tool or wrong gas, and what next?
Full theory: Refrigerants -
8 When it is presumed that the refrigerant leaks inside the condenser, following procedure to be taken; Close the condenser water inlet & outlet valves. Release the drain inside covers of condenser by removing the water pl
Also asked: “When it is presumed that the refrigerant leaks inside the condenser, following procedure to be taken; Close the condenser water inlet & outlet valves. Release the drain inside covers of condenser by removing the water plugs. And then carry a gas detector near the condenser tube plate (clad steel). Check the each cooling tube and tube plates for refrigerant leakage”
Answer
Suspected tube leak means isolate water first: shut sea connections, drain covers, then pump the charge to safety.
- Close condenser sea inlet and outlet valves at once so leaking refrigerant cannot reach overboard water.
- Crack the water-side drains with covers lifted to prove gas in water before opening further.
- Pump the charge to condenser and receiver with liquid outlet shut, then lift and pressure-test the nest.
- Restart only with water restored, air purged at the purge valve, and pressures proved normal both sides.
Surveyor follow-up: water in the refrigerant with the sea valves still open — isolate, sample or run on?
Full theory: Condensers, Evaporators and Heat Transfer -
9 With reference to main refrigeration plants give reason for each of the following operational irregularities and state how are these dealt with
Answer
Reefer faults read on gauges: undercharge runs hot and empty, overcharge runs high and trips, air and ice fake both.
- Undercharge shows hot compressor with high superheat, low suction and discharge, bubbles in the sight glass and long running.
- Overcharge shows high condenser level with high pressures tripping the HP cutout on excess refrigerant or trapped air.
- Air in the system mimics overcharge with high pressures; ice on the regulator starves the evaporator like a blockage.
- Cure by leak search at seals and glands, strainer and drier clearing, correct charging and air purging with rooms proved cooling.
Surveyor follow-up: high discharge with low suction and frost at the regulator — undercharge, overcharge or moisture first?
Full theory: Refrigeration Principles and the Vapour Compression Cycle -
10 How cooling is decided in evaporator?
Answer
The evaporator decides by how much refrigerant it is fed: more feed at the same pressure means a lower temperature and more cooling.
- The expansion valve meters flow to match the load, so the superheat at the evaporator outlet stays inside limits.
- Too little refrigerant and the coil runs dry, superheat climbs and the box loses its temperature.
- Too much and the coil floods, liquid refrigerant reaches the compressor and the cooling falls away.
- Evaporator pressure is the temperature dial — hold the evaporator pressure and the temperature follows it.
Surveyor follow-up: box warm with the coil clear and the superheat climbing — is the valve, the charge or the load?
Full theory: Condensers, Evaporators and Heat Transfer -
11 How meat room temperature is maintained at -15°C while your dairy room temperature is +4°C, by the same refrigerant
Answer
One refrigerant, two evaporators: the cold room runs at low evaporator pressure and the dairy at a higher one, each with its own circuit.
- The meat room evaporator operates at a low saturation temperature, so the same refrigerant boils there at −15°C.
- The dairy room runs at a higher evaporator pressure, so the same refrigerant boils there at about +4°C.
- Separate expansion valves feed each, and capacity control matches the compressor to whichever load is heavier.
- A direct-expansion chiller with two temperature rooms, or a booster and secondary circuit, does it either way.
Surveyor follow-up: dairy at +4°C but the meat room warming with both valves proved — is the valve, the charge or the capacity?
Full theory: Condensers, Evaporators and Heat Transfer -
12 Refrigeration system leak test
Also asked: “Refrigerant leak test”
Answer
A refrigeration system is proved by pressure with a dry charge or by holding vacuum: either way, by the test, not by a good feeling.
- Pressure test with dry nitrogen or a refrigerant charge holds the design pressure and shows where the gas goes.
- Soap solution on joints, valve stems and the compressor shell finds the small leaks a gauge cannot see.
- A vacuum test on the charging side proves the system holds vacuum — a system that will not hold vacuum will not hold charge.
- Find and fix the leak before charging; a lost charge is a symptom, and a system charged with the leak still in it is empty in a week.
Surveyor follow-up: system holding vacuum overnight with the compressor on the board — is the compressor, the drier or the suction line?
Full theory: Refrigeration Principles and the Vapour Compression Cycle -
13 Refrigeration unloader working
Answer
The unloader is capacity control: it holds the suction valve open or lifts it so a single-stage plant stops short of full pressure.
- On/off control cuts the compressor motor when the load is absent — the crudest form of capacity control.
- Variable motor speed, cylinder unloading and hot gas bypass are the other methods of matching capacity to load.
- An unloader that sticks open lets refrigerant back into the cylinder, so the box never gets cold with the compressor running.
- Prove it on the schedule: the unit must hold pressure at low load without the low-pressure cut-out tripping.
Surveyor follow-up: cold box running but the compressor tripping on low pressure — the unloader, the charge or the cut-out?
Full theory: The Refrigeration Compressor -
14 Refrigerator short cycling
Answer
Short cycling means the compressor cannot hold the box: low charge, a faulty cut-out, a clogged valve or a flooded evaporator.
- Low refrigerant charge, a choked expansion valve and frost or ice on the evaporator are the common causes.
- A faulty low-pressure cut-out or a low controller setting also cycles it, as does a high condenser pressure.
- A leaking master solenoid during the off cycle lets refrigerant through and cuts the unit in and out.
- Watch the pressures and the sight glass — a receiver low with big bubbles is a charge problem, not a control problem.
Surveyor follow-up: cycling every few minutes with the receiver full and the condenser clean — is the cut-out, the solenoid or the valve?
Full theory: Refrigeration Principles and the Vapour Compression Cycle -
15 Sketch and Describe and explain the operation of the domestic refrigeration system of a ship using freon 22 gas as refrigerant
Also asked: “Sketch, describe and explain the operation of the domestic refrigeration system of a 4 ship using R 22 gas as refrigerant. What kind of maintenance is essential for the system”
Answer
The vapour-compression cycle in four: compressor, condenser, expansion valve and evaporator, with R22 charged between them.
- The compressor raises the pressure of the vapour, and with it the saturation temperature, above the sea water or air cooling the condenser.
- At the condenser the hot refrigerant rejects heat and condenses to liquid at high pressure.
- The filter drier and liquid line pass it to the expansion valve, which drops its pressure and temperature to the evaporator level.
- At the evaporator the refrigerant boils and absorbs the heat from the space, and the low-pressure vapour returns to the compressor.
Surveyor follow-up: box warm with the compressor running and condenser gauges high — is it the charge, the condenser or the load?
Full theory: Refrigeration Principles and the Vapour Compression Cycle -
16 What is between reciever and expansion valve in domestic refrigerator?
Answer
The filter drier sits between the receiver and the expansion valve — and the sight glass, and often a solenoid, share that line.
- The filter drier traps moisture, acid and swarf before they reach the expansion valve or the small suction pipe.
- The sight glass on the liquid line shows whether the refrigerant is liquid or flashing to vapour.
- The liquid-line solenoid valve isolates the coil for defrost or shutdown.
- A blocked drier is a starved system with ice at the valve — so it is proved on every charge and changed on the schedule.
Surveyor follow-up: ice at the expansion valve with the drier new and the sight glass flashing — moisture, where is it coming from?
Full theory: Refrigeration Principles and the Vapour Compression Cycle -
17 how do you charge refrigerant
Answer
Charge by liquid into the system before the filter drier, and never past the evaporator, or liquid refrigerant damages the compressor.
- Charge liquid refrigerant into the system before the filter drier — never to the evaporator outlet.
- Liquid entering the evaporator will migrate towards the compressor and damage it, which is the rule being observed.
- Liquid charging is preferred because the charge stays liquid at any temperature and leakage losses are lower.
- For gas charging, a tee valve block connects the charging cylinder, vacuum pump and charging point, and the system is evacuated first.
Surveyor follow-up: charge cylinder connected to the evaporator by mistake and the compressor running — is the compressor the casualty?
Full theory: Refrigeration Principles and the Vapour Compression Cycle -
18 in refrigeration effect over a comparatively short period of time ;(4) " "
Answer
A refrigeration system that works well for a short time and then drifts is losing charge or capacity — the symptom points at a slow leak.
- Short-term performance that decays points to a refrigerant loss — a slow leak, a worn seal or a failed drier.
- Capacity that falls with a steady charge points to fouling, a short-circuited condenser or a failed control.
- A charge that is correct but the effect poor is a control fault — a drifting expansion valve superheat setting or a thermostat hunting.
- So trend suction pressure, discharge temperature and superheat on the same load: the drifting figure names the fault.
Surveyor follow-up: box cooling normally then failing after two hours with the charge proved — leak, control or fouling?
Full theory: Refrigeration Principles and the Vapour Compression Cycle -
19 refrigeration cycle explain
Answer
Four steps repeat for ever: compress the vapour, condense it, expand it, and boil it to take the heat out of the space.
- Compression raises the vapour pressure and its saturation temperature so it can be condensed against cooling water or air.
- Condensation rejects that heat and delivers liquid refrigerant at high pressure into the liquid line and receiver.
- Expansion through the valve drops the pressure sharply, and the flash of vapour that results flashes off as latent heat.
- Evaporation in the coil boils the remaining liquid and absorbs the load, and the low-pressure vapour goes back to the compressor.
Surveyor follow-up: compressor short-cycling with a steady charge — is it the load, the expansion valve or the control?
Full theory: Refrigeration Principles and the Vapour Compression Cycle
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