Refrigeration Cycle and Components — Reading the Plant From Its Two Pressures

What the rule requires

The loop and the line that divides it

Refrigeration moves heat from a cold space to a warmer one, against its natural direction, by evaporating and condensing a refrigerant in a closed cycle. It works because a liquid absorbs a large amount of heat — its latent heat — when it boils and gives that heat up again when it condenses; controlling the pressure makes the refrigerant boil at a low temperature in the space being cooled and condense at a higher temperature where the heat is rejected NAVEDTRA 14075 §6-1.

Four components connected in a loop: compressor, condenser, metering (expansion) device, evaporator. Low-pressure liquid boils in the evaporator inside the refrigerated space and leaves as low-pressure vapor. The compressor draws that vapor in and compresses it to high pressure, raising its temperature. The hot high-pressure vapor goes to the condenser, where seawater or air cools it enough to condense it back to high-pressure liquid. The liquid passes through the metering device, which throttles it to low pressure; the pressure drop chills it and lets it boil at low temperature again in the evaporator.

Two components form the boundary. The high side runs from compressor discharge through the condenser to the metering device; the low side runs from the metering device through the evaporator to the compressor suction. That line is what makes troubleshooting systematic rather than guesswork, and exam questions about where a component or a symptom belongs are answered from it directly.

The receiver — high side or low side?

High side. Liquid drains from the bottom of the condenser into the receiver, and the liquid line runs on from the receiver to the metering device NAVEDTRA 14075 §6-3. Since the metering device is the boundary, everything upstream of it — condenser, receiver, filter-drier, sight glass, liquid line — is high side.

Compressor: dry vapor only

The compressor draws low-pressure vapor from the evaporator and compresses it to a pressure high enough that the refrigerant will condense at the available cooling-water or air temperature, keeping the refrigerant circulating NAVEDTRA 14075 §6-2. Most marine plants use reciprocating machines: pistons draw vapor in through suction (reed) valves on the down-stroke and force it out through discharge valves on the up-stroke. They may be open, driven by an external motor through a coupling or belt with a shaft seal, or hermetic/semi-hermetic with motor and compressor sealed in one housing. Larger air-conditioning plants often use rotary screw or centrifugal compressors for smooth, high-capacity, continuous compression.

A compressor must receive dry vapor. Liquid returning to the suction — slugging, or liquid floodback — cannot be compressed and can break valves, rods, or the crankcase, so the system is arranged and controlled to ensure the refrigerant is fully evaporated and slightly superheated before it reaches the suction.

Three protective devices are examinable and easily confused: a high-pressure cutout stops the machine if discharge pressure rises dangerously, from a dirty condenser, air in the system, or lost cooling water; a low-pressure cutout stops it if suction pressure falls too low, from loss of charge or a starved evaporator; an oil-pressure safety switch protects the bearings. Capacity is matched to load by cycling on the pressure controls, unloading cylinders, or varying speed.

Condenser and receiver: where head pressure is set

The condenser rejects the heat absorbed in the evaporator plus the heat added by the compressor, condensing high-pressure vapor back to liquid . Marine plants commonly use water-cooled shell-and-tube condensers: refrigerant vapor fills the shell around a tube bundle carrying seawater, condenses on the outside of the tubes, and drains to the bottom of the shell. Smaller and air-conditioning units may use air-cooled condensers with a fan over finned coils.

Condenser performance directly controls the high-side pressure. Fouled or scaled tubes, warm or reduced cooling water, or non-condensable air trapped in the shell all raise head pressure and discharge temperature, the compressor works harder, capacity falls, and the high-pressure cutout may trip.

The receiver holds the liquid charge, supplies liquid steadily to the metering device regardless of load swings, and gives somewhere to pump the whole charge down into for servicing. The liquid line runs from receiver to metering device through a filter-drier that removes moisture and particles and a sight glass showing whether the liquid is solid or full of bubbles. Air that leaks in collects at the top of the condenser and receiver, raising head pressure without producing any cooling, and is removed by purging.

Metering device: superheat is the setpoint

The metering device separates high side from low side and controls how much liquid enters the evaporator NAVEDTRA 14075 §6-4. The thermostatic expansion valve (TXV) is the common marine control: a sensing bulb on the evaporator outlet reads the temperature of the leaving vapor and modulates the valve to hold a constant superheat, feeding more when the load is high and the outlet warm, less when the load falls. Holding superheat keeps the coil fully used while ensuring only vapor reaches the compressor.

Know the alternatives by what each holds constant. The automatic (constant-pressure) expansion valve holds a set evaporator pressure. The capillary tube is a fixed orifice used in small self-contained units. A float valve or level control keeps a flooded evaporator filled to a set level. Solenoid liquid-line valves, operated by a thermostat, start and stop flow to a coil and control a box by pumping down.

Feed too much and liquid floods back to the compressor; feed too little and the evaporator is starved, capacity drops and suction pressure falls with it.

Evaporator: where the useful work happens

Low-pressure liquid boils inside the coil and absorbs heat from the air, water, or brine around it, leaving as slightly superheated low-pressure vapor bound for the compressor suction NAVEDTRA 14075 §6-5. In a dry-expansion evaporator the metering device feeds just enough refrigerant to be fully evaporated by the coil outlet. In a flooded evaporator the coil is kept filled with liquid to a level set by a float control, giving very effective heat transfer, with vapor separated off the top.

Air-cooling coils are finned and usually fan-forced. Any coil running below freezing accumulates frost, and the frost layer insulates the coil and blocks airflow, cutting capacity steadily — hence periodic defrost by stopping the system, by electric or hot-gas heat, or by water. An air-conditioning coil runs above freezing and condenses moisture out of the air as water, which is drained away; that is the dehumidifying effect comfort cooling depends on.

Refrigerants, moisture, and handling

Older plants used R-12 and R-22, and some large refrigeration used ammonia (R-717); the ozone-depleting fluorocarbons have been phased out in favour of replacement blends, and the operating principles are unchanged NAVEDTRA 14075 §6-6. Ammonia is thermodynamically excellent but toxic and flammable, with a sharp warning odor.

Moisture is the enemy of any charge: water freezes at the metering orifice and reacts with oil and refrigerant to form acids and sludge. That is why filter-driers are fitted and why a system opened for repair is evacuated to a deep vacuum to boil off moisture before recharging.

Vapor can displace oxygen and suffocate in a confined space, and liquid causes severe frostbite; ventilate before entry, and detect leaks with electronic detectors, or by odor and chemical indicators for ammonia. Venting to the atmosphere is prohibited — refrigerant is recovered into cylinders for reuse or proper disposal. Cylinders are stored upright, secured, cool, never overfilled, never subjected to a flame. Charge level is judged by the sight glass, the operating pressures, and the superheat.

Air conditioning in one pass

Shipboard air conditioning is the same vapor-compression cycle applied to cooling and dehumidifying air NAVEDTRA 14075 §6-7. Refrigerant either chills air directly at a direct-expansion coil in an air handler, or chills water that is pumped to coils throughout the ship — a chilled-water system, favoured on larger vessels because cooling is distributed in water piping instead of long refrigerant lines. Thermostats control cooling by cycling the compressor, modulating chilled-water flow through a control valve, or staging capacity, while dampers set air volume and the recirculated/fresh-air mix. Dirty filters and coils choke airflow, and a blocked condensate drain overflows and causes water damage and mold.

Telling it apart

Every fault below is diagnosed from the same pair of readings. The criterion that separates them is which gauge is abnormal and in which direction, read together with the sight glass, the frost pattern, and what the compressor is doing NAVEDTRA 14075 §6-8.

Shortage of refrigerant (undercharge or leak) — suction low and head low, bubbles in the sight glass, poor cooling, short-cycling on the low-pressure cutout, frost forming only part way along the evaporator. Most often misfiled as a restriction, because both show bubbles and low suction.
Restricted liquid line — plugged filter-drier or moisture frozen at the expansion valve — suction low, poor cooling, and the giveaway: a temperature drop and frost at the restriction itself, often with the compressor running continuously instead of cycling.
Overcharge, or air (non-condensables) in the system — head pressure high and discharge temperature high, compressor overworked, possibly tripping the high-pressure cutout.
Dirty or scaled condenser, warm or reduced cooling water, fouled air-cooled coil — the same high head, checked by comparing condensing temperature against cooling-water temperature.
Expansion valve stuck open, or a bulb that has lost its charge — the evaporator floods and liquid returns to the compressor: frosted suction line right back to the compressor, slugging, valve damage.
Failed compressor with broken valves — suction will not pull down and head will not build, with little cooling.
Airflow fault — dirty coil, heavy frost, stopped fan — pressures normal, cooling lost. Nothing on the gauges will find this one.

Working a question

The ship's stores freeze box is climbing. Suction pressure reads low, head pressure reads low, the sight glass is full of bubbles, frost extends only part way along the coil, and the compressor keeps cutting out and restarting.

Read both gauges and locate the side. Both low, so the trouble is a low-side or charge problem; a high-side fault would show itself as high head.
Clear the condenser from suspicion. A fouled condenser, warm cooling water, or air in the shell raises head pressure. Head is low, so none of those explains this.
Separate shortage from restriction. Both give low suction and bubbles. A restriction announces itself with a localised temperature drop and frost at the drier or valve; find no local drop, and it is not a restriction.
Read the compressor's behaviour. Short-cycling on the low-pressure cutout fits loss of charge. A restriction typically leaves the compressor running continuously.
Name the single component whose fault explains every symptom before opening the system. Low suction, low head, bubbles, partial frost, and short-cycling are all one fault: the system is short of refrigerant.
Act on the cure, not the symptom. Find and repair the leak, then recharge — detecting with an electronic detector, ventilating the space before entry, recovering into cylinders rather than venting, evacuating to a deep vacuum to boil off moisture, and charging to the correct amount judged by sight glass, pressures, and superheat.

Distractors on this question type are usually "add refrigerant" or "adjust the expansion valve." Topping up a leaking system is not the required action.

Where candidates lose the point

Answering that the condenser rejects the heat picked up in the refrigerated space. It rejects that heat plus the heat of compression, which is why the discharge side runs hot and why condenser fouling drives head pressure up.
Choosing "low refrigerant charge" for a high head pressure. Undercharge drops both pressures. High head with high discharge temperature points to overcharge, non-condensables, a dirty condenser, or lost cooling water.
Saying the TXV holds a set evaporator pressure. That is the automatic (constant-pressure) expansion valve. The TXV senses the temperature of vapor leaving the coil at the bulb and holds constant superheat.
Reading "flooded evaporator" as a casualty. A flooded evaporator is a design, filled to a level by a float control for better heat transfer. Flooding the compressor is the casualty.
Treating bubbles in the sight glass as proof of undercharge. A restriction produces bubbles as well; the frost location and whether the compressor short-cycles are what settle it.
Adding refrigerant when cooling is lost but both pressures are normal. Normal pressures with poor cooling is an airflow problem — dirty coil, heavy frost, or a stopped fan.
Pumping the charge down and blowing it off before a repair. Venting to the atmosphere is prohibited; the charge is recovered into cylinders, or pumped down into the receiver.

Check yourself

Your R-22 chill-box plant trips repeatedly on the high-pressure cutout. Seawater inlet temperature is normal and the charge was checked last week. What do you suspect first?

Fouled or scaled condenser tubes, reduced seawater flow, or non-condensable air trapped in the top of the condenser and receiver. Any of them stops the condenser rejecting heat, raising head pressure and discharge temperature until the cutout trips. Compare condensing temperature against cooling-water temperature; if air is present, purge NAVEDTRA 14075 §6-3, NAVEDTRA 14075 §6-8.

The suction line is frosted all the way back to the compressor and you hear knocking on start-up. What is happening, and what is at risk?

The evaporator is flooding and liquid refrigerant is returning to the suction — an expansion valve stuck open or a sensing bulb that has lost its charge. Liquid cannot be compressed; slugging can break valves, rods, or the crankcase , NAVEDTRA 14075 §6-2.

Suction pressure is low, cooling is poor, the compressor runs continuously, and you find a cold spot with frost on the filter-drier. Shortage or restriction?

Restriction. A plugged filter-drier or moisture frozen at the valve orifice starves the evaporator, and the localised temperature drop and frost at the restriction is the tell. A shortage would also pull head pressure down and short-cycle the compressor on the low-pressure cutout .

Why must the refrigerant leaving the evaporator be slightly superheated?

So that only dry vapor reaches the compressor suction. The TXV holds constant superheat precisely to use the whole coil while keeping liquid out of the compressor NAVEDTRA 14075 §6-4, NAVEDTRA 14075 §6-5.

A freeze-room coil has lost capacity. Both pressures are normal and the charge is correct. What do you check?

Airflow over the coil — heavy frost buildup, a dirty coil, or a stopped fan. Frost insulates the coil and blocks airflow, so the coil is defrosted on schedule by stopping the system, by electric or hot-gas heat, or by water , .

You have opened a system for a compressor valve repair. What is done before the charge goes back in, and why?

Evacuate to a deep vacuum to boil off all moisture. Water in the system freezes at the metering orifice and reacts with oil and refrigerant to form acids and sludge; the filter-drier is the running defence, evacuation is the one after the system has been opened NAVEDTRA 14075 §6-6.

Air-conditioning suction pressure is low and the spaces are warm. Which two faults from the refrigeration side transfer directly?

Low refrigerant charge dropping suction pressure, and a fouled condenser raising head pressure — the same faults and the same high- and low-pressure cutouts protect the compressor in an air-conditioning plant. Add the air-side items: dirty filters and coils choking airflow, and chilled-water flow NAVEDTRA 14075 §6-7.