Air Conditioning, Ventilation, and Compressed Air

What the rule requires

The high side / low side split decides the fault questions

The dividing line runs from the compressor discharge through the condenser to the metering device (high side), and from the metering device through the evaporator to the compressor suction (low side); that line is fundamental to troubleshooting any system NAVEDTRA 14075 §6-1. Low-pressure liquid boils in the evaporator, absorbing heat from the space; the compressor draws that low-pressure vapour and raises it to high pressure and temperature; the condenser rejects to seawater or air both the heat picked up in the space and the heat added by compression; the metering device throttles the high-pressure liquid back down so it will boil cold again.

Read the gauge, name the side, then go to the charted causes NAVEDTRA 14075 Ch. 6 — Correcting Troubles:

High condensing pressure — air or non-condensable gas in the system (purge the condenser); condenser inlet water warm, or insufficient water flowing through the condenser (increase the quantity of condensing water); condenser tubes clogged or scaled (clean the water tubes); too much liquid in the receiver with the condenser tubes submerged in liquid refrigerant (draw the liquid off into a service cylinder).
Low condensing pressure — too much water flowing through the condenser, or water too cold (reduce the quantity of water); liquid refrigerant flooding back from the evaporator (change the expansion valve adjustment and examine the fastening of the thermal bulb); a leaky discharge valve (remove the head, examine the valves, replace any found defective).
High suction pressure — overfeeding of the expansion valve (regulate the valve, check the bulb attachment); a leaky suction valve (remove the head, examine and replace if worn).
Low suction pressure — restricted liquid line, expansion valve or suction screens (pump down, remove, examine and clean the screens); insufficient refrigerant in the system; too much oil circulating; improper expansion valve adjustment (adjust to give more flow); an expansion valve power element dead or weak (replace the power element).

Note that only one of the five charted causes of low suction pressure is a shortage of charge. The chart also opens with the instruction to consult the manufacturer's technical manual before any repair or adjustment.

The compressor's behaviour is charted separately from the gauge readings, and the four behaviours do not share causes:

Short cycles on the low pressure control — low refrigerant charge; thermal expansion valve not feeding (dirty strainers, moisture frozen in the orifice or the orifice plugged with dirt, dead or weak power element); water flow through the evaporators restricted or stopped, or evaporator coils plugged, dirty or clogged with frost; a defective low pressure control switch.
Runs continuously — shortage of refrigerant (repair the leak and recharge); leaking discharge valves (replace them).
Short cycles on the high pressure control switch — insufficient water flowing through the condenser or a clogged condenser (determine whether the water has been turned off, check for a scaled or fouled condenser); a defective high pressure control switch.
Will not run — seized compressor; cut-in point of the low pressure control switch set too high (reset it to cut in at the correct pressure); high pressure control switch does not cut in (check discharge pressure and reset it); or the electrical side — defective switch, power cut off, disconnect open, fuses blown, overload relays tripped, low voltage (should be within 10 percent of nameplate rating), motor trouble, control circuit trouble, or the motor stopped by the oil pressure differential switch.

Two more that get asked as symptoms rather than as pressures: sudden loss of oil from the crankcase comes from liquid refrigerant slugging back to the crankcase, corrected at the expansion valve; and a compressor that continues to operate unloaded points to a pressure regulating valve that is not closing, while one that continues at full or partial load points to a regulating valve not opening.

Your plant shows high condensing pressure and normal suction pressure. A shipmate suggests the charge is low. Is he right?

No. A shortage of charge is a low-side symptom — it shows as low suction pressure, and where it drives compressor behaviour it makes the machine run continuously or short cycle on the low pressure control. High condensing pressure with normal suction is a heat-rejection problem: non-condensables, warm or insufficient condensing water, scaled tubes, or liquid backed up in the receiver submerging the condenser tubes.

Air conditioning is the same cycle, applied to air

Shipboard air conditioning uses the vapour-compression cycle to cool and dehumidify air, either with a direct-expansion coil in an air handler or with chilled water pumped to coils throughout the ship — the chilled-water arrangement is favoured on larger vessels because cooling is distributed in water piping rather than long refrigerant lines NAVEDTRA 14075 §6-7. Air handlers take a mixture of recirculated and fresh outside air across the coil, where it is cooled and dehumidified, and deliver it through ducts; the condensed moisture is collected and drained. Heating coils, filters and fans in the same handlers condition the air in all seasons.

Control holds temperature and humidity together. Thermostats sense space temperature and act on the cooling by cycling the compressor, modulating chilled-water flow through a coil with a two-way or three-way control valve, or staging capacity, while dampers set the volume and mix of air. The coil is deliberately run cold enough to wring moisture out of the air, and the air is then delivered at a comfortable temperature.

The maintenance list is short and it is examinable: keep filters and coils clean, because dirty filters and coils choke airflow and cut capacity; keep the refrigerant charge and chilled-water flow correct; keep condensate drains clear, since a blocked drain overflows and can cause water damage and mould; and keep fresh-air ventilation adequate. The faults and the protections are the same as refrigeration — fouled condensers raise head pressure, low charge drops suction pressure, and the same high- and low-pressure cutouts protect the compressor.

Air conditioning aboard ship is not only for the crew. Mechanical cooling or ventilation is provided in ammunition spaces to prevent deterioration of ammunition components, in gas storage spaces to prevent excessive pressure buildup in containers and contamination from gas leaks, and in electrical and electronic equipment spaces to hold the ambient temperature and humidity specified for the equipment NAVEDTRA 14104 Ch. 10 — Air Conditioning.

Chilled-water plants and capacity control

Two basic types of chilled-water plant are in use: the vapour compression unit, in which the primary refrigerant (R-11 or R-114) chills the secondary refrigerant, water, which is then circulated to the cooling coils; and the lithium bromide absorption unit, which runs on the absorption cycle using water as the primary refrigerant and lithium bromide as the absorbent NAVEDTRA 14104 Ch. 10 — Systems ¶1. Vapour compression plants are used in most ships; lithium bromide plants are used in submarines because they require no compression and are therefore quieter. The absorption cycle uses heat energy in place of mechanical energy — steam heat replaces the compressor.

In the large centrifugal plant, condensed refrigerant drains into a float chamber and a float-operated valve (an orifice in some R-11 units) admits it to the water chiller, where it sprays over the tubes carrying the water to be chilled. Load is met by adjustable prerotation vanes on the compressor suction, which throttle vapour flow and so change capacity without changing compressor speed, positioned automatically by an electropneumatic control that holds chilled-water outlet temperature at a preset value. Automatic shutdowns protect against high condenser pressure, low compressor lube oil pressure, loss of seawater to the condenser, loss of chilled water, low refrigerant temperature, low chilled water temperature, and high discharge temperature. The oil sump heater is energised when the compressor is off: unheated oil absorbs large quantities of refrigerant and foams excessively on start.

The compressor most commonly met on modern ships is a high-speed, continuous-running, variable-capacity multicylinder reciprocating machine with a positive unloader system built in to control capacity NAVEDTRA 14075 Ch. 6 — Compressors.

Compressed air

Compressed air starts large diesels, operates pneumatic controls and automation, runs air tools, sounds the whistle, and blows through and clears lines — high pressure, several hundred psi, for diesel starting air, and lower pressures for control and service air NAVEDTRA 14075 §7-1. Most units are positive-displacement reciprocating machines: a piston draws air in through a suction valve and forces it out through a discharge valve into a receiver. High-pressure air is compressed in two or more stages because reaching high pressure in one step generates excessive heat; an intercooler between stages removes the heat of compression, which also improves efficiency and condenses out moisture, and an aftercooler cools the final discharge. Rotary screw compressors are widely used for control and service air.

Because compression heats the air, wrings out water vapour and carries over oil mist, the system manages heat, moisture and oil: coolers control temperature; moisture separators, traps and receiver drains remove condensed water; air driers are fitted where control air must be very dry. Draining condensate from receivers, intercoolers and separators is a routine watch duty — water carried downstream fouls pneumatic controls and, in the starting system, defeats engine starting and causes corrosion.

Two safety points carry weight out of proportion to their length. Oil mist in hot compressed air can ignite or explode, so oil carryover is minimised, discharge temperatures are held within limits, and receivers and lines carry relief valves and, on high-pressure systems, fusible plugs or bursting discs that must be kept clear and tested. And because these are positive-displacement machines, a compressor must never run against a closed discharge: each stage has its own relief valve, and receivers are sized to give a set number of engine starts before recharging.

The second-stage relief valve on a two-stage starting air compressor lifts. Is the receiver relief valve sufficient protection for that stage?

No. Each stage has its own relief valve because a positive-displacement compressor must never run against a closed discharge — a shut valve, or a stage blocked downstream, isolates that stage from the receiver and its relief valve entirely. The stage relief, the fusible plugs and bursting discs on high-pressure systems, and holding discharge temperature within limits are separate protections, and all must be kept clear and tested.

What the regulations cover

The subpart applies to fixed refrigeration systems for air conditioning, refrigerated spaces, cargo spaces, and reliquefaction of low temperature cargo installed on vessels; it does not apply to small self-contained units 46 CFR §58.20-1. The trigger is fixed and installed — a small self-contained unit is outside the subpart.

Telling it apart

What actually reaches the space

Direct expansion — refrigerant itself chills the air at a coil in the air handler; no secondary fluid NAVEDTRA 14075 §6-7.
Chilled water, vapour compression — the primary refrigerant cools a secondary refrigerant, chilled water, which is circulated to the coils; heat from the space goes into the water and is removed from the water by the refrigerant in the chiller NAVEDTRA 14104 Ch. 10 — Systems ¶1.
Chilled water, lithium bromide absorption — water is the refrigerant, lithium bromide the absorbent, and heat energy replaces the compressor.

How capacity is trimmed

Multicylinder reciprocating — a positive unloader system built into the compressor unloads and loads cylinders; unloading faults trace to the capacity control valve hand stem or the pressure regulating valve NAVEDTRA 14075 Ch. 6 — Compressors.
Centrifugal — prerotation vanes throttle suction to change capacity at constant speed.

Which cooler you are being asked about

Intercooler — between stages; removes the heat of compression, improves efficiency, condenses moisture NAVEDTRA 14075 §7-1.
Aftercooler — after the final stage; cools the discharge before the receiver.

Working a question

Your R-12 plant is short cycling on the high pressure control switch and condensing pressure is running high. What do you do?

Name the side. The high pressure cutout and condensing pressure are both high side, so the low-side causes are out of play before you read any option NAVEDTRA 14075 §6-1.
Recognise the pair. High condensing pressure together with short cycling on the high pressure switch is the charted signature of an inadequate supply of water passing through the condenser NAVEDTRA 14075 Ch. 6 — Correcting Troubles.
Check the water first. The charted corrective is to determine whether the water has been turned off, then check for a scaled or fouled condenser. A secured or throttled valve is found in seconds; scale is found by comparing overboard temperature and flow.
Work down the remaining high-side causes. Warm inlet water calls for more condensing water; clogged or scaled tubes call for cleaning; air or non-condensable gas calls for purging the condenser; liquid backed up in the receiver submerging the tubes calls for drawing liquid off into a service cylinder.
Do not touch the cutout. The switch is doing its job. It is a charted cause only when the pressure is normal and the switch is defective, in which case it is repaired or replaced, not readjusted around the trip.
Consult the manufacturer's technical manual before any repair or adjustment.

Where candidates lose the point

Answering "add refrigerant" to anything. It attracts because a shortage of charge is genuinely a cause of low suction pressure and of a compressor running continuously. It is one of five charted causes of low suction pressure, and restricted screens, excess oil in circulation and a dead expansion valve power element are the others. If the stem gives you a restriction symptom, the charge is not the answer.
Treating a tripping cutout as a switch fault. Short cycling on the high pressure switch sends you to the condenser water and the condenser tubes. Adjusting or resetting the cutout suppresses the alarm and leaves the plant against high head pressure.
Confusing "runs continuously" with "will not run." Runs continuously is charted to a shortage of refrigerant or leaking discharge valves. Will not run is charted to a seized compressor, a low pressure switch whose cut-in point is set too high, a high pressure switch that will not cut in, or the electrical side — fuses, overload relays, voltage outside 10 percent of nameplate, control circuit, or the oil pressure differential switch.
Reading a too-high low-pressure cut-in setting as short cycling. The plausible-sounding answer is that it short cycles. Suction pressure never rises to a cut-in set too high, so the switch never closes and the compressor does not run; the corrective is to set the switch to cut in at the correct pressure.
Blaming the charge for unloaded running. A compressor that keeps running unloaded is charted to a pressure regulating valve that is not closing, and one stuck at full or partial load to a regulating valve that is not opening. Check that the capacity control valve hand stem is in the automatic position before condemning anything.
Ignoring the condensate side of an air conditioning question. A blocked condensate drain overflows and causes water damage and mould, and a dirty filter or coil chokes airflow and cuts capacity. Neither shows first as a pressure fault.
Applying the CFR subpart to a small self-contained unit. It covers fixed installed systems only.

Check yourself

Condensing pressure is low and the condenser water flow and temperature are correct. The options offered are: add refrigerant, purge the condenser, increase compressor speed, adjust the thermostatic expansion valve. Which?

Adjust the thermostatic expansion valve. With water quantity and temperature ruled out, the remaining charted cause of low condensing pressure that these options reach is liquid refrigerant flooding back from the evaporator, corrected by changing the expansion valve adjustment and examining the fastening of the thermal bulb. Purging is the corrective for high condensing pressure caused by non-condensables.

You find condensing pressure low and the condenser overboard water running cold with a full flow. What is the corrective measure?

Reduce the quantity of condensing water. Too much water flowing, or water too cold, are both charted causes of low condensing pressure and both are corrected by reducing the water quantity.

Sudden loss of oil from the compressor crankcase. Cause and corrective?

Liquid refrigerant slugging back to the crankcase, corrected by adjusting or replacing the expansion valve. Treat it as a low-side metering fault, not a lubrication-system fault; the oil left the crankcase because liquid returned to it.

Standing an engine room watch, why must you drain the intercooler and receiver of the starting air compressor?

Compression wrings water vapour out of the air, and water carried downstream fouls pneumatic controls and, in the starting system, defeats engine starting and causes corrosion. Draining receivers, intercoolers and separators is a routine watch duty.

A 150-ton centrifugal chilled-water plant needs to take on more load. How is capacity increased, and what does not change?

The adjustable prerotation vanes on the compressor suction open further, increasing refrigerant vapour flow into the suction. Compressor speed does not change — the throttling action at suction is what varies capacity, and the vanes are positioned automatically to hold chilled water outlet temperature at its preset value.

Air conditioning is fitted to a magazine and to a gas storage space. Why, in each case?

Cooling or ventilation in ammunition spaces prevents deterioration of ammunition components. In gas storage spaces it prevents excessive pressure buildup in the containers and contamination of the space from gas leaks. Neither is a comfort installation.

The refrigerant charge, condenser and expansion valve all check out, but the compressor will not start and the motor is cool. Where do you look?

The electrical side and the protective devices: defective switch, power cut off, service or disconnect switch open, blown fuses, tripped overload relays, low voltage — which should be within 10 percent of nameplate rating — motor trouble, trouble in the starting switch or control circuit, or the motor stopped by the oil pressure differential switch, in which case check crankcase oil level and oil pump pressure.