Refrigeration Principles and the Shipboard R-12 Plant

What the rule requires

Refrigeration is the removal of heat from a space, object or material and the maintenance of it below the temperature of the surrounding atmosphere. Heat on its own flows in one direction only, warmer to colder, so mechanical refrigeration is the artificial apparatus that transfers heat from one substance to another against that natural direction. 46 CFR §12.505 lists refrigeration principles and refrigeration systems among the subjects an oiler must prove knowledge of, and the examination is conducted only in English. NAVEDTRA 14104 Ch. 10 — Fundamentals of Refrigeration

The cycle works on latent heat: a liquid absorbs a large quantity of heat when it boils and surrenders that heat again when it condenses. Controlling pressure is what makes the same refrigerant boil cold inside the box and condense hot at the condenser. Four components in a loop do it — compressor, condenser, metering (expansion) device, evaporator. Low-pressure liquid boils in the evaporator and absorbs heat from the space; the compressor draws that low-pressure vapor and compresses it, raising its 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, which chills it and lets it boil at low temperature again. NAVEDTRA 14075 §6-1

Fix the two boundary points and the rest of the lesson has somewhere to attach. The compressor discharge is where the low side ends and the high side begins. The metering device is where the high side ends and the low side begins. Nothing else in the plant is a boundary.

The components and what each answers to

Evaporator — a coil of copper, aluminum or aluminum alloy tubing in the refrigerated space. Liquid R-12 enters at reduced pressure, therefore with a lowered boiling point, and heat flowing in from the surrounding air boils it off. Once entirely vaporized the refrigerant keeps absorbing heat until it is superheated by roughly 10°F; a range of 4° to 12°F of superheat is desirable, because it raises plant efficiency, evaporates all of the liquid, and prevents liquid carry-over into the compressor. NAVEDTRA 14104 Ch. 10 — Main Parts of the R-12 System ¶1
Thermostatic expansion valve (TXV) — meters liquid to the cooling coil according to the temperature of the space. The thermal control bulb is clamped to the coil near the outlet; when bulb temperature rises, the charge expands and transmits pressure that drives the diaphragm downward, opening the valve and admitting more refrigerant. When bulb temperature falls, pressure above the diaphragm decreases and the valve tends to close. The amount of superheat is set by the spring adjustment.
Compressor — essentially a pump, used to pump heat uphill from the cold side to the hot side. Because the available cooling medium is relatively warm, the only way to make the vapor condense is to compress it: raising pressure raises the condensing temperature, which is what allows seawater to be used in the condenser. It also keeps the refrigerant circulating and maintains the pressure difference between the two sides. Smaller machines use splash lubrication, which depends on a fairly high crankcase oil level; high-speed or large-capacity machines use pressure lubrication.
Condenser — the high-pressure, high-temperature discharge vapor gives up its superheat, that is its sensible heat, to the seawater, drops to the condensing point and condenses; the liquid is then subcooled slightly below its condensing point at the existing pressure to ensure it will not flash into vapor. Circulating water comes from a branch off the fire main or from an individual pump taking suction from the sea. The purge connection is on the refrigerant side, for removing air and other noncondensable gases that are lighter than R-12 vapor. Small units may instead be air-cooled, using externally finned tubing, usually with fans for positive air circulation. NAVEDTRA 14104 Ch. 10 — Main Parts of the R-12 System ¶2
Receiver — temporary storage space and surge tank for liquid refrigerant, and a vapor seal to keep vapor out of the liquid line to the expansion valve. Built for either horizontal or vertical installation.

The capacity control system is the sixth primary component in that manual's list, and it is oil-operated. It unloads, or cuts out of operation, cylinders following decreases in the refrigerant load, a cylinder being unloaded by a mechanism that holds the suction valve open so no gas can be compressed. Since oil pressure is required to load a cylinder, the compressor starts with all controlled cylinders unloaded, and they become operative once it comes up to speed and full oil pressure develops. Unloading reduces power consumption. Where numerous cooling coils are served by one compressor, the system keeps suction pressure from dropping to the low-pressure cutout setting, so the compressor is not stopped before all solenoid valves have closed.

Its working fluid is compressor oil pump pressure, metered into the system through an orifice, after which it becomes control oil and does work. The capacity control valve senses crankcase (suction) pressure and meters the oil bleed from the control oil side of the hydraulic relay back to the crankcase, which moves the relay piston and admits or cuts off oil to each unloader power element in steps.

Watch the component count on a question that asks you to list the primary components. One manual gives five — thermostatic expansion valve, evaporator, compressor, condenser, receiver NAVEDTRA 14075 Ch. 6 — Refrigeration and Air Conditioning — and the other gives six, the extra one being the capacity control system. Everything else is completion equipment: piping, pressure gauges, thermometers, control switches and control valves, strainers, relief valves, sight-flow indicators, dehydrators and charging connections.

The space warms up. Does the TXV open or close, and what senses the change?

It opens. The thermal bulb clamped to the cooling coil near the outlet warms, its charge expands, the transmitted pressure drives the diaphragm downward, and the valve admits more refrigerant to the coil. Temperature near the evaporator outlet is what controls the TXV, not the pressure in the box and not the compressor.

On the regulatory side, refrigeration machinery is acceptable for installation where design, material and fabrication comply with the applicable ABS Marine Vessel Rules, with minimum design pressures for all components taken from the piping table in ASME B31.5, and pressure vessels designed under 46 CFR part 54. Two hard limits sit inside that paragraph and both are exam-able: no pressure component may be designed for a pressure less than the setting of the system's safety devices, and for any system other than cargo reliquefaction, only the refrigerants listed in 46 CFR §147.90 are allowed. 46 CFR §58.20-5

Telling it apart

Three devices react to conditions in this plant and candidates blur them. The separating criterion is the signal each one senses and the single output it has.

Thermostatic expansion valve — senses temperature at the evaporator outlet through its bulb, and its output is refrigerant flow into the coil. Most often misfiled: cylinders cutting in and out while box temperature holds steady gets blamed on the valve, when suction pressure and the capacity control valve govern that. NAVEDTRA 14104 Ch. 10 — Main Parts of the R-12 System ¶2
Capacity control valve — senses crankcase (suction) pressure, and its output is loading and unloading of controlled cylinders in succession. It never stops the machine.
Low-pressure cutout switch — also senses suction pressure, but its output is stopping the compressor. Misfiled: credited with unloading. Unloading is what keeps suction pressure away from its setting in the first place. NAVEDTRA 14104 Ch. 10 — Main Parts of the R-12 System ¶1

Superheat and subcooling are the other pair worth separating, and each belongs to one side. Superheat is picked up on the low side, in the evaporator, after the liquid is entirely vaporized. Subcooling happens on the high side, in the condenser, after the vapor has condensed, and its purpose is to keep the liquid from flashing to vapor at the existing pressure.

Working a question

Your ship's service refrigeration plant is carrying a normal heat load and you have confirmed a sufficient charge. Compared with the figures logged when the unit was in good condition, the difference between the temperature corresponding to condensing pressure and the temperature of the outlet circulating water has widened. What is indicated, and what do you do?

Test the premises before the numbers. The comparison only points at the condenser water side if the plant is under a normal heat load and adequately charged. Fail either premise and the widened difference is telling you about the load or the charge instead. NAVEDTRA 14075 Ch. 9 — Engine Maintenance ¶13
Read condensing pressure off the high-side gauge. Gauges and thermometers are standard fittings on the plant for exactly this purpose. NAVEDTRA 14104 Ch. 10 — Main Parts of the R-12 System ¶1
Convert that pressure to its corresponding temperature. That is the temperature the refrigerant is actually condensing at inside the shell, and pressure is your only access to it.
Read the circulating water leaving the condenser — the outlet, not the inlet. Outlet water is what the refrigerant has finished transferring heat into; a fouled tube wall shows up as refrigerant sitting hotter than water that has already been warmed by it.
Subtract and compare to your own good-condition baseline. The band of degrees that the exam wants is a lookup; the skill the question is testing is knowing which two temperatures make the difference and in which direction it moves.
Clean the water side of the condenser. On an air-cooled unit the analogous fault is dirty exterior tube and fin surfaces restricting air circulation, and the treatment is on the air side, not inside the tubes.

The reasoning generalises. Any question that hands you a pressure asks you first which side of the plant that pressure lives on, then what the pressure implies about the temperature of the refrigerant there, and only then what component could have put it there.

Where candidates lose the point

Reads the bulb backwards and answers that a warming space closes the valve. The attraction is the word "thermostatic" and an expectation of a protective response. Bulb temperature up means diaphragm down means valve open, admitting more refrigerant to a coil that now has more heat to absorb. NAVEDTRA 14104 Ch. 10 — Main Parts of the R-12 System ¶1

Uses phase as the test for which side they are on: liquid must be low side, vapor must be high side. It costs marks on any question about the liquid line or the receiver, both of which are high side. The high side runs compressor discharge to metering device regardless of what state the refrigerant is in along the way. NAVEDTRA 14075 §6-1

Treats liquid flooding back to the compressor as a starved-coil symptom, and so picks a valve stuck shut or adjusted for too much superheat. It is the opposite: too little superheat means liquid is still present at the coil outlet, and the superheat range exists precisely because it evaporates all the liquid and prevents carry-over into the compressor. NAVEDTRA 14075 Ch. 9 — Engine Maintenance ¶13

Answers that the compressor starts with all cylinders working. Oil pressure loads a cylinder, and there is no oil pressure at the instant of starting, so it starts with all controlled cylinders unloaded and loads them once it is up to speed.

Purges the water side of the condenser. Air and noncondensables collect in the refrigerant space, being lighter than R-12 vapor, and the purge connection is fitted on the refrigerant side. NAVEDTRA 14104 Ch. 10 — Main Parts of the R-12 System ¶2

Assumes that compliance with the ABS rules settles refrigerant choice and design pressure. Refrigerant selection outside cargo reliquefaction is closed to the list in 46 CFR §147.90, and no pressure component may be designed below the setting of the system's safety devices. 46 CFR §58.20-5

Check yourself

You are asked where the low side of the plant begins. What is your answer, and what is the trap in the distractors?

At the metering device. The low side continues through the evaporator to the compressor suction. Distractors usually offer the receiver or the condenser outlet, which are on the high side, and they work on anyone who is sorting by temperature or by phase rather than by the two boundary points. NAVEDTRA 14075 §6-1

The plant is holding temperature but the compressor suction line is sweating and the crankcase is carrying liquid. Which adjustment do you look at first?

The TXV spring adjustment, which sets superheat. Superheat below the desirable 4° to 12°F band leaves unevaporated liquid at the coil outlet, and that liquid is carried over to the compressor. NAVEDTRA 14104 Ch. 10 — Main Parts of the R-12 System ¶1

Why does raising the pressure of the vapor allow seawater to condense it?

Raising pressure raises temperature, and with it the condensing temperature of the refrigerant. Once the condensing temperature is above that of the available cooling medium, seawater will take the heat. That is the compressor's primary function, along with circulating the refrigerant and maintaining the pressure difference between the sides.

A question asks what the receiver does. Two options are "stores liquid refrigerant" and "keeps vapor out of the liquid line". Which is correct?

Both are functions of the receiver: it is a temporary storage space and surge tank for liquid refrigerant, and it acts as a vapor seal keeping vapor out of the liquid line to the expansion valve. If the exam offers a combined option, take it. NAVEDTRA 14104 Ch. 10 — Main Parts of the R-12 System ¶2

Suction pressure falls away after the pulldown period and cylinders begin cutting out. Is the plant faulted?

No. Falling suction pressure after pulldown is a falling refrigerant load, and the capacity control valve responds by increasing the control oil bleed to the crankcase, dropping hydraulic relay pressure so the relay piston closes oil ports in succession. As oil pressure leaves a power element the suction valve rises and that cylinder unloads. Power consumption drops with it.

A vessel's engineer proposes charging the ship's refrigeration plant with a refrigerant not listed in 46 CFR §147.90, arguing the machinery meets the ABS rules. Correct?

No. For any refrigeration system other than one for reliquefaction of cargo, only the refrigerants under 46 CFR §147.90 are allowed. Compliance of design, material and fabrication with the ABS Marine Vessel Rules is a separate requirement and does not open the refrigerant list. 46 CFR §58.20-5