Refrigeration Systems and there operations in detail with diagrams.

Refrigeration Systems

Most shipboard refrigeration systems now use R-22 (chlorodifluoromethane) or as a refrigerant. R-22 has such a low boiling point that it cannot exist as a liquid unless it is confined in a container under pressure. The cycle of operation and the main components of R-22 systems are basically the same as those in other refrigeration and air-conditioning plants. R-12 refrigerant: Chemically dichlorodifluoromethane is no longer available. Refrigeration systems using R-12 are being replaced, or converted to R-22. (See Refrigeration Transition)

In this chapter most of the discussion and drawings refer to R-12 systems. Most of the USCG exam questions also refer to R-12 systems and haven't been up dated in many years. As stated above all refrigeration systems are basically the same.

 

FUNDAMENTALS OF REFRIGERATION

Refrigeration is a general term. It describes the process of removing heat from spaces, objects, or materials and maintaining them at a temperature below that of the surrounding atmosphere. To produce a refrigeration effect, the material to be cooled needs only to be exposed to a colder object or environment. The heat will flow in its NATURAL direction-that is, from the warmer material to the colder material. Refrigeration, then, usually means an artificial way of lowering the temperature. Mechanical refrigeration is a mechanical system or apparatus that transfers heat from one substance to another.

It is easy to understand refrigeration if you know the relationships among temperature, pressure, and volume, and how pressure affects liquids and gases.

 

THE  REFRIGERATION   TON

The unit of measure for the amount of heat removed is known as the refrigeration ton. The capacity of a refrigeration unit is usually stated in refrigeration tons. The refrigeration ton is based on the cooling effect of 1 ton (2,000 pounds) of ice at 32°F melting in 24 hours. The latent heat Latent (hidden) heat when added or removed changes the state of a substance with no change in temperature or pressure. Example, the heat added to water to boil it into a gas (steam) The temperature remains at 212°F throughout the process. of fusion of ice (or water) is 144 BTU's. Therefore, the number of BTU's required to melt 1 ton of ice is 144 x 2,000= 288,000. The standard refrigeration ton is defined as the transfer of 288,000 BTU's in 24 hours. On an hourly basis, the refrigeration ton is 12,000 BTU's per hour (288,000 divided by 24).

The refrigeration ton is the standard unit of measure used to designate the heat-removal capacity of a refrigeration unit. It is not a measure of the ice-making capacity of a machine, since the amount of ice that can be made depends on the initial temperature of the water and other factors.

 

MECHANICAL REFRIGERATION SYSTEMS

Various types of refrigerating systems are used for shipboard refrigeration and air conditioning. The one usually used for refrigeration purposes is the vapor compression cycle with reciprocating compressors.

The figure left shows a general idea of this type of refrigeration cycle. As you study this system, try to understand what happens to the refrigerant as it passes through each part of the cycle. In particular, you need to understand (1) why the refrigerant changes from liquid to vapor, (2) why it changes from vapor to liquid, and (3) what happens in terms of heat because of these changes of state. In this section, the refrigerant is traced through its entire cycle, beginning with the thermostatic expansion valve ( TXV Thermostatic Expansion Valve ).

Liquid refrigerant enters the TXV that separates the high side of the system and the low side of the system. This valve regulates the amount of refrigerant that enters the cooling coil. Because of the pressure differential as the refrigerant passes through the TXV, some of the refrigerant flashes to a vapor. From the TXV, the refrigerant passes into the cooling coil (or evaporator). The boiling point of the refrigerant under the low pressure in the evaporator is about 20°F lower than the temperature of the space in which the cooling coil is installed. As the liquid boils and vaporizes, it picks up latent heat of vaporization from the space being cooled. The refrigerant continues to absorb latent heat of vaporization until all the liquid has been vaporized. By the time the refrigerant leaves the cooling coil, it has not only absorbed this latent heat of vaporization. It has also picked up some additional heat; that is, the vapor has become superheated. As a rule, the amount of superheat is 4° to 12°F.

The refrigerant leaves the evaporator as low-pressure superheated vapor. The remainder of the cycle is used to dispose of this heat and convert the refrigerant back into a liquid state so that it can again vaporize in the evaporator and absorb the heat again.

The low-pressure superheated vapor is drawn out of the evaporator by the compressor, which also keeps the refrigerant circulating through the system. In the compressor cylinders, the refrigerant is compressed from a low-pressure, low-temperature vapor to a high-pressure vapor, and its temperature rises accordingly.

The high-pressure R-12 vapor is discharged from the compressor into the condenser. Here the refrigerant condenses, giving up its superheat ( sensible heat Heat which causes a change in temperature of a substance. ) and its latent heat of condensation. The condenser may be air or water cooled. The refrigerant, still at high pressure, is now a liquid again. From the condenser, the refrigerant flows into a receiver, which serves as a storage place for the liquid refrigerant in the system. From the receiver, the refrigerant goes to the TXV and the cycle begins again.

This type of refrigeration system has two pressure sides. The LOW-PRESSURE SIDE extends from the TXV up to and including the intake side of the compressor cylinders. The HIGH-PRESSURE SIDE extends from the discharge valve of the compressor to the TXV.

 

MAIN PARTS OF A REFRIGERATION SYSTEM

The main parts of a refrigeration system are shown diagrammatically in the figure below. The six primary components of the system include the thermostatic expansion valve, evaporator, capacity control system, compressor, condenser, and receiver. All refrigeration systems must also be fitted with a relief valve. The system below shows a relief valve installed on the discharge side of the compressor which relives pressure to the suction side when the compressor discharge pressure exceeds the relief valve preset pressure.

 

Thermostatic Expansion Valve (TXV)

The TXV regulates the amount of refrigerant to the cooling coil. The amount of refrigerant needed in the coil depends, of course, on the temperature of the space being cooled.

The thermal control bulb, which controls the opening and closing of the TXV, is clamped to the cooling coil near the outlet (tail coil), and before the back pressure regulating valve if installed. The substance in the thermal bulb varies, depending on the refrigerant used. The expansion and contraction (because of temperature change) transmit a pressure to the diaphragm. This causes the diaphragm to be moved downward, opening the valve and allowing more refrigerant to enter the cooling coil. When the temperature at the control bulb falls, the pressure above the diaphragm decreases and the valve tends to close. Thus, the temperature near the evaporator outlet controls the operation of the TXV.

Flash gas formed in the liquid line of a refrigeration system due to low refrigerant may cause expansion valve pins and seats to erode. A leaking expansion valve could result in excessively low temperature to the space regulated.

 

Evaporator

The evaporator consists of a coil of copper, aluminum, or aluminum alloy tubing installed in the space to be refrigerated. Aluminum tubing with copper fins are used in ammonia systems. The liquid R-12 enters the tubing at a reduced pressure and, therefore, with a lower boiling point. As the refrigerant passes through the evaporator, the heat flowing to the coil from the surrounding air causes the rest of the liquid refrigerant to boil and vaporize. Refrigerant temperature in an evaporator is directly related to refrigerant pressure. After the refrigerant has absorbed its latent heat of vaporization (that is, after it is entirely vaporized), the refrigerant continues to absorb heat until it becomes superheated by approximately 10°F. The amount of superheat is determined by the amount of liquid refrigerant admitted to the evaporator. This, in turn, is controlled by the spring adjustment of the TXV. A temperature range of 4° to 12°F of superheat is considered desirable. It increases the efficiency of the plant and evaporates all of the liquid. This prevents liquid carry-over into the compressor (flooding back).

Excessive circulation of the lubricating oil will cause the evaporator temperature to increase. The main cause of slugging is improperly adjusted thermal expansion valve.

Defrosting of evaporator coils of a multi-box, direct expansion type refrigeration systems, and ice machines can be accomplished by passing hot vapors from the compression cycle through the coils. A re-evaporator is one way to overcome the possibility of a large slug of liquid refrigerant entering the compressor suction when hot gas defrosting a refrigeration system.

 

Compressor

The compressor in a refrigeration system is essentially a pump. It is used to pump heat uphill from the cold side to the hot side of the system. The heat absorbed by the refrigerant in the evaporator must be removed before the refrigerant can again absorb latent heat. The only way the vaporized refrigerant can be made to give up the latent heat of vaporization that it absorbed in the evaporator is by cooling and condensing it. Because of the relatively high temperature of the available cooling medium, the only way to make the vapor condense is to compress it.

When we raise the pressure, we also raise the temperature. Therefore, we have raised its condensing temperature, which allows us to use seawater as a cooling medium in the condenser. In addition to this primary function, the compressor also keeps the refrigerant circulating and maintains the required pressure difference between the high-pressure and low-pressure sides of the system. Many different types of compressors are used in refrigeration systems. The designs of compressors vary depending on the application of the refrigerants used in the system. The figure below shows a motor-driven, single-acting, two-cylinder, reciprocating compressor.

Compressors used in refrigeration systems may be lubricated either by splash lubrication or by pressure lubrication. Refrigeration compressors require a lubricant with a low pour point, and low wax content to keep any oil leaving the compressor from  congealing in the evaporator. Splash lubrication, which depends on maintaining a fairly high oil level in the compressor crankcase, is usually satisfactory for smaller compressors. High-speed or large-capacity compressors use pressure lubrications systems.

The sudden reduction of pressure occurring within the crankcase of a refrigeration compressor during starting causes the release of refrigerant from the oil/refrigerant mixture. Foaming of the oil in a refrigeration compressor crankcase is caused by refrigerant boiling out of the lube oil. The oil in the sump of a secured refrigeration compressor is heated to reduce absorption of refrigerant by the oil. Excessive oil foaming in the crankcase of a refrigeration compressor at start up can cause compressor damage from improper lubrication. The oil level in a refrigeration compressor, the most accurate reading is obtained immediately after shutdown following a prolonged period of operation. The refrigerant has had time to separate from the oil.

Refrigerant entering the compressor should be superheated vapor. Its possible to have liquid refrigerant returned to the suction side of a compressor due to a faulty or improperly adjusted expansion valve.  A flapper valve, also known as a beam valve, is frequently used in refrigeration compressor discharge valves, and is designed to pass liquid slugs. Some systems have devices installed in the compressor suction line to boil off liquid refrigerant returning to the compressor, such as liquid separators, liquid accumulators, economizers, and heat exchangers.

 

Capacity Control System

Most compressors are equipped with an oil-pressure-operated automatic capacity control system. This system unloads or cuts cylinders out of operation following decreases in the refrigerant load requirements of the plant. A cylinder is unloaded by a mechanism that holds the suction valve open so that no gas can be compressed.

Since oil pressure is required to load or put cylinders into operation, the compressor will start with all controlled cylinders unloaded. But as soon as the compressor comes up to speed and full oil pressure is developed, all cylinders will become operative. After the temperature pull-down period, the refrigeration load imposed on the compressor will decrease, and the capacity control system will unload cylinders pressure accordingly. The unloading will result in reduced power consumption. On those applications where numerous cooling coils are supplied by one compressor, the capacity control system will prevent the suction pressure from dropping to the low-pressure cutout setting. This will prevent stopping the compressor before all solenoid valves are closed.

Several designs of capacity control systems are in use. One of the most common is shown in figure above. The capacity control system consists of a power element and its link for each controlled cylinder, a step control hydraulic relay, and a capacity control valve.

The system's components are all integrally attached to the compressor. The suction or crankcase pressure of the refrigeration plant is sensed by the capacity control valve to control the system. In other words, a change in the refrigeration load on the plant will cause a change in suction pressure. This change in suction pressure will then cause the capacity control system to react according to whether the suction pressure increased or decreased. The working fluid of the system is compressor oil pump pressure. Compressor oil pump pressure is metered into the system through an orifice. Once the oil passes the orifice, it becomes the system control oil and does the work.

Another type of capacity control uses a solenoid valve, in conjunction with an unloader head. The solenoid valve allows the refrigerant to pass from the suction chamber to the top of the unloader piston, causing the piston to lift and unload the cylinder.

 

Locate the following components on figure 10-6, and refer to them as you read the next two paragraphs.

1. Compressor oil pump pressure tap-off

2. Control oil strainer

3. Hydraulic relay

4. Hydraulic relay piston

5. Unloader power element

6. Unloader power element piston

7. Lifting fork

8. Unloader sleeve

9. Suction valve

10. Capacity control valve

11. Crankcase (suction) pressure sensing point

The following functions take place when the compressor is started with a warm load on the refrigeration system.

Compressor oil (A) is pumped through the control oil strainer (B) into the hydraulic relay (C). There the oil flow to the unloader power elements is controlled in steps by the movement of the hydraulic relay piston (D). As soon as pump oil pressure reaches a power element (E), the piston (F) rises, the lifting fork (G) pivots, and the unloader sleeve (H) lowers, permitting the suction valve (1) to seat. The system is governed by suction pressure, which actuates the capacity control valve (J). This valve controls the movement of the hydraulic relay piston by metering the oil bleed from the control oil side of the hydraulic relay back to the crankcase.

Suction pressure increases or decreases according to increases or decreases in the refrigeration load requirements of the plant. After the temperature pull-down period with a subsequent decrease in suction pressure, the capacity control valve moves to increase the control oil bleed to the crankcase from the hydraulic relay. There is a resulting decrease in control oil pressure within the hydraulic relay. This decrease allows the piston to be moved by spring action. This action successively closes oil ports and prevents compressor oil pump pressure from reaching the unloader power elements. As oil pressure leaves a power element, the suction valve rises and that cylinder unloads. With an increase in suction pressure, this process is reversed, and the controlled cylinders will load in succession. The loading process is detailed in steps 1 through 7 in the figure above.

 

SHAFT SEALS

Where the crankshaft extends through the crankcase, a leakproof seal must be maintained to prevent the refrigerant and oil from escaping and also to prevent air from entering the crankcase when the pressure in the crankcase is lower than the surrounding atmospheric pressure. This is accomplished by crankshaft seal assemblies. There are several types of seals including the rotary seal, and the diaphragm.  The rotary seal shown right consists of a stationary cover plate and gasket, a rotating assembly which includes a carbon ring, a neoprene seal, a compression spring, and compression washers. The sealing points are located (1) be-tween the crankshaft and the rotating carbon rings, and sealed by a neoprene ring; (2) between the rotating carbon ring and the cover plate, and sealed by lapped surfaces; and (3) between the cover plate and the crankcase, and sealed by a metallic gasket. The seal is adjusted by adding or removing metal washers between the crankshaft shoulder and the shaft seal compression spring.

A stationary bellows seal is illustrated left. It consists of a bellows clamped to the compressor housing at one end to form a seal against a rotating shaft seal collar on the other. The sealing points are located (1) between the crankcase and the bellows, and sealed by the cover plate gasket; (2) between the crankshaft and the shaft seal collar, and sealed by a neoprene gasket; and (3) between the surface of the bellows nose and the surface of the seal collar, and sealed by lapped surfaces. The stationary bellows seal is factory set for proper tension and should not be altered.

The rotating bellows seals, figure right, consists of a bellows clamped to the crankshaft at one end to form a seal against a stationary, removable shaft seal shoulder on the other end. The sealing points are located (1) between the crankshaft and bellows, and sealed by a shaft seal clamping nut; (2) between the removable shaft seal shoulder and the crankcase and sealed by a neoprene gasket; and (3) between the bellows nose piece and the shaft seal collar, and sealed by lapped surfaces. This type seal is also factory set.

 

 

Condenser

The compressor discharges the high-pressure, superheated refrigerant vapor to the condenser, where it flows around the tubes through which water is being pumped. As the vapor gives up its superheat (sensible heat) to the seawater, the temperature of the vapor drops to the condensing point. The refrigerant, now in liquid form, is sub-cooled slightly below its condensing point. This is done at the existing pressure to ensure that it will not flash into vapor. A water-cooled condenser is shown in figure left. Circulating water is obtained through a branch connection from the fire main or by means of an individual pump taking suction from the sea. Sea water condensers have zinc anodes in the end covers to protect against corrosion.

 Newer ships use a closed fresh water  system, consisting of circulating pump, and keel-cooler. The purge connection (fig. left) is on the refrigerant side. It is used to remove air and other non-condensable gases that are lighter than the refrigerant vapor.

If a large difference exists between the compressor discharge pressure and the pressure corresponding to the existing condensing temperature the system, should be purged.

Most condensers used for shipboard refrigeration plants are of the water-cooled type. However, some small units have air-cooled condensers. These consist of tubing with external fins to increase the heat transfer surface. Most air-cooled condensers have fans to ensure positive circulation of air around the condenser tubes.

 

Receiver

The receiver (fig. below right) acts as a temporary storage space and surge tank for the liquid refrigerant. The receiver also serves as a vapor seal to keep vapor out of the liquid line to the expansion valve. A pressure drop in the liquid line of a refrigeration system may cause the liquid refrigerant to flash to gas. Receivers are constructed for either horizontal or vertical installation.

 

In addition to the five main components of a refrigeration system, a number of controls and accessories are required. The most important of these are described briefly in the following paragraphs.

 

Dehydrator

Systems opened for repairs or service are vulnerable to moisture contamination. The maximum level of moisture permitted in an operating refrigeration system is 15 parts per million.

 A dehydrator, or dryer, containing silica gel or activated alumina, is placed in the liquid refrigerant line between the receiver and the TXV. In older installations, bypass valves allow the dehydrator to be cut in or out of the system. In newer installations, the dehydrator is installed in the liquid refrigerant line without any bypass arrangement. A dehydrator is shown in upper left figure . A refrigeration system contaminated with moisture can be affected by, acid formation, sludge formation, ice in the expansion valve, and corrosion.

If a liquid drying agent is used in a refrigeration system already equipped with a solid drying agent, the liquid drying agent will release the moisture already trapped in the solid drying agent.

 

Moisture Indicator and Liquid Eye

A moisture indicator is located either in the liquid refrigerant line or built into the dehydrator. The moisture indicator contains a chemically treated element that changes color when there is an increase of moisture in the refrigerant. The color change is reversible and changes back to a DRY reading when the moisture is removed from the refrigerant. Excessive moisture or water will damage the moisture indicator element and turn it gray, which indicates it must be replaced. In an operating refrigeration system low on refrigerant, a liquid line sight glass will show bubbles.

 

Solenoid Valve and Thermostatic Control Switch

A solenoid valve is installed in the liquid line leading to each evaporator. Figure right shows a solenoid valve and the thermostatic control switch that operates it. The thermostatic control switch is connected by long flexible tubing to a thermal control bulb located in the refrigerated space. When the temperature in the refrigerated space drops to the desired point, the thermal control bulb causes the thermostatic control switch to open. This action closes the solenoid valve and shuts off all flow of liquid refrigerant to the TXV. When the temperature in the refrigerated space rises above the desired point, the thermostatic control switch closes, the solenoid valve opens, and liquid refrigerant once again flows to the TXV.  This is is an example of two position control.

The solenoid valve and its related thermostatic control switch maintain the proper temperature in the refrigerated space. You may wonder why the solenoid valve is necessary if the TXV controls the amount of refrigerant admitted to the evaporator. Actually, the solenoid valve is not necessary on units that have only one evaporator. In systems that have more than one evaporator and where there is wide variation in load, the solenoid valve provides additional control to prevent the spaces from becoming too cold at light loads.

In addition to the solenoid valve installed in the line to each evaporator, a large refrigeration plant usually has a main liquid line solenoid valve installed just after the receiver. If the compressor stops for any reason except normal suction pressure control, the main liquid solenoid valve closes. This prevents liquid refrigerant from flooding the evaporator and flowing to the compressor suction. Extensive damage to the compressor can result if liquid is allowed to enter the compressor suction.

 

Refrigeration Valve

Refrigeration valves are used to add and remove refrigerant from the system. Most systems have refrigeration valves installed in both, high and low pressure sides of the system.

Like most valves used in refrigeration they are double seating. When the valve stem is rotated counter-clockwise to the fully opened position the upper seat seals the valve from leakage. Double seating valves should be used either fully opened or closed. Double seating valves permit repacking under pressure. A cap is provided to reduce the possibility of loss of refrigerant from the system. The valve shown has a gauge connection.

 

Modulating Valves

Modulating valves are similar to solenoid valves, as they control the liquid refrigerant to the TXV. A liquid line solenoid valve is either completely opened or closed, whereas a modulation valve is positioned according to the strength of the applied electrical signal. The movement of the armature within a modulating valve is controlled by the electromagnetic force of the coil and opposed by spring pressure. This arrangement will cause the valve to the open position due to the spring pressure acting upon the armature if the coil fails.

 

Evaporator Pressure Regulating Valve

In some ships, several refrigerated spaces of varying temperatures are maintained by one compressor. In these cases, an evaporator pressure regulating valve is installed at the outlet of each evaporator EXCEPT the evaporator in the space in which the lowest temperature is to be maintained. The evaporator pressure regulating valve is set to keep the pressure in the coil from falling below the pressure corresponding to the lowest evaporator temperature desired in that space. The evaporator pressure regulating valve is used on;

• water coolers

• units where high humidity is required (such as fruit and vegetable stow spaces)

• installations where two or more rooms are maintained at refrigeration unit. different temperatures by the use of the same

A cross section of a common evaporator pressure regulating valve (commonly called the EPR Evaporator Pressure Regulating Valve valve) is shown in figure 10-11. The tension of the spring above the diaphragm is adjusted so that when the evaporator coil pressure drops below the desired minimum, the spring will shut the valve.

The EPR valve is not really a temperature control; that is, it does not regulate the temperature in the space. It is only a device to prevent the temperature from becoming too low.

 

BACK PRESSURE REGULATING VALVE

Also known as an evaporator pressure regulating valve, is used to maintain a minimum evaporator pressure.   In a direct expansion type cargo refrigeration system, a box is normally changed from chill to freeze by adjusting the back pressure regulating valve.

 

Low-Pressure Cutout Switch

The low-pressure cutout switch is also known as a suction pressure control switch. Pressure acting on a bellows opens and closes contacts causing the compressor to go on or off as required for normal operation of the refrigeration plant. It is located on the suction side of the compressor and is actuated by pressure changes in the suction line.

When the solenoid valves in the lines to the various evaporators are closed, the flow of refrigerant to the evaporators is stopped. This action causes the pressure of the vapor in the compressor suction line to drop quickly. When the suction pressure has dropped to the desired pressure, the low-pressure cutout switch stops the compressor motor. When the temperature in the refrigerated spaces rises enough to operate one or more of the solenoid valves, refrigerant is again admitted to the cooling coils. This causes the compressor suction pressure to buildup again. At the desired pressure, the low-pressure cutout switch closes, starting the compressor, and the cycle is repeated again. The pressure range between the system cut in and cut out pressures in a refrigeration unit is known as differential. The low pressure cutout switch used on a refrigeration system compressor is set to cut in at approximately 5 Psig and cutout at .5 Psig.

 

High-Pressure Cutout Switch

A high-pressure cutout switch is connected to the compressor discharge line to protect the high-pressure side of the system against excessive pressures. The design of this switch is essentially the same as that of the low-pressure cutout switch. However, the low-pressure cutout switch is made to CLOSE when the suction pressure reaches its upper normal limit, while the high-pressure cutout switch is made to OPEN when the discharge pressure is too high. As you already have learned, the low-pressure cutout switch is the compressor control for the normal operation of the plant. On the other hand, the high-pressure cutout switch is a safety device only. It does not have control of the compressor under normal conditions.

 

Oil separator

Oil separators or traps, if supplied are located between the compressor discharge and the condenser. Oil separators serve to return oil entrained in refrigerant vapor back to the compressor crankcase.

 

Water Failure Switch

A water failure switch stops the compressor if there is a circulating water supply failure. The water failure switch is a pressure-actuated switch. Its operation is similar to the low and high pressure cutout switches previously described. If the water failure cutout switch fails to function, the refrigerant pressure in the condenser quickly builds up to the point that the high-pressure switch stops the compressor.

 

Strainer

Because of the solvent action of refrigerant, any particles of grit, scale, dirt, or metal that the system may contain are circulated through the refrigerant lines. To avoid damaging the compressor from foreign matter, a strainer is installed in the compressor suction connection.

 

Water Regulating Valve

A water regulating valve controls the quantity of circulating water flowing through the refrigerant condenser. The water regulating valve is actuated by the refrigerant pressure in the compressor discharge line. This pressure acts upon a diaphragm (or, in some valves, a bellows arrangement) that transmits motion to the valve stem.

The primary function of the water regulating valve is to maintain a constant refrigerant condensing pressure. Basically, the following two variable conditions exist:

1. The amount of refrigerant to be condensed

2. Changing water temperatures

The valve maintains a constant refrigerant condensing pressure by controlling the water flow through the condenser. By sensing the refrigerant pressure, the valve permits only enough water through the condenser to condense the amount of refrigerant vapor coming from the compressor. The quantity of water required to condense a given amount of refrigerant varies with the water temperature. Thus, the flow of cooling water through the condenser is automatically maintained at the rate actually required to condense the refrigerant under varying conditions of load and temperature.

 

Heat Interchanger

The function of a heat interchanger is to lower the temperature of liquid refrigerant before entering the expansion valve, reduce the possibility of liquid refrigerant from flooding back to the compressor, and minimize sweating of the suction line.

 

Pressure Gauges and Thermometers

A number of pressure gauges and thermometers are used in refrigeration systems. The figure left shows a compound R-12 gauge. The temperature markings on this gauge show the boiling point (or condensing point) of the refrigerant at each pressure; the gauge cannot measure temperature directly. The red pointer is a stationary marker that can be set manually to indicate the maximum working pressure.

A water pressure gauge is installed in the circulating water line to the condenser to indicate failure of the circulating water supply.

Standard thermometers of appropriate range are provided for the refrigerant system.

 

DETECTING AND CORRECTING PROBLEMS

A number of symptoms indicate faulty operation of refrigeration and air-conditioning plants. The table below list some possible causes  and corrective measures and includes recommended test procedures that may be used to isolate the problems.

 

Trouble	Possible Cause	Corrective Measure
High condensing pressure.	Inlet water warm.	Purge air from condenser
	Air on non-condensable gas in  system.	Increase quantity of condensing water.
	Insufficient water flowing through condenser.	Increase quantity of water.
	Condenser tubes clogged or scaled.	Clean condenser water tubes.
	Too much liquid in receiver, condenser tubes submerged in liquid refrigerant.	Draw off liquid into service cylinder.
Low condensing pressure.	Too much water flowing through condenser.	Reduce quantity of water.
	Water too cold.	Reduce quantity of water.
	Liquid refrigerant flooding back from evaporator.	Change expansion valve adjustment, examine fastening of thermal bulb.
	Leaky discharge valve.	Remove head, examine valves. Replace any found defective.
Frosting or sweating of a liquid line.	Refrigerant line restriction.	Check for partially closed stop valve, or stuck solenoid valve.
	System low on refrigerant.	Check for leaks, add refrigerant.
High suction pressure. Compressor crankcase sweating	Overfeeding of expansion valve.	Regulate expansion valve, check bulb attachment.
	Leaky suction valve.	Remove head, examine valve and replace if worn.
Low suction pressure.	Restricted liquid line and expansion valve or suction screens.	Rump down, remove, examine and clean screens,
	Insufficient refrigerant in system.	Check for refrigerant storage.
	Too much oil circulating in system.	Check for too much oil in circulation. Remove oil.
	Improper adjustment of expansion valves	Adjust valve to give more flow.
	Expansion valve power element dead or weak	Replace expansion valve power element.
Compressor short cycles on low- pressure control.	Low refrigerant charge.	Locate and repair leaks. Charge refrigerant.
	Thermal expansion valve not feeding properly. Dirty strainers. Moisture frozen in orifice or orifice plugged with dirt. Power element dead or weak	Adjust, repair or replace thermal expansion valve. Clean strainers. Remove moisture or dirt (use system dehydrator). Replace power element.
	Water flow through evaporators restricted or stopped. Evaporator coils plugged, dirty, or clogged with frost.	Remove restriction. Check water flow. Clean coils or tubes.
	Defective low-pressure control switch.	Repair or replace low-pressure control switch.
Compressor runs continuously.	Shortage of refrigerant.	Repair leak and recharge system.
	Leaking discharge valves.	Replace discharge valves.
Compressor short cycles on high- pressure control switch.	Insufficient water flowing through condenser, clogged condenser.	Determine if water has been turned off. Check for scaled or fouled condenser.
	Defective high-pressure control switch.	Repair or replace high-pressure control switch.
Compressor will not run.	Seized compressor.	Repair or replace compressor.
	Cut-in point of low-pressure control switch too high.	Set L. P. control switch to cut-in at correct pressure.
	High-pressure control switch does not cut-in. 1 .Defective switch. 2. Electric power cut off. 3. Service or disconnect switch open. 4. Fuses blown. 5. Over-load relays tripped.   6. Low voltage.   7. Electrical motor in trouble. 8. Trouble in starting switch or control circuit.     9. Compressor motor stopped by oil pressure differential switch.	Check discharge pressure and reset P. H. control switch. 1. Repair or replace switch. 2. Check power supply. 3.Close switches. 4. Test fuses and renew if necessary. 5. Re-set relays and find cause of overload. 6. Check voltage (should be within 10 percent of nameplate rating). 7. Repair or replace motor. 8. Close switch manually to test power supply. If OK, check control circuit including temperature and pressure controls. 9. Check oil level in crankcase. Check oil pump pressure.
Decreased capacity of the compressor.	High vapor superheat.	Adjust or replace expansion valve.
Sudden loss of oil from crankcase.	Liquid refrigerant slugging back to compressor crank case.	Adjust or replace expansion valve.
Capacity reduction system falls to unload cylinders.	Hand operating stem of capacity control valve not turned to automatic position.	Set hand operating stem to automatic position.
Compressor continues to  operate at full or partial load.	Pressure regulating valve not opening.	Adjust or repair pressure regulating valve.
Capacity reduction system fails to load cylinders.	Broken or leaking oil tube between pump and power element.	Repair leak.
Low discharge pressure with high suction pressure.	Discharge relief valve leaking back to the suction side.	Replace relief valve.
Compressor continues to  operate unloaded.	Pressure regulating valve not closing.	Adjust or repair pressure regulating valve.
Compressor oil brownish in color	Copper plating caused by moisture in the system.	Change filter drier, or dehydrator.
Compressor oil gray or metallic.	Compressor bearing wear or piston scoring.	Replace or overhaul compressor.
Compressor oil black	Carbonization resulting from air in the system.	Remove air from system.

 

CHARGING THE SYSTEM

One of the test questions states, "Before charging a refrigeration unit, the refrigerant charging lines should be purged with the refrigerant". New laws regarding refrigerants require all charging lines or hoses to be equipped with valves which seal the lines when they are not connected.

The amount of refrigerant charge must be sufficient to maintain a liquid seal between the condensing and the evaporating sides of the system. When the compressor stops, under normal operating conditions, the receiver of a properly charged system is about 85% full of refrigerant. The proper charge for a specific system or unit can be found in the manufacturer's technical manual or on the ship's blueprints. A refrigeration system must have an adequate charge of refrigerant at all times; otherwise its efficiency and capacity will be impaired.

Low side passive charging of a refrigeration system may be speeded up by warming the service cylinder with hot water to help boil off the liquid. The safest and quickest method of adding refrigerant to a refrigeration system is to add the refrigerant through the charging valve as a liquid.

A refrigeration system should not be charged if there are leaks or if there is reason to believe that there is a leak in the system. The leaks must be found and corrected. A system should be checked for leaks immediately following, or during the process of charging.

 

Container Refrigeration

If an evaporator or condenser coil of a container refrigeration system becomes dirty and requires cleaning, one of the suggested methods is to use the high pressure wash system.

When a thermostatic expansion valve is installed in a container refrigeration system, the sensing bulb may not require insulation if the bulb is installed outside of the cooled air stream. If the evaporator coil horizontal return line of a container refrigeration system is less than 0.874" (2.21 cm) in diameter, the thermostatic expansion valve sensing bulb should be placed on the upper surface of the line.

 

SAFETY PRECAUTIONS

Secure and tag the electrical circuit of the system before working any shipboard system, to prevent damage to the equipment, and injury to personal.

On all vessels equipped with refrigeration units of over 20 cubic foot capacity, a gas mask suitable for protection against each refrigerant used, or a self contained breathing apparatus must be provided.

Use chemical safety goggles or a full face shield and rubber gloves while handling refrigerant.

Ammonia vapors in a low concentration can cause death, will dissolve in perspiration and cause caustic burns, and can burn or explode.

Coast Guard Regulations (46 CFR) require a method for the relief of an over-pressurized refrigeration system. A rupture disk may be fitted in series with the relief valve.

Overfilling a refrigerant container is extremely dangerous due to the high pressures generated by hydrostatic pressure of the expanding liquid.

Low pressure refrigeration containers used for transportation are not refillable. They rupture disc set for 15 Psig, and are not to be heated, or stored in temperatures over 125°F. Containers are to be pumped down to 0 Psig or below, and disposed of with the valve opened.

 

 

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