chapter 6 full

7/25/2019 Chapter 6 Full

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Chapter 6

Commercial Systems

Construction Of Refrigerating Mechanisms

Operating fundamentals of domestic refrigeration systems also apply to commercial systems. But

many commercial systems using mechanical cycle mechanisms differ in some way from the

domestic mechanism. These differences are chiefly in the following:

1. The number of evaporators connected to one condensing unit

2. Compressor design and size

3. Condenser unit design and size4. Motor controls both temperature and pressure

5. Refrigerant controls both liquid and vapor

6. Piping

7.

Variety of evaporator designs8.

Defrosting systems

9. Variety of refrigerants used

Complete Mechanical Mechanism

Fig 6-1:- An open top display cabinet with a self contained refrigeration unit.

Fig 6-1 shows a single unit mechanism. It include:

1. High-pressure sidea. Compressor usually hermetic

b. Condenser usually air-cooled

c. Liquid receiver when a thermostatic expansion valve or automatic expansionvalve is used

d. High-pressure safety motor control


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e. Liquid line with dryer and sight glass

The refrigerant control is at the division point between the low side and the high side

of the system. It will consist of an automatic thermostatic expansion valve or capillarytube.

2. Low-pressure side:

a.

Evaporatorb.

Low pressure or temperature motor control

c. Suction line some with filter-driers and surge tanks

The multiple mechanism includes :1.

High Pressure-side

a. Compressor often with an oil separator

b. Condenser water or air cooled

c. Liquid receiverd. High pressure motor control

e. Liquid lines with a drier and a sight glass

f. Water valve used with water-cooled units

2.

Low-pressure sidea.

Refrigerant controls, two or more , usually of the thermostatic expansion valve

typeb.

Evaporators two or more. These may be any of several types: natural

convection, forced convection or submerged.

c. Motor control, which is usually of the pressure type

d. Suction line with drier and suction pressure regulatore. Two temperature valves for multiple temperature installation

f. Surge tanks for reducing rapid pressure changes

g. Check valves for multiple temperature installations3. There are many varieties of commercial systems. A water chiller system is shown in Fig

6-2.

Condensing units are normally mounted on a steel base. In the external drive unit, the motor is

mounted outside the compressor and drives the compressor either directly or with one or more

belts as in Fig 6-3.

Fig 6-2:- Water chiller. Hot gas bypass keeps low-side pressure high enough to prevent

freezing of chilled water. Liquid injection keeps suction vapor cool enough to prevent

compressor motor overheating.


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Fig 6-3:- Condenser with belt-driven, four-cylinder, air-cooled compressor. Unit is

mounted on steel base.

Fig 6-4:- Bolted type field serviceable eight cylinder hermetic motor compressor.


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Fig 6-5:- Serviceable commercial hermetic condenser. Note the electrical panel. Wiring is

enclosed in the flexible metal conduit. The unit is spring-mounted to allow movement. Note

also the pressure motor control mounted next to the electrical control.

In the hermetic unit, the motor is connected directly to the compressor. It is designed for low-temperature work and has a 40,600 Btu/hr, capacity at -52F. (-46C).

For large commercial installations, compressors are made with three, four, five, six, seven ormore cylinders.

The cylinder arrangement is another method of naming compressors: vertical single, horizontal

single, 45 deg. single (inclined), vertical two cylinder, V-type two cylinder, W-type threecylinder, radial three cylinder, vertical four cylinder and V-type four cylinder, have all been

used.

As with cylinder arrangements, there are many crankshaft arrangements. One type of serviceable

hermetic motor compressor has an eccentric type crankshaft. The inside construction of a

multiple cylinder serviceable hermetic motor compressor is shown in Fig 6-4.

To increase the cooling of the compressor and motor of a serviceable motor compressor, one

company uses a fan and motor. This manufacturer can convert the motor compressor easily intoan external drive compressor.

A six cylinder compressor with an external drive motor has a crank throw type crankshaft.

A steel frame is used to hold the shell and tube condenser on the hermetic unit shown in Fig. 6-5.

Note water cooling coil around the electric motor, the oil level sight glass in the lower right of

the compressor and the spring mounted hermetic unit.


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Commercial Hermetic units

Fig 6-6:- Commercial hermetic condensing unit with air-cooled condenser. Separate motor

is used to drive condenser fan.

Fig 6-7:- Four cylinder hermetic compressor. This type is used on air conditioning, heat

pump, and commercial condensers. A- Crankshaft B-Connecting rod. C- Pistons. D- Motor

Windings. E-Electrical Terminals. F- Suction and discharge openings.

Several companies now produce hermetic units of 20 hp or more. Some of the units are thebolted assembly type. These are often called field serviceable or accessible. Some units aresealed in a welded casing. Both types are equipped with a service valve. They may be connected

to any type of evaporator and used for many different applications.

An advantage in the use of hermetics in the commercial field is the elimination of the crankshaft

seals and belts. Because any trouble in the compressor mechanism involves both the compressor

and the motor, the service technician must be very careful when working on these hermetic units,to keep moisture and dirt out of the system.


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A serviceable motor compressor equipped with a fan condenser, a shroud, and service valves is

shown in Fig 6-6. The inside of a welded hermetic motor compressor is shown in Fig 6-7.

Smaller units have single-phase motors. Units over hp generally have three-phase motors.

The condensing units may be installed in many different ways. Some are mounted on the roof,

some on the same floor level with the evaporator but in different rooms or outside the building.Note the suction service valve in the compressor, the liquid receiver service valve on the

receiver, and the forced convection condenser.

A factory assembled condensing unit and evaporator combination can be installed. These

straddle or in plug-in units eliminate the need to put in piping between the condensing unit and

the evaporator. The installation, therefore, consist of preparing the opening, mounting the unit,

running the electrical lines, and opening the shutoff valves.

For large installations, two-motor compressor designs have been developed to provide greater

refrigeration capacity. They are:

1.

Tandem assembly motor compressors

2. parallel assembly motor compressors

The tandem design connects two motor compressors together at the motor end. These units can

be run separately for low load or together for full load. But if, one motor compressor fails and

must be replaced, the complete system must be shut down during the service.

Parallel design connects two or more units in parallel by piping (Fig 6-8). The units also require

a compressor oil piping system to make certain that all the compressors have the correct amountof oil in each crankcase while in operation.

Fig 6-8:- Simple diagram of parallel motor compressor assembly. Two motor compressors

are used. Note oil separator, oil reservoir, venting, suction accumulator, and oil level

regulator.


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Outdoor air-cooled condensing Units

Fig 6-9:- Condensing unit during on cycle. Liquid refrigerant is stored in liquid receiver.

Fig 6-10: Condensing unit during off cycle. Liquid refrigerant fills condenser tubes.


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To save space in commercial buildings and in homes (when air conditioning), there is anincreasing use of outdoor air-cooled condensing units. Air cooled units save the cost of plumbing

required for water cooling.

These units may be mounted outside the building, on the roof, on the outside wall or at ground

level. Four major concerns:

1.

There must be a head pressure control if the unit is exposed to outdoor weather that may

go below the operating cabinet temperature.

2. A method of preventing short cycling must be designed into the system

3. A means must also be provided to prevent dilution of the compressor oil by liquidrefrigerant.

4. The completed condensing unit must be constructed and installed so it is virtually

weatherproof.

Low ambient temperatures will cause low head pressures. This pressure may drop so low that it

may even stop the flow of refrigerant. Four different methods to maintain pressures:

1. Partially fill the condenser with liquid refrigerant

2. Stop or slow the condenser fans.

3. Partially close or completely close the ambient air louvers4. Heat the condenser

Outdoor units require about 1000 cubic feet per minute (cfm) of condenser air circulation perhorsepower. They are less costly to operate than indoor air-cooled units.

One of the main problems with outdoor units is to keep the thermostatic expansion valveoperating at full capacity during cold weather. Capacity depends on the pressure difference

across the valve. If condensing pressure reduces from 102 psi, 90F. (32C.) for R-12 to 56 psi,

30F. (-2C.), the valve capacity will drop. (Not enough liquid refrigerant will flow). The

refrigerated fixture temperatures may then rise too high. Also, a small pressure difference maycause short cycling of the condensing unit.

The condensing temperature and pressure may be kept at a proper operating level by a designchange. The unit is made to nearly fill the condenser tubes with liquid. Just enough condensing

surface is left to maintain the pressure.

The pressure on the limiting valve on the outlet of the condenser will not open (and allow liquid

to leave the condenser) until the condensing pressure reaches the proper level. The limiting valve

is found at the outlet of the condenser. It opens when pressure rises.

The installation specifications must be carefully checked. The receiver must hold enough liquid

refrigerant to flood most of the condenser and still safely hold the refrigerant when required.


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During the on part of the cycle, the unit operates as a normal system, but never at a temperature

les than 80F. (27C). This is shown diagrammatically in Fig 6-9. During the off part of the cycle,the condenser fills with the liquid refrigerant (still at the 80F temperature pressure relationship).

This condition prevents short cycling and also stops refrigerant from moving out of the low side,

through the compressor, into the high side during off cycle. See Fig 6-10.

The condensing pressure may also be controlled by limiting the airflow to the condenser.

Another way to keep the pressures up is to close the condenser housing airflow louvers as the

pressure drops. A pressure-sensitive device does this. It is connected into the condenser tubing.

This head pressure device will move the rod out as pressure increases. The rod will open the

louvers. As head pressures decreases, the rod will move back and start to close the louvers.

The condenser fans may either operate when louvers are closed or they may be shut off when the

louvers near the closing point.

Other systems shut off the condenser fan when the condenser pressure falls to a minimum level.

Some shut off one or more of several fans. Others lower the fan speed when the head pressuredrops by using electrically controlled modulated fan speeds. This system operates with a

thermistor type sensor on the condenser, a special fan motor, and a timing device. It permits the

system to operate for a few minutes on start-up even though the low-side pressure is low.

To keep the receiver temperature warmer than the cabinet temperature, electric heating elements

are some times placed in or round the receiver. If allowed to become too cold, the receiver would

act like a condenser.

Some systems use a bypass from the compressor to the receiver. This bypass feeds a certain

amount of hot refrigerant vapor to the receiver to keep it warm. The bypass has a check valvemounted in it to ensure only one-way flow.

The compressor itself must be kept warm enough during cold weather to prevent dilution of the

oil by the liquid refrigerant. Heat is provided by electric heating elements in or around the motorcompressor. These are thermostatically operated to energize the heating elements at about 50F.

(10 C). This heater usually has a 100W to 200W capacity.

The built in capacity to overcome low ambient temperatures may not be enough if the outdoor

unit is exposed to above normal winds. Windy conditions can prevent damper and fan operation.

Or the cooling effect may be more than the electric heating element can overcome. The unit mustbe installed in a position to avoid the harmful effects of any high-velocity cold winds.

The unit should be weatherproofed.

Head pressure control valves are often used. These are usually thermostat operated. A low-

pressure switch may not cut in because condenser pressures are below cut-in pressures. All

refrigerant may then transfer to the condenser, because it will be the coldest part of the system. A


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check valve is often used in the condenser outlet to prevent flow of refrigerant to the cold

receiver.

As noted before, some systems flood the condenser with liquid refrigerant during ambient

temperatures to maintain high enough condensing pressures. Such systems require valves. When

the condenser pressure decreases, one valve closes the outlet of the condenser and the condenserstarts to fill with liquid refrigerant. This action reduces the condensing surface and condensing

pressure will rise.

This system which uses a valve designed to open as the pressure in the receiver falls also allows

hot gas to bypass into the receiver (at about 20 psi pressure difference). This raises the receiver

pressure and increases the flow of liquid refrigerant to the evaporators.

The valve has two openings. If located in a cold place, the receiver may need an electric heating

element to help the system operate efficiently. These valves must be sized to the capacity of the

system. Avoid using excessive pressures when testing for leaks. The valve bellows may suffer

damage. Keep the pressure at or below 200 psi.

The system usually is charged with twice as much refrigerant as the system would need withoutthe condenser flooding feature. To permit year-around operation, the receiver should be twice the

normal size in order to hold all the refrigerant.

The compressor may also collect liquid refrigerant during the off cycle. A trap may be needed inthe compressor discharge line and an inverted trap at the condenser outlet.

The Compressor

Commercial compressors are two general types:

1.

External drive

2. Hermetic

These are several types of commercial hermetic compressors. Some are part of a bolted(serviceable) hermetic two cylinder compressor with a force feed lubrication system.

Some large units have either hydraulic or electric unloading devices to control the number ofcylinders which are pumping. The higher the load, the more cylinders used to pump the vapor.

Fig 6-11 is a schematic of the oil circuit and describes how it is controlled to operate the

compressor unloader.

Another type of hermetic compressor unit is the welded motor compressor design, which is non-

filled and serviceable. These units are built in sizes from 1/6 hp up to approximately 2 hp.Internal design varies with size and manufacturer. Some are spring mounted internally, while

some use outside (external) mounting springs. The smaller units usually have one cylinder, while

larger units (1/2 hp and above) have two or more cylinders. Small unit motors may be either two

or four pole (single phase). Three phase motors are generally used in the larger units.


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Many combination, type and sizes of compressors can be combined to provide the pumping for a

great variety of evaporator type and sizes. Each compressor is most efficient under definitelimits. Each compressor has a minimum and maximum:

1.

Revolutions per minute (rpm) for efficiency2.

Compression ratio (a maximum pressure difference between low side and high side).

3. Discharge temperature

4. Volume of gas it can pump.

Before using a certain compressor, the manufacturers operating specifications must be known.

Many low temperature systems use a cascade system. The first stage compressor may be areciprocating type, but rotary units are also used. The rotary compressor pressure limit is about

45psi across the compressor. It works very well with a compression ratio of about 4:1 and with a

discharge temperature of about 200F (93C.).

The rotary system has a high volumetric efficiency. A check valve is usually placed in the

discharge side to prevent back flow of refrigerant during the off cycle. A check valve should beplaced in the oil lines for the same reason.

Compressors can have from one to twelve cylinders in many different cylinder arrangements :

vertical V,W,Y,X or radial.

Internal unloaders are usually operated by oil pressure. A spring holds the intake valve open until

the oil pressure builds up causing all intake valves to operate. It is also used to ensure the oilpressure builds up causing all intake valves to operate. It is also used to reduce pumping capacity

during low-load periods. Solenoid valves are mounted in the oil lines to unloaders. When the

solenoid closes, the oil pressure drops in the unloader while the intake valves are kept open.

Low side pressure switches operate the solenoids. A timer bypass pressure switch is used to

operate the system at full capacity for about a minute each hour or two. External unloaders use a

bypass to the evaporator inlet to make sure suction vapor is cool (de-superheating).


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Fig 6-11:- Oil circuit of an eight cylinder compressor with oil take-off to operate

compressor unloader. A-Oil circuit. B-Electrical control. C-Pneumatic control.


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Air Cooled Condenser

Air-cooled condensers are quite common in commercial systems. Cooling water may be tooexpensive or corrosive. Smaller units use static condensers with thermal airflow.

Larger condensers may be cooled by a big fan built into the motor or into the compressorflywheel on external drive units. Larger hermetic units use separate motors to drive the fans.

The efficiency of the fan on an air-cooled condenser may be increased by placing a metal shroudaround it. Air can be drawn, induced (led into) or forced through the condensers. These

condensers have fins and frequently use a double or triple row of tubes. Many fin arrangements

and constructions have been used.

To cool the compressor head and valves, a double air-cooled condenser is sometimes used.

Refrigerant leaves the compressor and passes through one condenser. It is then led back through

the motor compressor to help cool it. From there it goes into the second condenser, where it is

condensed (cooled) into liquid.


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Cooling Towers

Fig 6-12:- Six different types of cooling towers. A, B, C show three different airflow

patterns. D, E, and F show three different ways to vaporize some of the water.

In some areas, water contains chemicals causing it to be unsuitable for use as a coolant. In otherlocalities, water may be very scarce, expensive, or its use may be limited by law.

A variety of cooling tower designs are shown in Fig 6-12. Cooling towers can be rather noisy.

They should, therefore, be located away from noise-sensitive areas such as offices, restaurants

and living quarters.

One system connects the water lines of the condenser to a water coil in an enclosure. A pump

forces the water through the condenser and then through the tower. The tower coil is piercedwith holes, and the water is sprayed onto the enclosure.

Air rushing through the sprayed water evaporates some of it. Evaporation cools the remainingwater to the outdoor temperature or even lower (wet bulb temperature).

Motor driven fans control air flow through a cooling tower.

Cooled water collects in the bottom of the enclosure and passes through a screen to remove

leaves or other foreign material. Then it is recirculated through the condenser.


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A float controlled valve in the lower water pan adds more water as needed. This float operates

like a refrigerant low-side float mechanism. A drain continually bleeds some water out of the

water pan to keep water hardness to a minimum. Chemicals may be added to the water to retardrust formation, algae, fungus growth and the like.

Cooling towers are made of corrosion resistant materials such as FRP.

The more water which comes in contact with the air flowing through a cooling tower, the more

efficient the cooling action. Most towers have some arrangement so that water flows overmaterials in thin films. This material, usually called in-fill, provides a surface for this. These fills

are made of many materials, e.g. FRP. The shapes of the surface vary from honeycomb,

embossed, flat sheet to corrugated sheet. The cellular (honeycomb) fill is becoming very popular.

The distribution system (nozzles, troughs, V-notches) must be kept clean and must distribute the

water evenly to prevent scale buildup.

Use strainers on pump inlet and be sure all suction lines are below water level in the coolingtower, otherwise air may enter the suction line causing pump volume to drop. Pumps can be

damaged in this way.

Put fine strainers on pump outlets. The water pump should push water through the system to

prevent low water pressures push water through the system to prevent low water pressures in the

condenser tubes or pipes.

The cooling tower has overflow pipes that carry any excess water to the drain system of the

building. Some have air control flow to prevent freezing if wet bulb or dry bulb temperature goesbelow 32F. (0C). The air outlet thermostat operates the fan dampers.

Cooling towers evaporate about two gallons of water every hour for each ton of refrigerant. One

gallon of water weighs about 8.3 lb. About 1000 Btu are needed to evaporate 1 lb. of water.Then, for one gallon of water, it will require 8.3 x 1000 = 83000 Btu. For two gallons, it would

require 8300 x 2 = 16,600 Btu.

Compressor Protection Devices

Many reciprocating compressors are damaged when liquid refrigerant accidentally flows into the

compressor from the suction line.

The refrigerant must be in a vaporous state. This means that the vapor temperature must be

higher than the temperature of the evaporating liquid in the evaporator. This increase intemperature means the vapor is superheated.

Many devices have been used to prevent or minimize entry of suction line liquid refrigerant intothe compressor:

1. An accumulator in the suction line

2. Hot gas bypass valves to move hot gas into the suction line where it can evaporate any

liquid.


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3. Temperature sensing devices are solenoid valves.

4. Heat exchangers to warm the suction line vapor liquid

5. Electrical heaters to warm the suction line vapor liquid6. An evaporator (blower coil) in the suction line

When a system is using a hot gas defrost or when a system has a sudden load change, drops andslugs of liquid refrigerant may travel in the suction line. This liquid may damage the compressor.

Many systems have an accumulator in the suction line to reduce the danger of flow of liquid

refrigerant into the compressor. See Fig 6-13.

The accumulator will lengthen the cycling interval (gas storage). It usually has aspirating

(suction) devices to return oil.

Fig 6-14 shows an internal design of an accumulator.

Fig 6-13:- Suction line accumulator. (1)- Mixture of refrigerant vapor liquid and oil enters.

(2)- Swirling motion created on entering mixture. (3)- Liquid strikes inside wall. (4)-

Refrigerant vapor and mist drawn upward, vertical motion, and then downward into

tubing. (5)- Flow turns 1800 and upward through orifice, drawing measured amount of

liquid refrigerant and oil from bottom. (6)- Combination of refrigerant vapor, compressor

lubricating oil, and refrigerant flow vertically, forming a mist before entering. (7)- The

compressor suction.


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Fig 6-14:- Inside of an accumulator. A- Suction gas in. B- Suction gas out. C- Oil return

aspirator hole. D- Liquid refrigerant trapped until it can evaporate.

The hot gas bypass device depends on the use of a temperature sensor attached to the suction

line, when there is danger of liquid refrigerant flowing into the compressor.

Electrical heaters may be attached to the suction line as a temperature sensing element used to

turn on the current when heating of the suction line is needed.

The blower evaporator is usually operated by a temperature sensing element attached to the

suction line. The fan in the blower coil will be turned on when the suction line temperature

indicates danger of liquid refrigerant flowing into the compressor.

The biggest problem is to have a temperature-operated device or a pressure device which will

detect the presence of liquid quickly enough to turn on mechanisms which will stop the liquidfrom reaching the compressor.

The best sensor is a thermistor which can react very quickly. It can be connected to an alarmcircuit or operating circuit to stop the compressor and eliminate any danger of damage.


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Oil Separators

It is very important that the compressor be kept lubricated. It pumps a certain amount of oil alongwith the refrigerant vapor and there is a possible chance that too much oil may leave the

compressor. Therefore, it is important to return the pumped oil to the compressor as quickly as

possible.

Refrigerant systems work better when the oil is kept in the compressor. The capacity of each part

of the system is increased by 5 to 15 percent. Oil in the condenser and evaporator will reduce theefficiency of the unit.

It is important to keep the oil from circulating in low temperature installations. It thickens in very

low temperatures and becomes difficult to move out of the evaporator.

Oil separators remove the oil from the hot compressed vapor as the vapor leaves the compressor.

The oil will separate because the vapor flow slows down as it arrives in the separator. See Fig 6-

15. The oil will collect in the separator until a certain level is reached in the oil separator. Then, afloat opens a needle valve. This allows the oil to return to the compressor crankcase.

Oil separators are also placed between the compressor and the condenser (fig 6-16). Fig 6-17

illustrates the inside of a typical oil separator. Note the separator is insulated. This prevents it

from acting as a refrigerant condenser.

Many oil separators are serviceable (bolted construction). On hermetic systems, the oil return

line is usually connected to the suction line near the motor compressor.

Fig 6-15:- Cutaway of an oil separator. Bag-like attachments are screens.


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Fig 6-16:- Oil separator installation. Oil is removed from high temperature, high pressure,

refrigerant and returned to compressor.

Fig 6-17. Internal construction of an oil separator.

Liquid refrigerant may collect in the oil separator during long off cycles or during long manual

shutdown. This liquid refrigerant, as it returns by way of the oil return line to the compressor,

may cause oil pumping. This action may damage the compressor. A check valve in the vaporoutlet of the oil separator will reduce this danger. A filter in the oil return line will help keep the

oil clean.

A solenoid valve in the oil return line is sometimes used to keep oil or refrigerant returning to the

crankcase during the off-cycles. A thermostat controls the solenoid. The thermostat will close

only when the oil separator is warm (100 to 130F) (38 to 54C).

A capillary tube is sometimes employed to carry the oil from the oil separator to the compressorcrankcase. It reduces oil (or liquid refrigerant) flow to the crankcase and also reduces the chance

of oil slugging and damage to the compressor.

Large units up to 150 tons capacity can use the oil separator. It has a 16-in diameter shell.

Note oil return is located at the bottom.


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Filter-Dryers (Liquid Line)

Fig 6-18:- This filter dryer is designed for use in liquid lines. It has a solid core and flared

connection.

The efficient operation of a commercial system depends, to a great extent, on the internalcleanliness of the unit. Only clean, dry refrigerant and clean dry oil should circulate in the

system.

Practically all dirt and water must be removed or must be trapped in some part off the systemwhere they can do no harm. Screens, filters and water adsorbents are used. These devices may be

in separate units or may be built in into a single unit which filters and adsorbs.

A common method of removing moisture is with a liquid line dryer (Fig 6-18). If enough drying

material is used it can keep the refrigerant both clean and dry.

The conventional straight through dryer is a cylinder (brass, copper or steel) filled with a

chemical (desiccant) such as activated alumina or silica gel. These materials remove moisture by

adsorption. Both ends of the cylinder usually contain filter elements. The end caps are fitted witheither flare or soldered connections.

One design of liquid line dryers allows the casing to stay in the line. Only the dryer cartridge

needs to be changed.

The most common desiccants (chemicals in shells) are combinations of activated alumina, silica

gel and molecular sieves. These chemicals can adsorb 12 to 16 percent of their weight in water.Dryers are usually installed in the liquid line.

The refrigerant should be dried below 15 parts of water per million if R-12 is used, below 25

ppm if R-22 or R-500 is used and 5 ppm for R-502. The beginning of corrosion in R-12 is 25ppm of water, 120 ppm for R-22 or R-500, and 15ppm for R-502. Experience shows that

corrosion, oil breakdown and motor burnouts are almost eliminated if the refrigerant has the safe

(or dry) amount of moisture in the system.

When cleaning a refrigeration system, four basic things are to be done:

1. Water removal2. Acid removal


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3. Filtering out of circulating solids

4. Some means of indicating when the drying job is complete.

Dryers will do the first three. A moisture indicator is required to do the fourth.

Dryers should be left in the system permanently since oil loses its moisture slowly. Also,insulation in hermetic compressors and in small crevices may release moisture over a long period

of time. A dryer is like a sponge, however, it can become saturated and leave the refrigerant still

wet if the dryer is too small. A moisture indicator is the only sure means of recognizing a wetcondition.

Remember that R-22 dryers must be three to five times as large as those needed for an equal

quantity of R-12. The greater the ability of a refrigerant to hold water, the larger the dryerrequired. R-500 dryers need to be as large as R-22 dryers and R-502 dryers need to be as large as

R-12 dryers.

Most dryers have shaped cores of two or three dryer (desiccant) materials. The cores aredesigned for efficient flow and efficient drying. They are shaped, then fused together at high

temperatures into a porous ceramic structure which must be:

1. Capable of absorbing moisture

2. Noncorrosive

3. Nonsoluble4. Nonreactive with oil

5. Neutralize for hydrochloric and hydrofluoric acid

6. Capable of filtering out particles down to 10 micron in size (0.025 in.).

Filter-Dryers (Suction Line)

Filter-driers are often mounted in the suction line to prevent foreign particles of over 5 microns

in size as well as acids, sludge and moisture, from entering the compressor. Strainers (screens)

are usually made of Monel metal.

Only two things should be allowed inside a refrigeration system: clean, dry refrigerant and good,

dry oil. A system which is clean, dry and acid-free will run almost indefinitely without corrosion,

freeze-ups, oil breakdown or hermetic motor burnouts. In such a system, there is nothing to filterand plugging is impossible. A clean, dry, acid-free system remains factory bright and trouble free

in operation.

A normal system is completely clean. A dirty system is faulty and must be regarded as a

mechanical failure, just as much as a faulty valve plate or connecting rod. A large capacity

suction line filter-dryer is shown in Fig 6-19.


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Fig 6-19:- A large capacity suction line filter dryer. Note Schrader valves at both end.

These are used to check the pressure drop through the filter dryer.

A suction line filter-dryer should be replaced if pressure drop is excessive. Replace for R-12 and

R-500 refrigerants if it exceeds 2 psi for low-temperature units, up to 8 psi for high-temperature

units.

Low Temp. psi Medium Temp psi High Temp. psi

R-12 and R-500 2 6 8R-22 and R-502 3 9 14

Replace for R-22 and R-502 refrigerants if pressure drop exceeds 3 psi for low-temperature

units, up to 14 psi for high-temperature units.

The density of the gas increases more with a pressure increase than it does with a temperature

rise. The best way to know that a system is dry is to use and depend on a moisture indicator.

chapter 6 full

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