Refrigeration and Air Conditioning Equipment Cooling Episode 2 docx

5 Frontseat the compressor discharge and suction shutoff valves.. Purge the lines and valves; then open two turns off the backseat the liquid line shutoff valve.. Start the compressor an

Figure 19 Single-phase wiring diagram for a semihermetic condensing unit.

gauge (registering low side pressure) registers 2 p.s.i.g

(4) Stop the compressor and watch the gauge If

the pressure rises, pump down again Repeat the

operation until the pressure remains at 2 p.s.i.g

(5) Frontseat the compressor discharge and suction

shutoff valves

(6) If the compressor is to be left pumped down for any period, tag the disconnect switch to prevent accidental starting of the unit

5 If the compressor is the only component to be removed, pumping down the crankcase will be sufficient This may be done by front-seating the suction shutoff

valve and completing steps (1)-(5) listed under pump-down procedure.

You must stop the compressor several times during

pump-down to prevent excessive foaming of the oil as

the refrigerant boils out since the foaming oil may be

pumped from the crankcase

6 Breaking refrigerant connections When it

becomes necessary to open a charged system, the

component or line to be removed or opened should be

pumped down or evacuated to 2 p.s.i.g You must allow

enough time for all adjacent parts to warm to room

temperature before you break the connection This

prevents moisture from condensing on the inside of the

system

7 After the component has warmed to room

temperature, you are ready to break the connection and

make the necessary repairs

8 Cleaning the expansion valve strainer To clean

the expansion valve strainer, you must close the liquid

line shutoff valve and pump down the system to 2 p.s.i.g

Disconnect the valve and plug the tube ends Remove

the screen and clean it with a recommended cleaning

solvent After the screen is clean and dry, reinstall it in

the valve and connect the valve in the system Purge the

lines and valves; then open (two turns off the backseat)

the liquid line shutoff valve

9 Cleaning suction strainers Most suction strainers

are located in the suction manifold on the compressor

Pump down the compressor to 2 p.s.i.g and frontseat the

discharge shutoff valve At this point, you must check

the manufacturer’s handbook to locate the strainer

Remove and clean it with solvent After the strainer

drys, replace it, purge the compressor, and start the unit

Figure 20 shows two different types of strainers, basket

and disc, and their location in the compressor motor

10 Purging noncondensable gases Noncondensable

gases (air) collect in the condenser (water-cooled) above

the refrigerant The presence of these gases cause

excessive power consumption, a rise in leaving water

temperature, and high compressor discharge pressure

11 To purge these gases from the system, stop the

compressor for 15 to 20 minutes Then open the purge

cock (if available) or loosen a connection at the highest

point of the condenser for a few seconds After purging

is completed, close the purge cock (or tighten the

connection) and run the compressor If the discharge

pressure is still high, repeat the procedure until the

discharge pressure returns to normal

12 Adding oil Add only the recommended oil

listed in the manufacturer's handbook The oil should be

taken directly from a sealed container Do not use oil

that has been exposed to the atmosphere because it may

contain some absorbed moisture

13 To add oil, pump down the compressor to 2 p.s.i.g Remove the oil filter plug (if available) or disconnect the pressurestat connection on the suction manifold Insert a funnel and pour in the oil Hold the oil container close to the funnel to minimize contact with the air The correct amount of oil needed can be estimated by observing the oil sight glass (one-third to two-thirds full) After sufficient oil is added, connect the pressurestat or replace the oil filler plug, purge the compressor, and start the unit

14 Removing oil To remove excess oil from the

crankcase, pump down the compressor to 2 p.s.i.g Loosen the oil plug (if available), allowing the pressure to escape slowly Then use a hand suction pump to remove the desired amount of oil If a filler plug is not available, loosen the bottom plate or drain plug Retighten the plate or plug when the oil assumes a safe level in the crankcase one-third to two-thirds full Purge and start the compressor

15 Testing for leaking valves Leaky compressor

valves will cause a serious reduction in the capacity of the system Install a manifold and gauge set Start the compressor and allow it to run until it is warm; then frontseat the suction shutoff valve Pump down the compressor to 2 p.s.i.g Stop the compressor and quickly frontseat the discharge shutoff valve Observe the suction and discharge gauges If a discharge valve is leaking, the pressures will equalize rapidly The maximum allowable discharge pressure drop is 3 p.s.i.g per minute

16 There is no simple method of testing suction valves If there is an indicated loss of capacity and the discharge valves check properly, you must remove the head and valve plate and check the valves physically

17 Disassembly, inspection, and reassembly of valve plates Pump down the compressor to 2 p.s.i.g and

remove the compressor head capscrews Tap the head with a wooden or plastic mallet to free it if it is stuck and remove the cylinder head

18 Remove the discharge valves and valve stops as shown in figure 21 Free the valve plate from the dowel pins and cylinder deck Many valve plates have tapped holes The capscrews are screwed into them and function as jacking screws Now you can remove the suction valves from the dowel pin Figure 22 shows the suction valve and suction valve positioning spring Inspect the valve seats and valves If the valve seats look worn or damaged, replace the valve plate assembly (fig 21)

19 It is preferable to install new valves with a new valve plate If new valves are not available, turn the old valves over and install them

12

Figure 20 Suction strainers

with the unworn seat toward the valve seat If the valve

seats and valves are not noticeably worn, it is still good

practice to turn the discharge valves; otherwise they may

not seat properly

20 The suction valves are doweled and may be

reinstalled as they were originally You must never

interchange valves Be careful when replacing the suction

valves The positioning springs must be placed on the

dowels first Place them with their ends toward the

cylinder deck and the middle bowed upward

21 Worn valves may be reconditioned by lapping

them, using a fine scouring powder and a piece of glass

Mix refrigerant oil with the powder to form a liquid paste Then move the valve in a figure 8 motion over the paste and glass After the valve is reconditioned, clean and reinstall it

22 Use new valve plate and cylinder head gasket when you install the valve plate and cylinder head

23 Disassembly, inspection and assembly of the oil pump and bearing head Remove the oil pump cover,

shown in figure 23 This will free the oil feed guide retainer spring and the oil feed guide Then remove the oil pump drive segment

Figure 21 Valve plate assembly.

24 After you remove the bearing head you can

remove the plunger snaprings which hold the plunger,

plunger spring, and guide spring in the pump plunger

cylinder Snapring or jeweler's needle-nose pliers are

recommended for removing the shapings

25 Push the pump rotor out of the bearing head by pressing against the bearing side of the rotor The rotor retaining ring will come out with the rotor Installing a new pump and bearing head is the only positive way of eliminating oil pump trouble However, if the cause of the trouble is determined, replacement parts are available for almost all compressors

26 The first step in installing the oil pump and bearing head is to install the rotor retaining ring in the ring groove of the rotor, with the chamfered edge toward the compressor Compress the retaining spring and insert the pump rotor into the bearing head

27 The plungers (flat ends in), plunger springs, spring guides, and snaprings are installed in the plunger cylinders Compress the snaprings and force them into their grooves Place a new bearing head gasket and the bearing head into position and bolt them to the crankcase Install the drive segment Be careful not to forget the lockwashers (shown in fig 23) Insert the oil feed guide with the large diameter inward Place the guide spring so that it fits over the

Figure 22 Suction valve positioning spring.

Figure 23 Compressor breakdown.

small diameter of the oil feed guide; then install a new

pump cover gasket and pump cover

28 Disassembly, inspection, and assembly of the

eccentric shaft and pistons Remove the oil pump and

bearing head previously described Remove the motor

end cover, being careful not to damage the motor

windings Do not allow the cover to drop off You must

support it and lift it off horizontally until it clears the

motor windings Remove the bottom plate and block the

eccentric so that it will not turn Remove the equalizer

tube and lock screw assembly from the motor end of the

shaft Look at figure 23 for the location of these components

29 Pull the rotor out, using a hook through the holes on the rotor Do not hammer on the motor end of the shaft or rotor since this may cause the eccentric straps or connecting rods to bend

30 Remove the bolts holding the counterweights and eccentric strap shields onto the eccentric shaft (Refer to fig 24 during these procedures.) Remove the eccentric strap side shields and the pump end counterweight through the

Figure 24 Removing counterweights and eccentric strap shields.

bearing head opening The motor end counterweight will

hang on the eccentric shaft until the shaft is removed

Pull the eccentric shaft through the bearing head opening

Rotate the shaft, tapping it lightly to prevent the eccentric

straps from jamming Guide the straps off the shaft by

hand The eccentric straps and pistons are removed

through the bottom plate opening

31 The piston pin is locked in place with a lockring

The pin can be removed by tapping lightly on the

chamfered end of the pin (the end not having a lockring)

32 Examine the parts to see that they are not worn beyond the limits given in the manufacturer's handbook

To reassemble, follow the disassembly instructions in reverse order

33 Terminal assembly Refer to figure 25 for the

relative positions of the parts The washers

Figure 25 Terminal block breakdown.

are usually color coded and slightly different in size

Assemble them as shown

34 The terminal mounting plate assembly is

originally installed with a small space left between the

outer terminal block and the surface of the mounting

plate This provides further tightening of the terminal

bushing in case of a leak To stop a leak, tighten the

terminal block capscrews only enough to stop the leakage

of gas Do not tighten the capscrews so that the terminal block is flush with the mounting plate If further tightening will cause this situation, the terminal assembly must be replaced

35 To replace the assembly, pump down the compressor to 2 p.s.i.g and remove the assembly Install the new assembly, using the recommended

torque on the capscrews (1.5 ft lbs.); purge and start the

compressor Avoid excess torque since terminal block

and components are generally constructed of plastic or

bakelite

36 Testing capacitors and relay The starting

capacitor used in single-phase units is wired as shown in

figure 19 Capacitors are connected in series with one

power lead to the motor starting winding These

capacitors may fail because of a short or open circuit If

they are short circuited, the starting current draw will be

excessive The compressor may not start and will cause

fuses to blow because of the increased load If it is

connected in a circuit feeding lights, the lights will dim

A humming sound from the compressor motor indicates

improper phasing between the starting and running

windings caused by an open-circuited capacitor To check

starting capacitors, replace them with good capacitors and

observe the operation of the unit

37 The running capacitors are connected across the

running and starting terminals of the compressor If

short circuited, they will allow an excessive current to

pass to the start winding continuously The compressor

may not start If it does, it will be cut off by the motor

over-load switch If they are open, the compressor will

operate, but will draw more power than normal when

running and will stall on heavy loads To test for

open-circuited capacitors, an ammeter should be connected in

series with one power lead With good running

capacitors, the current requirement will be less than it is

when the capacitor is disconnected An open capacitor

will cause no change in current draw when it is

disconnected

38 The relay is the potential or voltage type The

contacts are normally closed when there is no power to

the unit and open approximately one-fifth of a second

after power is applied The operation of the relay

magnetic coil is governed by the voltage through its

windings Upon starting, the counter EMF of the motor

builds up, causing a rise in voltage through the relay coil

As the voltage across the coil rises, the magnetic

attraction of the relay arm overcomes the spring tension

This causes the arm to move and force the relay contacts

open The starting capacitors, which are in series with

the starting winding when the relay contacts are closed,

are disconnected from the circuit

39 If the relay fails with the contacts open, the

starting capacitors will not be energized The compressor

motor will hum but will not start After the power has

been on for 5 to 20 seconds, the overload relay will cut

off the power to the compressor motor

40 To check the relay for contacts that fail to close,

put a jumper across the relay contacts and turn on the

power If the unit starts with the jumper, but will not start without it, you must replace the relay

41 When the relay fails with the contacts closed, the starting capacitors will continue to be energized after the compressor has come up to speed The compressor will start but will run with a loud grinding hum The overload relay will shut the compressor off after the compressor has run for a short time due to the extra load

of the start winding This type of relay failure can cause damage to the motor windings and the running capacitor

42 A visual inspection will determine if relay contacts fail to open Remove the relay cover and observe its operation If it does not open after the power has been applied for a few moments, you must replace the relay

43 Oil safety switch Many units have oil safety

switches which protect the compressor from low or no oil pressure This control has two circuits-heater and control

44 This switch measures the difference between oil pump discharge pressure and crankcase pressure If the net oil pressure drops below the permissible limits, the differential pressure switch energizes the heater circuit which will cause the bimetal switch in the control circuit

to open in approximately 1 minute Low oil pressure may result from the loss of oil, oil pump failure, worn bearings, or excessive refrigerant in the oil Figure 26 shows a typical oil pressure safety switch

45 The differential pressure switch is factory calibrated to open when the oil pump discharge pressure

is 18 p.s.i.g greater than the crankcase pressure It will close when the difference is 11 p.s.i.g Its adjustment should not be attempted

Figure 26 Oil pressure safety switch.

in the field If the differential pressure switch functions

properly and the compressor continues to run after 1

minute, the time-delay heater circuit is defective and the

oil pressure safety switch should be replaced The switch

should be checked monthly for correct operation

46 Troubleshooting One of your most important

responsibilities is the troubleshooting and correction of

malfunctions of these systems Throughout this chapter

we have given basic principles of D/X systems Using this knowledge and the information that we have provided in tables 1 through 10, you should have little trouble in achieving the desired skill levels

TABLE 1

TABLE 2

TABLE 3

19

TABLE 4

TABLE 5

TABLE 6

TABLE 7