This document discusses refrigeration system controls and compressor service valves. It describes how technicians use compressor service valves to check compressor reed valves, isolate the compressor, and check for compressor inefficiency. The document explains that front-seating the suction service valve allows technicians to perform troubleshooting procedures like checking the condition of reed valves, checking for high back-pressure overloading, isolating the suction side of the compressor for oil changes, and isolating both sides for valve plate inspection and compressor replacement. Safety precautions for isolating and replacing compressors are also outlined.
4. Compressor Service Valves
➢Compressor service valves are used for much
more than just checking system pressures.
➢Technicians also use compressor service valves
for checking compressor reed valves and isolating
the compressor.
➢Figure – 8 shows a larger-than-normal view of a
suction service valve for illustration purposes.
➢The position of the stem in this figure is called
back-seated, because the back of the valve stem
flange is against the seat.
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5. Service Valves Function
➢ All valves have a packing around the stem to prevent
refrigerant leaks.
➢ Some valves have a packing nut that should be
loosened a quarter turn before turning the valve stem
and then tightened afterward.
➢ The cover for the valve stem is not shown, but it is
called a dust cap. It keeps the valve stem clean.
➢ If dirt or rust collects on the valve stem, it will damage
the packing as the stem is turned in and out.
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6. Compressor inefficiency
➢One way to check for an inefficient compressor
is to front-seat the suction service valve and pull
the compressor into a vacuum.
➢A rise in crankcase pressure may be an indication
of bad reed valves inside the compressor.
➢A leaking service valve would also cause the
pressure to rise, giving a false diagnosis of leaking
reed valves.
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7. Fig.9(a): Valve is cracked or mid-seated
• In Fig.9(a), the valve is
cracked or mid-seated.
• By turning the valve
clockwise, the valve stem
• rotates inward, causing the
back of the valve flange
• to move away from its seat.
System pressure rushes
• into the area around the
stem and out the gauge
• port to the pressure gauge.
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8. Figure 6–9(b): Suction Valve Front Seated
• Fig.9(b) shows the
suction service valve
front-seated.
• The front of the valve
flange is seated, which
completely blocks off
the suction line to the
compressor.
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9. Trouble Shooting
In this position, you can perform the
following four troubleshooting procedures:
1. Check the condition of reed valves.
2. Check for high back-pressure overloading.
3. Isolate the suction side of the compressor
for oil changes.
4. Isolate both sides for valve plate inspection
and compressor replacement.
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10. Check the condition of reed valves
• Run the compressor until it pulls a vacuum in
the crankcase.
• Turn the compressor off and watch the
suction gauge.
• If the pressure starts rising, the suction valves
are leaking through or the
discharge vapour is leaking past the piston
and cylinder wall.
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11. Check for high back-pressure overloading
• Compressor overloading can occur on a
low-temperature unit during the initial start up.
• It can also happen in a freezer when it comes out
of defrost.
• In both the cases, the compressor starts, runs for
a short period, and then goes out on overload.
• The high back pressure (suction pressure) from a
warm evaporator (hot pull-down) is more than the
compressor can handle.
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12. Check for high back-pressure overloading
• By starting the compressor with the suction valve
front-seated, the compressor is starting
unloaded.
• With the compressor running, open the suction
service valve one turn at a time as you check the
compressor’s amperage.
• Stop opening when the amperage reaches the
compressor’s rated load amps (RLA), as listed on
the compressor’s electrical rating plate.
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13. Check for high back-pressure overloading
• As the box cools, the suction pressure and
amperage will fall.
• Continue opening the valve, while monitoring
RLA, until the valve is fully open.
• If the box cools down and if the compressor
cycles off on the t-stat and then restarts,
the compressor is probably going to be just
fine.
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14. Solution for Overloading
A compressor needs some help if it continues
to trip on overload when it is starting under high
suction pressure.
Solution
1. Install a crankcase pressure regulator
(CPR), or
2. Install a pressure-limiting thermal expansion
valve (TEV).
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15. Isolate suction side of compressor
for oil changes.
• If the suction valve is front-seated, all the
refrigerant in the low-pressure (LP) part of the
system is on the other side of the valve flange.
• The small amount of remaining refrigerant between
the service valve and the cylinder can be vented.
• The oil can now be changed, if the compressor is a
semi-hermetic.
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16. Isolate for valve plate inspection and
compressor replacement.
Fig. 10 – Isolated Compressor
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17. Isolated compressor
• In Fig. 10:
1. The suction service is front seated.
2. If the discharge service valve is also front
seated.
3. The pressure in the head is bled off.
4. The compressor is completely isolated from
both sides of the system.
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18. • Safety note:
• Never run the compressor with the discharge
service valve front-seated.
• It will immediately blow the valve plate
gasket.
• Or worse, it could pump dangerously high
pressure into your gauge hose if it is
connected to the service valve.
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19. • Safety note:
➢Make sure all the pressure is bled off the
compressor before unbolting the
compressor head.
➢It only takes a few pounds of pressure to blow
the head into the air.
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20. Compressor Replacement
1. Unbolt the service valves from the compressor.
2. Remove the old compressor.
3. Set the new compressor in place.
4. Re-bolt the service valves.
5. Pull a vacuum on the new compressor.
6. Back-seat the service valves.
7. You’re ready to start it up.
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21. Review Questions
(1) State the procedures for isolating a
compressor for:
a. Valve plate inspection
b. Compressor replacement
(2) What safety precautions should be followed
when isolating or replacing a compressor?
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