Page 185 - Boiler_Operators_Handbook,_Second_Edition

170 Boiler Operator’s Handbook

Back Pressure Control Valves cycling on and off on the low-pressure switch because
If the system has various loads back pressure con- there is inadequate refrigerant in the system. It’s readi-
trol valves can be used to maintain a higher saturation ly resolved by adding refrigerant but doing so requires
pressure in evaporators that do not maintain freezing that EPA license and you may need to bring in a licensed
temperatures. That prevents the need for defrosting technician to do it. Checking for spots of oil that are in-
those evaporators. Back pressure control valves are also dicative of a refrigerant leak before calling that techni-
used in temperature sensitive applications where the cian can help you reduce the cost of his services because
product being cooled is damaged by colder tempera- he’ll know the extent of work necessary if you spot a
tures. Back pressure control valves simply restrict the leak. A leak means a considerable amount of work has
flow of gas (vapor) from the evaporator to keep the pres- to be done to return the system to normal.
sure, and therefore the related saturation temperature, A high pressure switch is normally connected to
above the preset minimum value. The throttling device the hot gas line near the compressor discharge. You
controls the flow of refrigerant independent of the back may hear this referred to as a high head pressure switch
pressure control valve. I’ve had to reset the valves sev- because they normally sense the pressure at the com-
eral times with changes in cargo. pressor head. In systems with multiple compressors
you will normally find a combination high and low gas
Limit and Operating Controls pressure switch mounted at each compressor with con-
Loss of refrigerant, accidental crimping of tubing, a nections inside each compressor’s isolating valves for
dirty condenser, an iced up evaporator, and loss of con- exclusive control of that compressor. One purpose of
denser air or cooling water flow can result in damage to the high-pressure switch is to shut down the compres-
the equipment, especially the compressor. Limit controls sor before the motor is overloaded. The high-pressure
are provided on systems to prevent damage to equip- switch should always be set at a value equal to or lower
ment in the system by shutting down the compressor than the maximum design pressure of the compressor
whenever operating pressures or temperatures exceed discharge, hot gas piping, condenser, and liquid line. A
preset limits. These devices are either mounted at strate- high-pressure switch also prevents accidental operation
gic locations in the system or are incorporated in remote continuing when a discharge isolating valve is closed—
sensors wired to the system controls. You can think of just in case you forgot to open it. Regardless, you should
each of them as a device operating a simple switch that make a habit of checking to ensure the valves are open
turns a compressor on or off. Process refrigeration may before trying to start the compressor. It’s a bit difficult
include other devices not listed here and you should, I’ll with packed valves because you have to remove the
say it again, read the instruction manual so you under- caps but my experience tells me it’s always advisable
stand their purpose and operation. to check, especially if you’re not the only one operating
The most common limit device is a low-pressure that equipment. The combination high and low pressure
switch which is connected near the outlet of the evapo- switch also shuts down the compressor when you forget
rator and opens an electrical circuit when the pressure to open the suction isolating valve.
in the evaporator falls below the switch setting. When A high pressure switch may be provided on the
evaporator loads are low it works more like a control condenser in systems with multiple compressors and
switch. A typical situation would be a windshield de- condensers that can be isolated. In the event of a failure
frosting condition in your automobile in the wintertime of a reversing valve they’re also used on heat pumps.
because the air conditioning system is operated even if You might also encounter a high temperature switch
the temperature inside the vehicle is below the setting that senses the temperature of the refrigerant in the
of a thermostat. Of course that assumes you have a ther- compressor head and shuts down the compressor if that
mostat to control your automobile air-conditioning. temperature gets too high. Low system pressure, loss of
In larger systems a second low pressure switch lubrication, or loss of refrigerant flow through the com-
may be included, called a “low-low” or “limit” switch pressor could produce high head temperatures without
and that switch would require manually resetting it to a high head pressure.
restore system operation. Low oil pressure switches are provided on almost
Operation of the low-pressure switch when a sys- all large compressors. They are occasionally incorporat-
tem is not maintaining the set temperature is indicative ed with the combination high and low refrigerant pres-
of insufficient refrigerant. Checking you should find that sure switches into one assembly. Equipment such as re-
evaporator pressure remains low in the system while ciprocating compressors may have an integral oil pump