What categories are special refrigeration system components divided into?
Controls and accessories.
What categories are control components divided into?
Mechanical, electrical and electromechanical.
What do mechanical controls generally do and how do they do it? Where are they usually found?
They stop, start or modulate fluid flow and are operated by pressure, temperature, and electricity. They are usually found in the piping.
Describe an EPR valve.
An evaporator pressure-regulating valve is a mechanical control that keeps refrigerant pressure in the evaporator from going below a predetermined point.
What is another name for a holdback valve?
An EPR is called a holdback valve because it holds refrigerant back in the evaporator in order to keep the suction pressure above a set point.
How does an EPR work?
There is a spring attached to a disc. When the pressure is above the set point, there is enough pressure to push the disc and spring in and keep the EPR open and refrigerant can flow freely from the evaporator to the suction line. If the pressure drops be
What are the two forces in an EPR?
The evaporator pressure and the pressure of the spring.
T or F - The spring pressure is a opening force.
False, it is a closing force.
For a system with multiple evaporators, what is used instead of an EPR on the lowest temperature evaporator to keep refrigerant from flowing into it?
A check valve.
What is an EEPR and how does it differ from an EPR?
EEPR stands for electric evaporator pressure regulating valve. It is operated by a bipolar step motor that provides very close temperature control of the discharge air in supermarket display cases. They can move in very small increments, either opening or
What is a CPR?
It stands for crankcase pressure-regulating valve. It is in the suction line close to the compressor and is used to keep low-temperature compressors from overloading during a hot pulldown.
What are the three occurrences for a hot pulldown?
1. When the compressor has been off for a long time and the foodstuff has a rise in temp
2. On startup of a warm box
3. After a defrost period.
What are relief valves and what types do they come in?
Relief valves are designed to release refrigerant from a system when predetermined high pressure exists. They come in spring-loaded type (that reseats itself) and one-time type (that does not close back).
What is the purpose of a low-ambient control?
Its purpose is to keep head pressure up when the ambient air temperature is low like during the winter.
Name the four methods for low-ambient control
1. Fan cycling using a pressure control
2. Fan speed control
3. Air volume control using shutters and fan cycling
4. Condenser flooding devices
Describe fan-cycling pressure control
Using pressure and temperature controls, a system will turn off condenser fans (if there are multiple fans) when head pressure/temperature is below the preset limit. As pressure starts to rise, it will close the pressure control and start the fan(s) as ne
Describe fan speed control
As temperature drops on a cool day, the condenser fan motor speed is reduced. Less cfm will increase head pressure as less heat is rejected. As the temperature increases, the speed will increase.
Describe Air volume control
Using shutters and dampers, the amount of air that moves across the condenser coil can be decreased when the head pressure drops. As the pressure increases, the dampers will open. This system results in steady head pressure with no large fluctuations that
Describe condenser flooding for controlling head pressure
Used in mild and cold weather that causes the refrigerant to move from an oversized receiver into the condenser. This acts like an overcharge of refrigerant and causes the head pressure to be much higher. This system gives a very steady control with no fl
What is the most frequently used component to control fluid flow? Describe it.
A solenoid valve. It is a valve with a magnetic coil that can be energized to lift a plunger into the coil to control liquid or vapor flow. They are either normally open or normally closed.
What is the name of the solenoid used for larger applications?
A pilot acting solenoid.
Name four type of pressure switches.
1. A low-pressure switch- closes on a rise in pressure
2. A high-pressure switch- opens on a rise in pressure
3. A low-ambient control- closes on a rise in pressure
4. An oil safety switch- has a time delay and opens on a rise in pressure
What are two applications that a low-pressure switch does?
1. Low-charge protection - it cuts out when pressure is below for a typical operating evaporator. Usually when there is a loss of refrigerant and is needed to keep the low side from going into a vacuum.
2. Control of space temperature - As the temperature
T or F - An automatic pumpdown system consists of a normally opened liquid-line solenoid valve.
False- it consists of a normally closed liquid-line solenoid valve. When the t-stat is satisfied, it will open and deenergize/close the solenoid valve. The compressor continues to run and will evacuate refrigerant from the solenoid to the compressor, stor
On a call for cooling, what does the liquid line solenoid do?
It energizes and sends liquid refrigerant to the expansion valve and into the empty evaporator.
When leak testing a system with an automatic pumpdown, why should the liquid-line solenoid be energized?
To prevent the trapping of refrigerant in the condenser and receiver.
What are the three main reasons that automatic pumpdown systems are used on refrigeration systems?
1. To rid the evaporator, suction line and crankcase of refrigerant before the off/defrost cycle so migration of refrigerant to the compressor or crankcase is prevented.
2. To prevent surges of liquid refrigerant from entering compressor (slugging) during
What is the basic duty of a high-pressure control?
It keep the compressor from operating with a high head pressure.
Would you more likely find a high-pressure control on an air-cooled system or a water-cooled system?
You would find it more likely on a water-cooled system since it is more likely to have an interruption of water than air.
How does a high-pressure control work? What would be an example that would cause it to trip?
It opens on a rise in pressure. It must be set above the typical highest pressure that a system would normally encounter. An example would be if the condenser coil became very dirty and was not able to reject heat. The temperature in the coil would rise a
What does the low-ambient fan control do?
It stops and starts the condenser fan in response to head pressure. If the head pressure is below the cut-in pressure, the low-ambient fan control would stop the condenser fan to allow the head pressure to build up.
Where do you usually find oil pressure safety controls and how do they work?
They are normally found on larger compressors that have forced oiling systems. Since these systems have oil pumps that force oil through holes and onto the crankshaft and bearings, the oil pressure safety control ensures that the compressor has oil pressu
What is the equation for net oil pressure?
OIl pump discharge pressure - crankcase pressure = net oil pressure. It usually ranges from 20 to 40 psi. It is measured in psid.
Why is there a time delay built into an oil pressure safety control?
It is built into the control to allow the compressor to get started and up to speed and to prevent any premature and unneeded cutouts when oil pressure may vary for only a moment.
What does a current sensing relay do with an oil safety controller?
It allows the compressor to cycle on the internal overload while not affecting the operation of the oil safety controller.
How does a current sensing relay work with a oil safety control?
If there is a brownout and a lowering of voltage going to the compressor, the oil safety switch would trip due to the drop in oil pressure. With a current sensing relay, when there is a drop in voltage that causes the oil pressure to drop, the relay check
What is the high, medium, and low temperature ranges?
High is 47 - 60 degrees, medium is 28 - 40 degrees and low is 0 - (-20) degrees.
What is the rule of thumb for how much lower the coil temperature is below the evaporator temperature?
15 degrees.
What is off-cycle defrost?
The compressor is turned off, the evaporator fans will continue to run and the air in the cooler (that should be warmer than the ice on the coil) will defrost the coil.
What is a planned defrost?
By using a timer, the compressor is forced to shut down for short periods of time so that the air in the cooler can defrost the ice from the coil. It is usually programmed to defrost when the cooler is under the least amount of load.
Why can't an off-cycle defrost be used in a low-temperature cooler?
Since a low-temperature evaporator operates well below freezing, the air inside the box is also well below freezing. Heat must be added to the evaporator for defrost.
How does internal heat method defrost work?
It uses the hot gas from the compressor. It can be introduced into the evaporator from the compressor discharge line to the inlet of the evaporator and allowed to flow until it is defrosted. It is an energy saving option. It is started with a timer and te
What is an external heat defrost?
External heat defrost uses electri heating elements that are factory mounted next to the evaporator coil. It is usually a planned defrost that is controlled by a timer. It is not as efficient as an internal defrost since it has to use purchased energy for
What are the 4 steps for a electric defrost?
1. Evaporator fan stops
2. Compressor stops
3. Electric heaters energize
4. Drain pan heaters may be energized
Why is there an evaporator fan delay after a defrost?
It is to allow the heat in the evaporator coil to leave and become chilled again. That way the heat will not be distributed into the cooler.
What component can store all of the systems refrigerant and still have room for 20% more?
A receiver.
Where is a receiver located and what does it do to the refrigerant besides the ability to store it?
It is located on the liquid line. It assures that the refrigerant is 100% liquid before it gets to the metering device.
Where is a king valve located?
It is located on the outlet of the receiver.
What does front seated, cracked, and back seated mean to a king valve.
A front seated king valve means that the valve is closed off and no refrigerant can leave the receiver. A cracked king valve means that the valve is partially open and that gauges attached to the king valve can read the liquid line pressure. A back seated
Where is a liquid line filter drier usually found?
It can be found anywhere after the king valve.
What is the job of the filter drier?
It removes foreign matter from the refrigerant such as dirt, moisture, acid, solder beads and filings.
What are the three types of filtering agents found in filter driers and what do they take care of?
1. Activated alumina- absorbs acid and some water removal
2. Molecular sieve - best for water removal
3. Silica gel - used for water removal
What does a check valve do?
It allows refrigerant to flow in only one direction.
Where is the best location for a sight glass to ensure 100% liquid is going to the TXV.
Place it right before that TXV on the liquid line.
What component is used with multicircuit evaporators?
A refrigerant distributor. It is fastened to the outlet of the expansion valve.
What is a heat exchanger and where is it located?
It serves two purposes. It improves the capacity of the evaporator by subcooling the liquid refrigerant entering it. The cooler the refrigerant entering the expansion device, the more net refrigeration effect in the evaporator. It also prevents liquid ref
T or F - the main purpose of a suction-line accumulator is to prevent liquid refrigerant from passing into the compressor.
True- the accumulator collects the liquid refrigerant before it gets to the compressor and gives it a place to boil off to a gas before continuing to the compressor.
What is used in the J tube of an accumulator to prevent liquid refrigerant from being sucked into the compressor?
A pressure equalization orifice near the top of the tube prevents the liquid refrigerant from going up the J tube and into the compressor.
What component is usually attached to the front of a compressors?
A suction service valve.
If the discharge service valve is front seated and the compressor is turned on, what damaging event can happen to the compressor?
The head of the compressor may blow off.
If both service valves are front seated, what can be done to the compressor?
The compressor is isolated and can be removed.
What and where is an oil separator?
It is installed in the discharge line to separate the oil from the refrigerant and return the oil to the compressor crankcase.
What part of an oil separator keeps hot gas from being transferred to the compressor crankcase?
A small float at the bottom of the separator keeps the return line opening covered and only opens when the oil level is high enough to float it. This ensures that only oil will be returned to the crankcase.
If the float is damaged or collapsed, what can happen to the oil.
It will cause the oil return orifice to stay closed and excessive amounts of oil to accumulate in the separator. This can cause a drop in oil lubrication in the compressor and damage it.
What can be used to prevent damage to tubing connected to large compressors?
Using vibration eliminators installed close to the compressor can stop much of the vibration associated with compressor action. They are made of bellows-type lining and a flexible outer protective support.
How can heat be applied to the compressor to keep the refrigerant from migrating to the oil during the off cycle or long periods that the system is turned off?
Use crankcase heat. One option is a built-in crankcase heater that connects to the contactor. It is normally closed when the contactor is open. The other option is an external heater that is placed around the base of the compressor.