Modern Refrigeration - Part 3
Cryogenic fluids range in temperature from what?
-250 degrees Fahrenheit to absolute 0, or -460 degrees Fahrenheit. Such low temperatures can easily be reached by evaporating cryogenic fluids like Hydrogen or R-702,
The oil, with R-22, will remain fluid enough to flow down the suction line at temperatures down to what?
-40 degrees. However, around this temperature or not far below it, the oil will begin to separate from the refrigerant. Because oil is lighter, it will collect on surface of liquid refrigerant.
R-22's boiling point at atmospheric pressure is what?
-41 degrees Fahrenheit. The latent heat of R-22 at 5 degrees Fahrenheit is 93.2 BTUs per pound.
The temperature of a typical frosting-type evaporator, a domestic type, will vary from what to what?
0 degrees Fahrenheit to 25 degrees, and the refrigerant temperature will be about 10 degrees lower than this, or in the range of -10 to -15 degrees, while unit is running.
A replacement motor may have a max current draw of what?
10% larger, but never less than the motor being replaced
A thermistor is located in the front of the ice cube tray. It senses the temperature at that location. When the thermistor senses a reduction in temperature below what?
13 degrees, it sends a signal back to the solid state control circuit. The control system then triggers a relay which completes the circuit to drive the motor, initiating the harvest cycle.
Considered to be a low-pressure refrigerant, R-11 has a low-pressure of what?
24 inch vacuum at 5 degrees Fahrenheit, and a high-side pressure of 18.3 p.s.i.a. or 3.6 p.s.i.g. at 86 degrees. As much as 35 pounds of this refrigerant may be used for
The starting winding should only be energized for how long?
3 to 4 seconds at a time. If current flows through it for a longer period, the winding may overheat.
The temperature of the refrigerant in an air-cooled condenser is approximately how much warmer than the room temperature?
30 to 35 degrees. The temperature of the refrigerant in the condenser will be about the same as that of the cooling medium after the unit has been shut off for 15 to 30 minutes.
Describe the characteristics of R-717 or ammonia?
About 0.35 volumes per 100 volumes of air is the strongest concentration one can bear to breathe for any length of time. At 3 to 5 p.p.m., it is identified by smell.
What are some of the characteristics of R-12?
Boiling point of -21.7 degrees Fahrenheit at atmospheric pressure. It is soluble, or able to be dissolved, down to -90 degrees. This helps the oil flow in very
The connections, for a thermostat install, should be cleaned with what?
Clean steel wool before installing wires
Which has a greater coefficient of expansion, copper or steel?
Copper, in a bimetal, has a greater coefficient of expansion than steel, meaning it expands more with an increase in temperature than does steel.
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Current relays are sometimes called amperage relays, since it is the amp draw on the circuit that operates the relay.
To change p.s.i.g. to kilo pascals, first change p.s.i.g. to p.s.i.a. by adding 14.7. Then do what?
Divide the p.s.i.a. value by 14.7 and multiply by 101.3. Most technicians use the shortcut though of just multiplying ps.i.a. by 6.9
Water pipes, radiators, heating plants and the like are grounded. If the "live" wire, or black wire, comes in contact with one of these grounds, what happens?
Either a circuit breaker or a fuse will be blown. A person touching both a live wire and a ground will receive a very severe shock.
For a refrigerator, in the evaporator, the liquid refrigerant does what?
Expands and becomes a vapor. This vapor absorbs heat from the foods or beverages in the cabinet.
Frequently, a unit will short cycle because the thermostat is what?
Exposed to vibration, possibly on a shaky wall or stairs. Be sure to mount the T-stat to something solid.
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Has a boiling temperature at -452 degrees at atmospheric pressure. Carbon dioxide or R-744 has a boiling temperature of -109 degrees at atmospheric pressure.
Potential relays, sometimes called voltage relays, are usually used with what?
High-torque, capacitor-start motors. The contact points remain closed during the off-part of the cycle. As the motor speed increases, higher voltage from the start winding
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If the container is broken, the food rapidly spoils.
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If the freezer control cuts out before the desired cutout temperature is reached, the control may be adjusted to cut out at a lower temperature.
An electrical resistance heat wire, also called a perimeter drier, is located where?
In the trim of the freezer door, and operates all the time, providing enough 'warming effect' to stop condensation on the exterior of the cabinet and around the freezing compartment door
Describe R-500s mixture?
It is an azeotropic mixture of 26 percent R-152a and 74 percent R-12. The cylinder color is yellow.
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It is the right size and distance from the bimetal strips so that its heat opens the start winding contacts points as soon as the motor reaches its proper operating speed.
How can you reduce the interference of a radio or TV when a refrigerator starts or stops?
It may be reduced by grounding the frame of the motor to a water pipe or by placing a capacitor between the frame of the motor and the ground. Excessive radio interference
Fluids used as a refrigerant should have certain properties. These are what?
It shall be nontoxic, so not harmful if inhaled or spilled onto the skin, and nonpoisonous. It should be nonexplosive, noncorrosive and nonflammable.
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It should be easy to detect, and easy to locate. It should operate under a low pressure, or low boiling point. It should be a stable gas. Refrigerator or compressor parts
What does a heat anticipator do?
It's a small heater that warms the thermostat faster than the room air does. This enables the heating thermostat to shut off the furnace sooner than it otherwise would
The shaded pole produces a what?
Moving magnetic field perpendicular to the field pole. Approximately half of each pole face has a small copper plate insert, A, with a small winding. This insert slows down
If the resistance of the thermistor goes down as temperature increases, does it have a negative or positive temperature coefficient?
Negative temperature coefficient, or N.T.C. There are also P.T.C.s, where their resistance increases as temperature increases.
Is the insulation for the fresh food compartment and frozen food compartment both the same?
No, frozen food compartment must have thicker insulation.
Does the starting circuit, for a motor, throw the motor directly on the line, upon startup?
No. It brings into use a resistance or induction unit which restricts the flow of current at the instant of starting but allows an increase later as the motor speed increases.
If one must work in a damp or wet room, stand where?
On a dry, insulated platform. Rubber gloves and rubber-soled shoes or rubber boots, or rubbers provide considerable added protection.
Whatever moisture there is in the refrigerator, which is dry air, tends to collect and condense where?
On the evaporator surfaces. Therefore, food containers should be covered and as airtight as possible to keep foods moist. Temp. between 35 and 45 degrees.
To test a freezer t-stat, with the freezer plugged in and operating, do what?
Place the control bulb in the control tester. Adjust the refrigerant flow and pressure until the desired cutout temperature is indicated on the thermometer.
Refrigerator cabinets are made of what?
Pressed steel. The insulation is installed between the outer and inside shell. Urethane foam, when used, is expanded in-place and, therefore, fits with no crevices.
Sometimes, to free up a fan, you can do what?
Reverse the unit to make it run backward. This is accomplished by putting a capacitor in series with the run winding.
Is a positive temperature coefficient, or P.T.C. type thermistor connected in series or parallel with the motor windings?
Series. When an overload condition exists, the temperature increases and causes the resistance of the thermistor to go up.
What do brushless motors operate with?
Silicon rectifiers, transistors, and special circuitry
Oil which contains moisture or air will form what?
Sludge or varnish, and may cause damage to the unit. You want to fight against oil foaming.
If any of the cap tube is coiled, do what to prevent vibration?
Tape the coil.
A heavy rumbling sound at starting usually indicates what?
That the bearing is badly worn, even though the rotor may not be touching the field poles.
A current, or magnetic type, relay, uses what to operate it?
The electrical characteristics of the motor. As the rotor picks up speed, the magnetic fields build up and collapse in the motor. This produces a bucking, counter
A defrost control prevents what?
The formation of ice on the evaporator, by opening the circuit if the evaporator temperature is close to freezing.
In all cases, the condensing temperature will rise until the heat loss from the condenser equals what?
The heat input from the condenser. If condensing pressure is too high, the compressor has to work too hard. Too much vapor will be left in the compressor clearance pocket
What are the two basic systems used in automatic defrosting?
The hot gas system, through the use of solenoid valves, uses the heat in the vapor from the compressor discharge line and the condenser to defrost the evaporator.
How does food being in a refrigerator help keep that food fresh?
The lower temperatures slow down oxidation of the food, reduce the multiplication of the bacteria in the cells and fibers, and reduce evaporation, or loss of fluid, from the food.
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The magnetic current relay coil is then heavily magnetized. It lifts the weight or overcomes the spring pressure and closes the contacts. This action closes the starting winding
Ladder diagrams identify what?
The order of operation of specific devices, not their actual location.
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The other is an electrical heat strip.
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The pressure difference between evaporator pressure and condensing pressure should be as little as possible to increase pumping efficiency.
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The relay coil is made of small wire, so very little current passes through it. This minimizes the heating of the coil and core. Resistance of the relay coil to voltage must be
What are expendable refrigerants?
They are used once and then let out into the atmosphere. Liquid nitrogen or R-728 has a boiling temperature of -320 degrees at atmospheric pressure. Liquid helium or R-704
What are the characteristics of Group 3 refrigerants?
They form a combustible mixture when mixed with air. These are Butane or R-600, ethane or R-170, or propane or R-290.
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This control also serves as a safety cutout. If the motor should use too much current, the resistance wire will heat the bimetal strip. The heated bimetal strip will open the
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This decreases the magnetic strength of the magnetic current relay enough to allow the weight of the spring to open the points.
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This increase in resistance limits the current to the motor. After the motor and thermistor cool to a safe temperature, the motor can draw current to start up again.
Food has enzymes, colloids, water and a few microorganisms. What are enzymes?
Tiny particles of matter that exist in food substances. They are not destroyed by fast-freezing but their increase is slowed down by low temperatures. They seem to stimulate
What are some reasons moisture in the system is bad?
Water in a system may freeze at the refrigerant control and clog or partly clog it. Moisture and some refrigerants in the presence of high compressor temperatures may cause
On a thermal relay, what happens?
When cold, both sets of contact points are closed. A resistance wire is mounted near the bimetal strips. It is in series with both the start and run windings.
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When packaging food for the freezer, as much air as possible should be removed from the packaging.
High head pressure raises the temperature of the vapor and oil moving past the compressor exhaust valve to a point which what?
Which may cause oil and refrigerant breakdown. This condition is worsened if a little moisture and dirt are present. Carbon, acids and sludges may be formed.
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and prevents the room from becoming too warm. In a cooling system, heat is added to the T-stat during the off-cycle, before the room gets warmer than it should.
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circuit and the motor will quickly accelerate, or speed up, to 2/3 or 3/4 of the rated speed. As it does so, the amp draw of the run winding of the motor decreases.
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cold temperatures. The coil will begin to separate at this temperature and, because it is lighter than the refrigerant, will collect on the surface of the liquid refrigerant. Cylinder=white
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contact points and stop the motor.
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creates more magnetism in the relay coil, pulling the contact points apart, opening the start circuit. The relay coil is connected across the start winding.
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each 1,000 cubic feet of air conditioned space. This would be a room about 10 feet by 12 feet by 8 feet. Its cylinder code is orange.
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electromotive force, or e.m.f., or voltage on the run winding. The run winding consumes more current when the rotor is not running, or is turning slowly, than it does
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helium or R-704, Neon or R-720, Nitrogen or R-728, air, or R-729, Oxygen or R-732, and Argon or R-740.
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high enough to prevent the contact points from opening before the motor reaches 80 to 90 percent of its full rated speed. But such resistance must be low enough to
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in the rotor. The rotor turns as it is attracted by the magnetism. Movement of the rotor will continue as the A.C. changes the polarity of the poles and the rotor.
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lowering its volumetric efficiency. The temperature of the exhaust vapor will be too high and may cause oil contamination.
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moving in the fluid should be easy to lubricate. It should have a high liquid volume per pound to provide durable refrigerant controls. It should have a high latent heat
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of a continuing nature when the refrigerator motor operates, or when it starts, indicates a loose electrical connection or some fault in the mechanism of the motor.
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organic change. They are destroyed by pasteurization, which is the destroying of pathogens through addition of heat. Colloids seem to be cell containers or capsules.
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output voltage of the bridge changes.
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per pound to produce good cooling effect per pound of vapor pumped. It should have a low vapor volume per pound. This will reduce compressor displacement needed.
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positively open the points and remove the start winding from the circuit at the right time. If not, the motor will overheat.
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run winding, as it goes from a start condition to run. When the motor control, maybe a thermostat or pressure stat, contacts close, the high current flows into the run winding.
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sensor heater circuit. The harvest cycle is complete after 180 degrees of tray rotation.
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the building of the magnetic field through the copper plate just enough to cause a magnetic motion toward the copper plate. This action produces a lag for induced magnetism
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the refrigerant to break down and may form harmful acids. It may cause rusting, corrosion or oil slugging, which could results in motor burnout in a hermetic system.
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when it reaches full-speed. Current-operated relay switches, used to close and open the start winding, operate on the change in current flow to the
If the rotor is hitting the stator, then what?
Bearings are worn out, and must be replaced.
In an icemaker, during the freezing cycle, the temperature in the freezing compartment is about what?
8 degrees Fahrenheit or lower. After the water and temperature sensor are gradually cooled at the same rate to 11 degrees, the cold cut-in T-stat closes to start a harvest cycle.
One pound of water absorbs how many BTUs in changing from a liquid at 212 degrees to a vapor, or steam, at 212 degrees?
970 BTUs
What are the characteristics of Group 2 refrigerants, and name some?
Ammonia or R-717, Methyl chloride or R-40, and sulphur dioxide or R-764. These are toxic, irritating to breathe, and slightly flammable.
Describe R-502s mixture?
An azeotropic mixture of 49 percent R-22, and 51 percent R-115. The cylinder color is orchid.
Changes in a thermistor's resistance are detected by what?
An electronic circuit called a wheatstone bridge. As the resistance changes due to a change in temperature, the resistances in the bridge become unbalanced, as the
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At 15 p.p.m., the odor is quite irritating. At 30 p.p.m., the service tech will need a respirator. Leaks easily detected by white fumes it forms in the presence of sulphur candle or sulphur spray vapor.
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At 90 degree rotation, the sensor temperature rises to 35 degrees. At 120 degrees rotation, the temperature rises to 59 degrees and the T-stat opens the temperature
Foods to be kept for a year or more should be frozen at what temperature?
At least -20 degrees Fahrenheit or lower.
Oil gives lubrication and cools what?
The compressor's moving parts. It is cooled to low temperature, yet must withstand high temperatures in the compressor. It must remain fluid in all parts of the system.
A thermostat attached to the evaporator opens what?
The defrost heater circuit to end defrosting as soon as the evaporator reaches a temperature of 50 degrees Fahrenheit. It closes when it reaches 20 degrees.
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The drive motor starts and rotates the ice ray, at a speed of 1/4 R.P.M. A 22 watt heater in the temperature sensor is energized and heats the temperature sensor.