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What is the function of any type of refrigeration Compressor?
To compress low-temperature, heat-laden
gas into high-temperature, high-pressure
The process of a solid turninq directly into a
vapor without going to a liquid stage is called
Define a Btu and a calorie.
A Btu is the amount of heat required to raise one pound of water one degree Fahrenheit. A calorie is the amount of heat required to raise one gram of water one degree Celsius.
What are the disadvantages of superheated
a) The specific volume of the gas increases and has a direct effect on reducing compressor capacity
b) The temperature of the gas increases, with a resultant increase in the temperature of the compressor discharge gas.
c) Gas-cooled compressor motors need cool gas for extended life.
If a pound of refrigerant absorbs 50 Btu in the evaporator, how many pounds per minute are circulated in a 2-ton system?
8 200 btu's/min divided by 50 = 4,, 4 * 2 tons = 8 lbs/min
Define compression ratio
The ratio of absolute discharged pressure to,
absolute suction pressure, both in PSIA.
How many cubic inches of mercury equal one pound?
2.036 (By dividing 29.92 by 14.696 you come up with the correct answer)
How do you convert a pressure of 14.696 PSIA to inches of mercury?
2.036 * 14.696 = 29.921 inches of mercury.
How can barometric pressure in inches of
mercury be converted to PSIA?
Divide barometric pressure by 2.036.
What is the term applied to liquids that normally boil at temperatures, below room temperature?
In general practice, how much cooler is the condensing medium than the high-pressure gas in the condenser?
20 to 40°
Allowing a difference of 20° between condensing air and gas in the condenser, what is the condensing temperature if the available cooling air is 90°F?
What is the volumetric efficiency of a compressor that has a displacement of 20 cfm and is actually pumping 16 cfm?
16 / 20 = 0.8 * 100 = 80%
Give three reasons why it is desirable to have some suction superheating.
1) To assure against liquid slugging to the compressor.
2) To reduce the loss of refrigeration from the cold suction line to the warm room.
3) To prevent condensation of the suction line.
What is the condensing temperature of a gas if the discharge gas is 155 F, room temperature is 90°F,and condenser is 20°F higher than room temperature?
What factors affect volumetric efficiency?
The clearance volume of the cylinder. the ratio of compression, the discharge valve closing, the suction valve effect, and
How is compression ratio determined?
Condensing pressure (in psia) divided by suction pressure (in psia).
Find the compression ratio of a compressor that has a condensinq pressure of 160 psig and a suction pressure of 30 psig.
(160 + 14.7) (30 + 14.7) = 174.7/44.7 = 3.91
What should be the relationship between compression ratio and clearance volume?
The lower the clearance volume. The higher the compression ratio should be.
As temperature rises, the density of saturated vapor ________and the density of saturated liquid ________ Ref: 8
The correction factor for pressure readings amounts to _______ for Every 1,000 ft of elevation.
List four results of high compression ratio.
1) Higher discharge temperatures
2) Higher power consumption
3) Poor volumetric efficiency
4) Lower system capacity
In general, what are the maximum
compression ratios for reciprocating,
rotary, and centrifugal compressors?
10 to 12 (reciprocating), 5 to 6 (rotary), and 3 (centrifugal).
A refrigerant number in the 400 series designates a(n)
Zeotropes Blends that change volumetric composition and saturation temperatures as they evaporate or condense at a constant pressure.
The change in the proportions of the components of a refrigerant blend due to preferential evaporation or condensation of the more or less volatile component is called
At a constant pressure, a refrigerant being heated begins to boil at temperature T1. The boiling continues until the last liquid vaporizes, at which point the temperature has increased to T2· The range T2 - T1 is
What elements are present in CFC compounds? in HCFC compounds? in HFC compounds?
CFC: chlorine, fluorine. carbon
HCFC: hydrogen, chlorine, fluorine, carbon
HFC: hydrogen, fluorine, carbon.
What is the best kind of fire extinguisher to have on hand for refrigeration and air conditioning work?
A dry chemical fire extinguisher capable of controlling Class "B" and "C" fires.
What three factors determine the toxicity of a gas?
The nature of the gas, its concentration in air, and how long a time it is breathed.
Minimum suction gas temperature, entering the compressor, should be controlled at a minimum superheat of
What important item does an open-type compressor employ (as opposed to a hermetically sealed compressor)?
A shaft seal.
Determine the size of the motor pulley required to run a compressor at 625 RPM, if the flywheel diameter is 16 in. and the motor speed is 3,450 RPM.
What percentage of overall heat dissipated from the system comes from the compressor motor?
5 to 10%.
When a hermetic compressor designed for high or medium temperatures is applied to a low-temperature system, what is the resulting effect?
High discharge temperatures, resulting in impaired cylinder lubrication, and causing possible motor failure.
How many Btu of heat are added to the system by the work of the compressor?
2,546 Btu per hour for each horsepower.
On a system that has a 12,000 Btu load with the heat of compression added, what amount of additional capacity is required in the condenser?
Discharge lines are normally figured for a velocity of _________ft/min for vertical risers.
How much more water must be circulated in a cooling tower system than in a city water system?
Twice as much.
If the water flow through a system is doubled, what happens to the pressure drop?
It increases four times ..
Why is a trombone-shaped condenser limited to about 3 hp?
13. Because of the problem of bending larger tubes.
When an evaporator is designed for operation at 32°F and above, how many fins per linear inch of tubinq are generally used?
What is the disadvantaqe of close fin spacing on an evaporator coil?
The closer fins are to each other, the more rapidly the coil is reduced as frost build-up occurs.
For most low-temperature applications, 6 fins/inch spacing can be used as long as the evaporator capacity is reduced by _______ %
What is "unloading" and what are its advantages?
System pressures balance during the OFF cycle, thus permitting the use of less expensive motors with low starting torques
Why must a capillary tube system evaporator have a low-side accumulator?
To accept high-side refrigerant during the OFF cycle and to store excess refrigerant during certain operating conditions.
If excessive time is required for an off-cycle pressure balance to occur, what is the probable cause?
Liquid trapped in the high side, or a restriction in the capillary tube or the high side.
How can you locate El restriction in one of several capillary tubes on a multiple-circuit capillary tube system?
By using a thermocouple-type thermometer. The restricted tube will be warmer.
What does the term' cross charge" refer to?
The pressure-temperature curve of the refrigerant used crosses the pressure temperature curve of the refrigerant in the system.
What five factors determine the correct type and size of a TEV?
1) Pressure drop across the valve
2) Load requirements in Btuh
3) Temperature of the liquid entering the valve
4) Evaporator temperature
5) Refrigerant type
The saturated liquid temperature of a refrigerant is called the ________ . The saturated vapor temperature IS called the _____. The difference between these
two temperatures is known as _____
Bubble point, dewpoint, temperature glide.
When suction pressure decreases, the compressor discharqe temperature _________
Ref: 28/5 check on this
Replacement refrigerants for R-12 can increase TEV capacities by as much as _______. over those of R-12.
20 to 30%
What is the recommended dielectric strength for refrigeration oils?
25 kV or higher for all viscosities
Generally, when a system is running, will the oil temperature be higher or lower than the condensing temperature?
Ref: 14/2 Check on this
What is meant by recovery of a refrigerant?
Recovery means to remove the entire refrigerant charge from a system to an external container. Testing or processing is not necessarily implied.
What is meant by a recycled refrigerant?
A recycled refrigerant is one that has been cleaned by oil separation and at least one pass through a filter-drier to remove or reduce moisture, acidity, and contaminant particles. Recycling may be done on the job or at an off-site location.
What is meant by a reclaimed refrigerant?
A reclaimed refrigerant is used refrigerant that has been processed and tested for purity to meet ARI Standard 700. Reclaiming is usually done in a specialized facility, and may involve distillation and other chemical and physical treatments.
The maximum amount of current that a conductor can carry without excessive heat loss.
A transformer with a primary voltage of 120 V has 50 turns on the primary side and 10 turns on the secondary side. What is the secondary voltage?
24 V (50 -:- 10 is a 5:1 ratio, and 120 -:-5 = 24)
How do you convert inches of mercury
vacuum to psi a?
Subtract inches of mercury vacuum from 29.921 and multiply the result by 0.491.
How many horsepower are required to drive a compressor that requires 12,725 Btuh to compress and circulate the gas?
12,725/ 2,545 = 5 hp
Define heat rejection factor.
The percentage of total heat rejection that is represented by useful refrigeration performed by the system. Ref: 11/1
What factors comprise total heat rejection?
1) The latent heat to the evaporator.
2) The superheat in the suction line.
3) The heat of compression.
On what standards is a ton of mechanical refrigeration based?
5°F evaporator and 86°F condensing temperatures.
State Charles' Law regarding constant volume.
Absolute pressure and absolute temperature change at the same rate when volume is constant.
What is density?
The relationship between volume and weight of a material based upon weight per cubic foot.
What is specific gravity?
The weight of a material, especially a liquid, compared to the weight of 1 cubic foot of water, which has an assigned value of 1.
Charles' Law for a constant volume process states that the ________of a gas in a cylinder will change at the same rate as the ________
Absolute pressure, absolute temperature.
What did Boyle's Law establish?
The relationship between volume and pressure of a gas at constant temperature
State Charles' Law.
The volume of a dry gas is directly proportional to its absolute temperature if the pressure remains constant.
How should fuses be sized for a three-phase motor?
21. Select fuses rated at three times the full-load current rating per phase of the motor.
If the power factor is .mknown, how can power be determined?
Divide the true power (as measured with a wattmeter) by the apparent power (as calculated from ammeter and voltmeter
How does a thermal-only overload differ from
a thermal/current overload?
The thermal-only overload has two terminals, and the thermal/current overload has three. Ref: 11/2
A protective device designed to be mounted directly on a motor's windings is called a(n) _______overload.
A DOT-certified recovery cylinder may be filled to _____ % of capacity, provided
that it will not be exposed to temperatures in
excess of _____ "F.
When retrofitting with R-134a, you should use ________oil, with a minimum of _____ changes.
What is the recommended location for a TEV bulb on lines of 7\8 in. 0.D. and larger?
45° below the line's horizontal center.
Where should the entrance of the equalizing line of a TEV be located in relation to the TEV remote bulb?
12. The entrance of the TEV's equalizing line should be located at a point immediately downstream from the remote bulb.
What is the most important low-side pressure that affects the operation of a TEV?
The pressure drop between the valve outlet and the evaporator outlet.
What is the mathematical relationship among these three pressures?
7. Evaporator pressure + spring pressure = bulb pressure.
Why should a conventional large-bore liquid line to suction line heat exchanger never be used on a capillary tube system?
If the low side is even slightly overcharged. the system may lose all its capacity
What is the temperature point of separation between high-temperature and low temperature units?
Where are gravity fin coil units and gravity
booster units generally used?
22. In meat cutting and packing houses, flower storage facilities, and other high-humidity applications.
Name at least three types of refrigeration evaporators.
2. Gravity coil, brine, cold plate, shell-and-tube, blower.
Shell-and-coil condensers are usually limited to a capacity of ______ tons of refrigeration effect.
The average water consumption for
condensers cooled by city water is
gallons per minute per Ref: 5/3-5/4
ton of refrigeration.
1 1/2 gpm
If an air-cooled condenser rated at 1 ton at 9O°F ambient is used at 80°F ambient, what would the rating be? Consider a condensing temperature of 120°F
Why does the heat of compression increase
as the evaporator temperature decreases?
The compression ratio becomes greater Ref: 1212
For a given evaporating temperature or
suction pressure, which will have the greater capacity-a medium-temperature compressor or a high-temperature compressor?
A medium-temperature compressor.
When the same model of open-type compressor is used with both air-cooled and water-cooled condensing units, which is usually run at a slightly higher rpm?
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