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What are some common problems to look for when you troubleshoot the performances of shell-and-tube condensers?
Heat transfer between the refrigerant and the condenser water may be restricted due to fouling of the condenser tubes.
The condenser head gaskets may be dislocated, causing the tubes to be bypassed.
One millimeter equals _____ microns.
Start capacitors have typical tolerance ratings of ___%
±20 to ±25%
When suction risers are greater than 30 ft, why should traps be installed?
To prevent possible liquid slugging on start-up.
What must you do to avoid gas binding?
Install a condensate line large enough to allow vapor to pass back to the condenser, and to permit the flow of refrigerant to the receiver.
What is the best way to determine the direction of rotation of a PSC motor?
By direct observation of the driven fan. Look for dirt accumulation on the leading edge of the fan blade.
In addition to achieving the desired mixed-air temperature, why is a mixing box needed in
a dual-duct system?
To reduce the velocity of the air and prevent objectionable noise.
How does NEMA define the difference between integral-horsepower motors and fractional-horsepower motors?
Integral-horsepower motors are rated at 1 hp and above, fractional-horsepower motors are rated at less than 1 hp.
A typical start capacitor is designed for a ______ maximum number of starts per hour
The weight of water vapor per cubic foot of dry air is known as _____. humidity
What does the term "sealed VA" refer to as it applies to relays?
The volt-ampere rating tor the relay when the armature is pulled all the way down on the coil.
Can you use an ohmmeter to check the resistance in variable-speed motor windings?
In what respects are PSC motors more versatile than shaded-pole motors?
They are electrically reversible, have multiple speeds, and can run on multiple voltages.
When you must replace a motor rated at 1/4 hp or less, is it more accurate to match horsepower or current draw?
In heat load calculations, what does the letter "C" represent?
The temperature difference between room air and refrigerant temperatures is normally about ______
What is the maximum acceptable voltage drop across the contacts of a contactor?
3% of the rated line voltage.
When does "over-motoring" occur?
When a larger-horsepower motor is used to replace a smaller-horsepower motor. The result is a waste of electrical energy,
overheated wiring, and a loss of torque being supplied to the motor.
What does the term absolute humidity refer to?
The amount of moisture, by weight, actually present in one cubic foot of space.
What should the wiring for a unit be sized for.
125% of its FLA
Does a run capacitor reduce power
A motor start capacitor is a(n) ________capacitor.
With one exception, gases, liquids, and solids expand as they become warmer and contract as they become cooler. What is the exception?
Water between 32°F and 39.2°F
The resistance in the individual windings of a three-phase motor should be within _____ % of each other
3 to 5%
How do permanent split-capacitor motor starting currents compare with shaded-pole motor starting currents?
PSC motors usually draw half as much starting current as shaded-pole motors
What are the three types of winding connections generally used in three-phase motors?
Across-the-line (XL), part wind start (PWS), and wye-delta ("soft start).
The voltage imbalance between any two legs of the supply voltage applied to a three-phase motor must not exceed _____%
Motors used in refrigeration systems may need as much as________more current to start, than to run at design speeds.
3 to 6 times.
How many air changes per hour does an average house have?
What is the maximum amount of refrigerant that can be in a "small appliance?"
For a residential split system, the temperature drop of the air across an evaporator should be ________ .
Between 18 and 22°F.
What is the heat content of dry air per degree Fahrenheit change in temperature?
About 0.24 Btu/lb
A pilot-duty relay usually controls a load that draws less than
What is effective temperature?
An experimentally determined index that combines into a single value the effect of temperature, humidity, and air movement on
the human body.
What is cross phasing?
A condition in which some of the windings in a motor are trying to turn in one direction, while others are trying to turn in the opposite direction, thus working against each other and causing the motor to overheat.
What is the principle of the atomizing humidifier?
It atomizes water by throwing it from the surface of a rapidly revolving disk.
How is the motor heat generated in a hermetic compressor dissipated?
Through radiation, refrigerant vapor, and oil vapor.
In what types of applications should you use a constant-pressure valve that has rapid equalization features?
In applications with compressor motors that have low starting torques
What figure is used for estimating the heat gain produced by most common appliances?
3.412 Btuh per watt
What kind of acids will be present in the oil of a burned compressor motor?
Hydrochloric acid and hydrofluoric acid.
The density of air is the reciprocal of
What type of ac motor uses brushes?
A repulsion-induction motor.
List three ways in which a high-vacuum pump can be used to evacuate a system.
1) Straight vacuum
2) Multiple evacuation and blotter method
3) Combination of both.
The life of a motor start capacitor will be shortened if the voltage across the capacitor exceeds _____ % of the rated voltage
What is entropy?
The ratio of the heat content of a gas to its absolute temperature.
How many grains are there in 1 Lb of water?
In heat load calculations, what does the letter "K" represent?
The duct static value, for a residential split system, is the sum of the absolute values of the _____ and the _____
Supply duct pressure, return duct pressure.
What is temperature rise?
The amount by which a motor's temperature will increase over the ambient air temperature when operating at its rated load and speed.
When replacing a start capacitor, a variance in capacitance of ________ can usually be tolerated.
What is the difference between absorption and adsorption.
Absorption is a chemical process, and adsorption is a physical process.
What portion of the total pressure of the air comes from water vapor?
1/2 to 1.5 %
What two factors determine the size of a run capacitor?
Motor horsepower and applied voltage.
Name two limiting factors of the off-cycle defrost method.
Space temperature should be above 35°F The temperature surrounding the compressor should be warmer than the
temperature of the refrigerated space.
What is a simple way to check refrigerant charge and a window unit or Ptak?
Cover the entire face of the air filter with cardboard and close the fresh air damper. The coil should frost all over in about 10
minutes. If it does not, a shortage of refrigerant is indicated.
A potential relay has three active terminals. How are they numbered, and how are they connected?
Terminal 1 is always connected to the start capacitor. Terminal 2 is always connected to the start terminal. Terminal 5 is always connected to the common terminal.
The starting torque of a split-phase motor is ____ % of the full-load torque.
If a hermetic motor develops a locked-rotor condition, how soon can damage occur if the protector does not operate?
Within 20 seconds.
What is the capacity rating of a pan-type humidifier with a 1,000-W element?
3 to 4 Lbs/hr
(Ref: Figure 34-9) (1000 * 3.414) = 3414/970 =3.52Lbs/hr)
The only type of desiccant in use today is the type that removes moisture by
Adsorption. (A physical process)
What is the recommended value (in ohms) of a resistor used to discharge a capacitor?
Name two types of three-phase squirrel-cage motors that have comparatively low starting currents.
The double squirrel-cage motor and the increment-type motor.
When you use the ammeter/voltmeter method to determine capacitance, what is the approximate value of the capacitor if the ammeter reads 2.5 A and the voltmeter reads 245 V?
Approximately 27 Uf Uf = (2650 X cap amps)/voltage
What are the characteristics of a low pressure all-air system?
Duct static pressure does not exceed 2 in. w.g. Air velocity does not exceed 2,000 to 2,500 ft/min
What is an acceptable temperature rise for an open-frame motor'?
What does Dalton's Law state about the behavior of gases?
That two gases can occupy the same space at the same time, and that each gas behaves as if it occupies the space alone.
The production of most HCFC refrigerants is scheduled to be phased out by the year
2030 Check on
Describe the terminal wiring connections for a current relay.
The "L" terminal is connected to the line voltage. The "M" terminal is connected to the motor's run winding. The "S" terminal is connected to the motor's start winding.
The EPA requires that disposal records be kept on file for ______years.
What is the melting temperature of silver solder?
1,100 to 1,200°F
How is automatic adjustment to variable frost conditions accomplished?
By using time-initiated, temperature- or pressure-terminated devices.
Name five types of multi-zone systems.
Terminal reheat systems, variable air volume systems, induction systems, dual-duct systems, and multi-zone units.
How long should a sling psychrometer be whirled when taking a reading?
How much greater is the starting current than the running current of a three-phase squirrel-cage motor?
5 to 7 times
What is the term used to express the moisture capacity and efficiency of desiccants?
End-point dryness (EDP).
How is the rate of chemical reaction with oil and oxygen related to temperature rise?
For every 18°F rise in temperature above 200°F, the chemical reaction doubles.
Name the three types of circuit breakers in common use.
Thermal, thermal magnetic, and fully magnetic.
In general, an open-type motor is _____ heavier than a hermetic motor of the same horsepower.
What is the purpose of a run capacitor?
It improves the power factor of a motor.
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