AC Review HVAC
When testing the efficiency of a water cooled condenser, the refrigerant condensing temperature should be approximately _________ higher than the leaving water temperature.
10 degrees F.
Water will vaporize at ________ temperature in a 28 in Hg vacuum.
100 degrees F.
10 lbs. of refrigerant passing through a water cooled condenser will lose 20 Btus of heat per lb. If 10 lbs. of water passes through the condenser with an entering temperature of 90 degrees F. The temperature of the water leaving the condenser is;
100 degrees F°.
The R410A compound gauge reads 124 psig and the suction line temperature at the condensing unit is 54 degrees F. What is the operating system superheat?
12 degrees F.
Refrigerant R-134a vapor enters the condenser at 124 psig, 100 degrees F. It leaves as a liquid from the condenser sub-cooled 20°F. The pressure will be approximately;
124 psig.
The R134a high pressure gauge reads 146 psig and the liquid line temperature at the condensing unit is 95 degrees F. What is the operating system subcooling?
15 degrees F.
What is the maximum operating pressure setting for an acetylene torch?
15 psig.
If a 30 pound cylinder of reclaimed R-410A refrigerant has been stored overnight at 70°F, the pressure in the cylinder should be approximately;
201 psig.
When testing the efficiency of an air cooled condenser, the refrigerant condensing temperature should be approximately ________ higher than the entering air temperature.
30 degrees F.
What is the generally accepted range for the evaporator air flow in CFMs per ton of residential air conditioning?
350 to 450.
What is the recommended residential indoor relative humidity for summer?
50%.
When connecting copper to steel the recommended brazing filler alloy is a;
50/50 solder alloy.
If the evaporator pressure does not change when heat is added or removed from the thermostatic expansion valve bulb, the problem may be;
A faulty bulb or power head assembly.
A customer complains that their air conditioning system is running constantly, but not cooling enough. The suction line is very cold and the compressor is sweating. The most likely cause is;
A low refrigerant charge.
In a direct expansion (DX) evaporator system, the refrigerant enters the evaporator as;
A mixture of liquid with some "flash-gas".
Zero psia (pounds per square inch absolute) is;
A pressure which cannot be further reduced.
A temperature differential across a filter-drier indicates;
A restricted filter-drier.
A partially clogged capillary tube can cause;
A starved evaporator.
If a small air conditioner has only a very low, insignificant refrigerant pressure drop through the evaporator coil, the manufacturer may use;
A thermostatic expansion valve with an external equalizer connection.
A drain trap should be installed on a split system when the;
Air handler drain is on the negative air side.
An Energy Recovery Ventilator is;
An air to air heat-exchanger to recover heat.
What type of gauge is used to measure a vacuum of 500 microns?
An electronic vacuum gauge.
At a residential service call, "no cooling" complaint, the service technician finds the unit to be completely out of refrigerant. The technician decided to use a trace amount of refrigerant and ________ in order to pressurize the system.
An inert gas.
A partially iced over evaporator can be caused by;
An undercharge.
The heat removed from the refrigerant by the condenser is;
Both sensible and latent heat.
Solid contaminants such as a sludge and carbon, from a severe hermetic compressor motor burn out are best removed;
By flushing the system and then installing filter/driers in both the liquid and suction lines.
The temperature rise method for determining air flow is;
CFM = BTU ÷ (1.08 X Delta-T)
Which of the following types of air conditioning compressors is the most energy efficient?
Centrifugal.
A decrease in superheat causes the thermostatic expansion valve to begin to;
Close.
The sequential order of the steps in the vapor - compression processes are;
Compression, condensation, expansion, vaporization.
Moisture in a refrigeration system can cause;
Compressor failure.
A liquid line filter-drier should be placed between the;
Condenser and the metering device.
Which of the following is the most important consideration in the selection of an evaporator coil? Match the coil to the;
Condensing unit and the latent/sensible loads.
The movement of heat by direct contact is called;
Conduction.
A thermostatic expansion valve is designed to maintain;
Constant evaporator superheat.
The movement of heat by way of fluid (or air) is called;
Convection.
The first function of a condenser must perform is to ________ the refrigerant before condensing can occur.
De-superheat.
Which of the following represent the right order for the functions of a condenser?
De-superheats, condenses, subcools.
A customer reports that their residential split system had a compressor replaced last week by another service company. Now the unit is not cooling properly. When you arrive and inspect the unit, you find that the evaporator seems to be starved for the refrigerant. On examining the condensing unit, you find that the liquid line is sweating where it leaves the drier. This is most likely the result of;
Debris from the burnt out compressor has clogged the drier.
The temperature at which moisture will start to condense from the air is called;
Dew point temperature.
Which of the following is not a thermodynamic property of refrigerants?
Enthalpy.
The flow rate of a TXV used in R410A systems as compared to R22 systems is;
Equal.
A low pressure cut-off switch is used to;
Establish a minimum operating pressure in the evaporator.
There are three basic types of condensers, the air cooled, the water cooled, and the _____________ cooled.
Evaporative.
The thermostatic expansion valve has three operating pressures;
Evaporator pressure, spring pressure, and sensor bulb pressure.
The clearance space in a reciprocating compressor;
Has an effect on compressor capacity.
Which of the following is NOT a symptom of a hermetic system that is low on refrigerant?
High head pressure.
The state of refrigerant entering the condenser is;
High pressure/highly superheated vapor.
Which of the following symptom (s) would be indicative of an oil logged condenser?
High subcooling.
A faulty condenser fan motor on an air conditioning system will cause these symptoms.
High suction pressure and high discharge pressure.
A refrigerant overcharge in a capillary tube air conditioning system will cause these symptoms;
High suction pressure and high discharge pressure.
A TXV sensing bulb which becomes detached from the suction line will cause these symptoms;
High suction pressure and lower than normal discharge pressure.
A sight glass would normally be located;
In a liquid line, before the mtering device.
Fan airflow pressures are usually measured in;
Inches of water column (WC).
A liquid-to-suction heat exchanger is often used to;
Increase liquid sub-cooling
An overcharge of refrigerant will cause;
Increased head pressure.
An air conditioning system that is oversized can cause;
Inefficient dehumidification of the conditioned space.
When topping off an operating system containing a blended refrigerant;
Introduce the refrigerant as a liquid into the low side using a throttling valve.
A scroll compressor move refrigerant by means of;
Involute Spirals.
How does a TXV in an operating air conditioning unit react when the bulb is not attached to the outlet or suction line and hanging in the air?
It always floods the evaporator.
A refrigerant gives up heat when;
It condenses.
Which of the following is TRUE of a solenoid valve in a pump down system?
It is located before the metering device.
When mounting a thermostatic expansion valve sensor bulb, which of the following is NOT true;
It must be installed lower than the valve body.
If the cut-off plate is removed from a blower assembly, how would this affect blower performance?
It would reduce air flow.
In the evaporator;
Latent heat is transferred from the refrigerated space to the refrigerant.
heat added to substance without a rise in temperature; (example: liquid refrigerant to gas) this process is called?
Latent heat of vaporization.
Liquid Slugging is;
Liquid refrigerant in the compressor clearance space.
While troubleshooting a residential split system, the service technician finds low operating pressures and low system amperage draw. This could be an indication of;
Low airflow across the evaporator.
Bubbles in a sight glass could be an indication of;
Low compressor capacity.
A technician has found an air conditioning system with a leak in the liquid line. After the line is repaired, the system is evacuated and properly recharged. When the system is started, the compressor is noisy and running hot. The most probable cause of the noise is;
Low oil in the system.
The state of the refrigerant in the suction line should be;
Low pressure/low temperature vapor.
A compressor that has a broken valve will cause these symptoms;
Low suction pressure and low discharge pressure.
A dirty evaporator in an air conditioning system will cause these symptoms
Low suction pressure and low discharge pressure.
A low refrigerant charge in an air conditioning system will cause these systems;
Low suction pressure and low discharge pressure.
An inefficient compressor is usually indicated by ________ head pressure and _______ suction pressure.
Lower than normal/lower than normal.
An increase in head pressure causes _________ refrigerant to flow through a capillary tube.
More.
A thermal-electric (solid state) expansion valve is controlled by;
NTC thermistor.
A mechanical shaft seal is necessary on a;
Open drive compressor.
Back seating a two way service valve results in;
Open service port with normal system flow.
A compound pressure gauge measures;
Pressure and vacuum.
An accumulator is used to;
Prevent liquid refrigerant from reducing the compressor.
Why must refrigerant in the suction line be slightly warmer than the saturation temperature?
Prevent liquid slugging of the compressor.
After a system has been opened and the compressor changed, the service technician must;
Replace the filter drier, leak check, evacuate, and charge.
Heat which causes a change in temperature of a substance is called;
Sensible Heat.
When using an electronic leak detector, the sensor probe;
Should be moved approximately 1 inch for every two seconds.
____________ can be used for leak detection with All high Pressure Refrigerants.
Soap/bubble solution.
A liquid receiver is used to;
Store liquid refrigerant.
Heat added to the refrigerant in a vapor state after the evaporation process is called;
Superheat.
Frost on the suction line indicates;
That its temperature is below the dew point and below 32 degrees F.
The standard unit for measuring the quantity of heat in a substance is;
The British Thermal Unit (Btu).
The oil pressure safety switch is operated by;
The difference between crankcase pressure and oil pressure.
Zero psig (pounds per square inch gauge) is;
The same as the atmospheric pressure.
Which of the following is NOT true of a capillary tube metering device?
They maintain a constant evaporator temperature.
The evaporator;
Transfers heat from the refrigerated space to the refrigerant.
In order to remove as much moisture as possible from a system;
Triple evacuate the system to 500 microns with a deep vacuum pump.
In an air conditioning system that uses a thermostatic expansion valve, the temperature of the evaporator outlet will usually be;
Warmer than the evaporator inlet.
An automatic expansion valve is best suited;
When the system load remains fairly constant.
An overcharge of refrigerant will cause;
increased head pressure.
When a TEV or a capillary tube is used as a metering device, a low refrigerant charge will cause;
low head pressure.
Air or other non-condensable gasses in a capillary tube air conditioning system will cause these symptoms;
low suction pressure and high discharge pressure.
