unit 31-49 short answers

what are the two advantages of draining the condensed moisture to the slinger system on the condenser?

1) easy disposal of the condensate 2) improved performance by using the latent heat capacity of the water off the evaporator

a 40 va transformer is used on a control circuit to provide 24 volts to the relay and contractor coils of the system. The voltage across the secondary winding of the transformer is only 18 volts. What is most likely the problem?

1) the transformer could be defective 2)the control circuit may be drawing too much current,causing a drain on the voltage

if the suction pressure of an R-22 air-conditioing system is 67 psig and the evaporator outlet temperature is 51 degrees F, what is the evaporator superheat?

12 degrees

what is an air conditioning system's capacity in tons, if the supply air is moving through the 12"x24" main supply duct at a velocity of 400ft/min?

12x24= 288 288/2=2 tons

how much heat is being transferred if the water enters the heat exchanger at 50 degrees F and leaves the exchanger at 40 degrees F with a flow rate of 5 gallons per minute.

25,000 btu/h

In a lithium bromide system, the refrigerant boils at 40 degrees F. What is the corresponding pressure?

29.7 inches vacuum

a technician goes on a heat pump service call and finds the compressor is extremely hot and the suction line is also hot. What is the problem?

4 way valve is leaking internally

the air leaving an evaporator has a dry-bulb temperature of 85 degrees F and a wert-bulb temperature of 65 degrees F. The air entering the evaporator has a dry-bulb temperature of 95 degress F and a wet-bulb temperature of 75 degrees F. How many grains of moisture per pound of dry air were removed by the evaporator?

40 grains of moisture per pound of dry air

if the velocity of air though an 8"x 20" duct is 700 fpm, how many cfm of air is being carried by the duct?

8x20= 160 160/144=1.1 1.11X700=77 CFM

what are the three common evaporator designs used on air conditioning equipment?

A coil, slant coil and H coil

what is meant by the term "COP"?

Coefficient of performance the amount of heat supplied by the heat pump compared to the heat exchanger equivalent of the electrical energy required

why are multiple smaller compressors sometimes preferred over one large compressor?

If one of the cylinders of a large compressor fails, the entire system is brought off-line. If one compressor of many goes bad, the other compressors will still operate and handle the load

what are the two reasons for fastening the capillary tube to the suction line?

Improving evaporator capacity by sub-cooling the liquid refrigerant and provide proper superheat to the suction line

under what conditions can the wet-bulb temperature be higher than the dry-bulb temperature?

Never there's something wrong with the sling psychrometer

a field technician installed two side-air-discharge condensing units next to each other so that the discharge air of unit "A" feeds into condensing unit "B". What is likely to happen?

System A will work fine, but system B will run with extremely high head pressures and cut off on safety when outside ambient temperature is high

what is the reason to use cylinder unloading in high pressure chillers?

To provide capacity control for longer runtimes and closer control of chilled water temperatures

when the burners ignite, the burner flame is normal until the blower starts. At this time, continuous roll out occurs. What is the problem?

a hole in the heat exchanger

on a gas furnace, what energizes the blower motor in the heat mode?

a temperature sensing switch in the chamber

even though the cooling wire (usually the "Y" wire) runs from the thermostat to the air handler, it actually just continues on to the consenting unit. Why isn't the cooling wire run directly from the thermostat to the condensing unit?

a third set of wires needs to be run t-stat to condensing unit, t-stat to air handler, air handler to condensing unit. When using the air handler as a "junction box" only two sets of wires are ran and all voltage troubleshooting is performed at the air handler

air in homes tends to be drier in the winter. Why?

air at lower temperatures in the winter infiltrates into the house. As this air is raised in temperature, it expands. The portion of total volume the moisture occupies is less and the relative humidity is lower.

how is the oil cooled in a domestic refrigerator's compressor?

an oil cooler is used if this loop is traced out it will leave the compressor possibly go to the back of the unit and return to the compressor

what prevents the water in a cooling tower from freezing?

antifreeze, sump heater

where is the correct location for a sight glass in order to check the charge?

at the metering device on the condenser side

name two methods commonly used to unload a reciprocating compressor

blocked suction and lifting the suction line

define parallel alignment

both the center points of the motor shaft and pump shaft are lined up

define angular alignment

both the motor shaft and shaft pump are at the same angle as each other

what will happen if a technician uses a sling psychrometer that has a perfectly dry wick on the wet-bulb thermometer?

both thermometers will read the dry bulb temperature

how is the capacity of a screw compressor controlled?

by a slide valve that blocks off the inlets to screw involutes

why is it recommended to measure by weight the refrigerant charge into the system?

charging by weight of the refrigerant is the most accurate method

an oil burner equipped with the CAD cell relay will ignite, run for about ninety seconds, and cut off. What is the first thing the service technician should do?

clean the surface of the CAD cell

what can be done as an alternative to using a crankcase heater?

constant power can be fed to the compressor in the off cycle to allow a small amount of current flow through the windings, keeping them warm.

what should be done if the voltage to the unit is too low?

contact the power utility fro voltage check

branch ducts should always use what component to balance the air to the individual areas?

dampers

how can the installation of a humidifier help to reduce heating costs?

desired comfort levels can be reduced by increasing the humidity of the space and reducing the t-stat setting (lowering t-stat setting leads to shorter run time, saving money)

define drift

drift is the water that gets carried away with the air as it passes through the tower

superheated refrigerant is present where in a normally operating system?

evaporator, suction line, compressor, condenser and discharge line

what materials can be used successfully in an air-conditioning duct system?

flexible ducts, duct board and galvanized metal and aluminum

what tests can a technician perform on an oil furnace that will check the combustion efficiency?

flue stack test (net temperature), co2 test

how can a technician tell which port of the 4-way valve should be connected to which coil?

follow instructions carefully

which type of fan create more pressure from inlet to outlet and is more desirable for a commercial duct system?

forward curved centrifugal fan

what is the purpose of the heat anticipator in a room thermostat?

helps the unit from overshooting the set point

where is the thermostat's sensing bulb located on a window air conditioner?

in the return air stream

how does air movement improve comfort?

increases the latent heat removal from the body by increasing the evaporation rate of moisture off the skin

since there are no "valves" in a school compressor, what must be done to prevent liquid refrigerant from flowing backward when the compressor cycles off?

install a check valve

why should a technician visually inspect the equipment before installing gauges?

installation of gauges may not be necessary and refrigerant is lost whenever gauges are installed and removed

how are reciprocating,scroll and rotary compressors lubricated?

internal oil pumps using oil in the compressor crankcase

describe an atomizing humidifier

it discharges tiny water droplets into the air and they evaporate quickly

how is the water from the melting frost removed during an automatic defrost?

it goes into the condensate pan and evaporates off by compressor or condenser heat

what is the major advantage a top air discharge condensing unit has over a side air discharge condensing unit?

it makes less noise

for what is compressor crankcase heat used?

it prevents liquid refrigerant from accumulating under the oil in the crankcase

what is the purpose of a system pressure relief valve?

it protects the boiler from operating at high operating pressures

explain how a wet-bulb thermometer works.

it uses the humidity in the air with water to get a reading of the surrounding ambient air

how would too much refrigerant affect the operation of a system with capillary tubes?

it would raise the head pressure and suction pressure, reduce superheat and reduce system capacity

define atomization

it's the pressure of breaking a liquid up into small droplets. This is accomplished with the nozzle.

why does the hot gas from the compressor enter the condenser at the top and sub-cooled liquid leave the condenser from the bottom?

liquid refrigerant is denser than vapor. As the hot vapor condenses to a liquid, it pushes downward and aids refrigerant flow through the condenser. If the hot gas entered at the bottom of the coil, as it condensed it would push downward and slow the flow of refrigerant through the coil

before allowing the compressors of a water-cooled chiller to start up, what two things must be done?

make certain there's flow through chilled water circuit and condenser water circuit

if a duct system has a large air leaks at the connections, what can be used to seal them up?

mastick

why do the refrigerant and the water flow in opposite direction in a heat exchanger?

maximize heat transfer

what is the major advantage of a top air discharge condenser over a side air discharge condenser?

more outdoor debris will collect into the unit

centrifugal compressors are designed with the lubrication isolated from the refrigerant vapor. Why?

oil mixed with liquid refrigerant in the evaporator reduces the capacity of the system

when is it necessary to connect gauges onto a system?

only when temperature tests show a refrigerant quality or flow problem

name the six parts of an oil burner nozzle

orfice,body,filter,office disc,distributor,retainer

of what are the buried pipes usually made?

polyethylene and polybutylene

why should humidifiers be cleaned on a regular basis?

prevents contaminants and build up of mold and bacterias

why do propane and butane gas furnaces require 100% shut off in the event of pilot outage?

propane and butane gas are heavier than air and collects in low places.

why is inches of water column used to measure duct pressure instead of psig?

psig is a larger scale and the small duct pressures wouldn't register accurately

how can the cost of heating a home in the winter be lowered, and still have comfortable conditions in the home?

raising the humidity level and lowering the thermostat setting

how does adding humidity to the air in a residence affect the thermostat setting?

raising the relative humidity of the air allows comfort levels at lower thermostat settings

explain why chillers are desirable pieces of equipment for large air conditioning applications

reduced refrigerant used, reduced chances of refrigerant leaks, less piping to install and easier to maintain

what is the purpose of insulation on the suction line?

reduces condensation

a 24 ft. x 37ft. single story house with a basement has the heating unit in one end of the basement. What type of supply duct system would be used in this application?

reducing extended plenum system

on a multi-directiional air handler (an air handler that can be mounted as a right flow, left flow, or up flow) what must the installing technician remember to do before connecting?

relocate the condensate pan to its proper location

while the furnace is operating, heated air and flue products sill out of the burner compartment. What is the problem?

restriction in the heat exchanger

why do room air conditioners ice up easier than central air-conditioning systems?

room air conditioners operate with colder evaporators due to less air handled through the evaporator

why is 24 volts for the control circuit in residential and light commercial heating and air-conditioning systems?

safety and less energy used, installation costs less and they're more accurate

why are centrifugal circulating pumps relatively small in size?

since the system is filled with water, it doesn't take a lot of pressure to push the water around the loop

a duct system is pressurized by what three pressures?

static, velocity and total

while passing through the condenser, refrigerant is in three different states. What are they?

sub cooled liquid, superheated vapor and saturated

the ends of a chiller evaporator are closed off by standard or marine boxes. What is the difference?

supply and return piping is connected to a standard plate. When the end plates do not contain pipe connections they're called marine boxes and the ends can be removed easily

why are humidifiers usually installed in the supply duct of a gas-fired forced air heating system?

the air is heated and then humidified

floor diffusers along the outer wall of the room are the best way to introduce treated air into a room. Why?

the air is introduced at the point of the heating and or cooling load which counteracts the load before it has a chance to affect the comfort conditions in the area.

when a gas furnace ignites and is under "full-fire" why does the fan not come on immediately?

the chamber must warm up first or the blower will circulate cold air.

what is accomplished by having a compressor that is equipped with unloaders?

the compressor operates at different capacities in order to save energy and reduce the operating compression ratio of the system

explain the difference, with respect to superheat, between a dry type evaporator and a flooded evaporator

the dry type evaporator operates with a certain amount of superheat, while a flooded evaporator operates with no superheat

when the thermostat calls for boiler operation, the relief valve dribbles water until the water reaches a stable temperature. Then the dribbles stop. What is the most likely cause of the problem?

the expansion tank is water logged. There's no air in the tank.

how is manual defrost accomplished?

the food products should be removed first and then the refrigerator should be unplugged. Be sure to place towels down since all of the water will be leaking from the unit

when a filter rack return grill is located in the living space, why is it a good idea to make the return duct as large as possible?

the larger the duct the slower the air will move and the less noise it will make

define relative humidity.

the percent of moisture in air is as compared to the amount it could hold if it were saturated

why are open compressors often started up for a short time in the off season?

the shaft seal on the compressor is kept "wet" by the circulation of refrigerant oil over it. If the compressor is off for a long period of time, the shaft seal can dry up and eventually leak. Periodic running of the compressor helps keep the shaft seal in good shape

what would happen if the narrow spray angle nozzle were used in a round combustion chamber?

the spray would hit the rear of the combustion chamber and cause carbon buildup.

how can the heat pump melt the ice that forms on the outdoor coil?

the unit switches itself over to the cooling mode

if a window air conditioner is placed on a table and is allowed to run, will the unit ever cool the room? Why?

the unit will not cool the room, the condenser rejects more heat than the evaporator absorbs, so eventually the room will actually get warmer

describe vortexing

the whirlpool action of water entering a drain that occurs when the water level is too low to maintain even flow into the drain

what is the differnce in construction between a window air conditioner and a wall air conditioner?

the window air conditioner has the air intake for the condenser on the side of the unit while the wall air conditioner has the air intake for the compressor at the back of the unit (the wall unit has solid states)

the flame in a gas fired furnace is burning with heavy yellow tips. What is the problem?

there isn't enough air

if a high efficiency condemning unit is manufactured with one-speed condenser fan motor, what is likely to happen if the system is operating during mild weather?

there would be low head pressure, less system capacity and the expansion device will have a reduced pressure drop

what is the major objection to an atomizing type humidifier?

there's miners in the water

why is defrost not needed in geothermal heat pumps?

they operate at temperatures above freezing

why are high efficiency particulate arrestor (HEPA) filters desirable?

they trap particles that contain mold, dust mites, pollen and bacteria

why are variable speed condenser fan motors used?

they're used as a method of head pressure control. As the ambient temperature drops, the condenser fan motor will slow down to maintain head pressure

why are balancing valves used in hydronic systems?

to adjust the amount of hot water to the zone to handle the heat loss of the zone.

the water that leaves a cooling tower passes through a strainer. Why?

to catch any foreign matter that can easily find its way into the sump of the tower

why is the head pressure raised to 275 psig to check the sub cooling of the system?

to duplicate the conditions where the amount of sub cooling was established

summer cooling supply duct work should always be insulated. Why?

to eliminate condensation on the duct and capacity loss

why are crisper compartments semi-sealed off from the main compressor?

to maintain high humidity in the crisper compartment

why must the thermometers in a sling psychrometer be swung in order to obtain readings?

to obtain the necessary evaporation rate of moisture off the wet bulb thermometer

why must the expansion tank always be located higher than the boiler?

to permit the air in the connecting line to rise to the tank without being trapped in the line

if the suction line and compressor are sweating, what is the problem?

too much refrigerant

a split central cooling system is installed with the control transformer located in the attic hung air handler and the condensing unit located outside. How many low voltage conductors need to be run to the condensing unit?

two

what is the first indication that a refrigerator is running low on refrigerant charge?

unit runs constantly and the coil is only partially frosted

how can a technician tell the difference between an upflow furnace and a downflow furnace?

upflow: blower located below the heat exchanger, air is pushed upward downflow: blower located above the heat exchanger, air is pushed downward

how much does sunshine effect the operation of a condensing unit?

very little

a home on a concrete slab has a perimeter loop air distribution system. What is the important advantage of this system over an overhead system?

warm air at the outer edge of the slab keeps the outer area of the slab warm

what is meant by the term "blowdown?"

what is drained from the tower sump and replaced by a constant supply of fresh water to reduce mineral and contaminate content in the sump water

when is insulation required on air ducts?

when there's a 15 degree difference from inside and outside the duct

what would happen if a hydronic system was installed without an expansion tank?

when water is heated, it expands a great deal, if the expansion tank wasn't there, the relief valves would open and cause constant flooding.

how are centrifugal compressors lubricated?

with a separate oil circulating system then isolated from the refrigerant system

what is the difference between a yellow gas flame and an orange gas flame?

yellow: insufficient amount of air carbon dioxide is produced orange: dust being burnt

Why should sharp objects not be used to chip away ice from an evaporator?

you could make a hole in the evaporator and there would be a chance of refrigerant to leak out of the system

why is "inches of water column" used to measure gas pressure instead of psig?

"inches of water column" is a smaller scale and will give more accurate measurements