Unit 37 Exam Air Distribution & Balance

What is an air conditioning system's capacity in tons, if the supply air is moving through the 12" x 24" main supply duct at a velocity of 400 ft/min?

Answer : 12x24=288/144= 2 tons

In a typical air conditioning system, ___ cfm of air is moved through the evaporator for each 12,000 Btu/h cooling capacity of the system.

Answer : 400 cfm

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

Answer : 8x20=160/144=1.11x700=778

When is insulation required on air ducts?

Answer : A 15 degree temp. difference from the inside to the out -side of the duct is the maximum difference before insulation is required.

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

Answer : Balancing Dampers

In a residential air conditioning system, the only air moving device that is used is a _____________.

Answer : Centrifugal Fan

The free area of most registers and grills is 65% of the total face area.

Answer : True

The main drawback of the extended plenum system is that the velocity of the air is lower as the air reaches the last takeoffs on the extended plenum

Answer : True

The static pressure in a duct is the total pressure less the velocity pressure

Answer : True

To put trunk duct or extended plenum sections together, the device used is called a drive clip.

Answer : True

The________ of a section of sheet metal is directly related to its weight.

Answer :Thickness

If the equivalent length of a transition piece of ductwork is 10 feet and the main duct in series with it is 20 feet, what is the length used to estimate the total frictional loss?

a. 10 feet b. 30 feet c. 60 feet d. 20 feet Answer : B

By design, which of the following air ducts will allow 250 cfm to flow with a velocity of 500 ft/min?

a. 9" ´ 8" b. 16" ´ 9" c. 10" ´ 9" d. none of the above Answer : A

Technician "A" says that if the air filter on an air-conditioning system is totally blocked with dirt, the amperage draw of the evaporator fan motor will go down. Technician "B" says that if you close down one supply register on a duct system, the volume of air feeding the other locations will increase slightly. Which of the following statements is correct?

a. Technician "A" is correct. Technician "B" is correct. b. Technician "A" is correct. Technician "B" is incorrect. c. Technician "A" is incorrect. Technician "B" is correct. d. Technician "A" is incorrect. Technician "B" is incorrect Answer : A

The pressure in a duct system is measured in:

a. psig b. psia c. " Hg d. " WC Answer : D

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

Answer : Centrifugal Fan

Because the blower in the air handler (furnace) draws the air down the return air duct system, the return air duct is the same size as the supply duct system.

Answer : False

The volume of air that an air conditioning system can deliver can be increased by adjusting the belt tension on the blower.

Answer : False

Over-sized duct systems cost more to fabricate and install and will also result in the slowing of the air as it flows through the duct.

Answer : True

To connect a branch duct to the main duct, a ___________ fitting is used.

Answer : Take Off

The fitting at the end of a round duct that connects the duct to the floor register is called a(n) ____.

a. boot b. reducer c. take off d. stack head Answer : A

When a canvas collar (vibration eliminator) is properly installed between two duct sections, the metal surfaces of the duct-work should be rubbing against each other.

Answer : False

A 24 ft. x 37 ft. 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?

Answer : Reducing extended plenum

A duct system is pressurized by what three pressures?

Answer : Static, Velocity & Total Pressure.

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

Answer : The Pressure is too small to be measured by PSIG.

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?

Answer : The larger the duct the slower the speed of the air, the less noise it will produce.

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?

Answer : The loop has a constant pressure around the system & provides the same pressure to all outlets.

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

Answer : This is where the heat exchange occurs, placed next to the outside wall to accomplish the load cancelling effect. cold air kept in & hot air kept out. depending on the season.

Ideally, the sum of all the air volumes feeding each area of a conditioned space should equal the tonnage of the system multiplied by four hundred

Answer : True

If the resistance against a propeller fan is doubled, the cfm output decreases, but the amperage draw of the motor ____.

a. decreases b. increases c. stays the same d. overheats the motor Answer : B

If the resistance against a centrifugal blower is doubled, the cfm output decreases, and the amperage draw of the motor ____.

a. decreases slightly b. increases slightly c. stays the same d. overheats the motor Answer : A

The reason for using a reducing extended plenum system is to ____,

a. save metal by using a smaller duct b. reduce the operating cost of the system c. provide easier means of fastening the take off fittings d. maintain a more even pressure throughout the entire length of the trunk duct Answer : D

Which of the following is typically measured with an inclined water manometer?

a. static pressure in a duct system b. pressure in a refrigeration system c. manifold pressure in a gas fired furnace d. nozzle pressure of an oil burner Answer : A

One reason for using a reducing plenum system is to keep ____.

a. the velocity of the air constant b. the noise level low c. the duct pressures constant d. both a and c Answer : D

Measuring velocity pressure in a duct indicates the ____.

a. volume of air through a duct b. speed at which the air is moving c. pressure against the sides of the duct d. weight of the air moving through the duct Answer : B