Chapter 29 Questions

Using Figure 29-30, what should the liquid line temperature be for a model 10ACB30 unit operating in an ambient temperature of 97°F?

10°F approach 97°F + 10°F = 107°F liquid line

Using Figure 29-22, what would be the proper superheat for a system with an ambient temperature of 95oF and a return air wet bulb of 70°F?

18°F

Using Figure 29-28, what would be the proper subcooling for a system with an ambient temperature of 115oF and a return air wet bulb of 62°F?

4°F subcooling

What is the standard AHRI design condition used for rating unitary air conditioning equipment?

95°F outdoor ambient temperature, 80°Fdry bulb, 67°F wet bulb indoor temperature

Using the information from question 19, what charge adjustment needs to be made if the liquid line temperature is 110°F?

Add charge until the line temperature drops to 107°F

Using Figure 29-18, the suction pressure is 140 psig, the liquid pressure is 370 psig, and the ambient temperature is 105°F. What charge adjustment needs to be made?

Add charge, the system is undercharged.

How does an overcharge affect the superheat of a fixed restriction system?

An overcharge will decrease system superheat.

How does an overcharge affect the subcooling of a thermostatic expansion valve system?

An overcharge will increase system subcooling.

How does an undercharge affect the subcooling of a thermostatic expansion valve system?

An undercharge will decrease system subcooling.

How does an undercharge affect the superheat of a fixed restriction system?

An undercharge will increase system superheat.

What is the best method for charging a large system that holds 50 pounds of refrigerant?

Liquid into the king valve with the king valve front-seated and the system operating.

Give an example of a condition that can make a system appear overcharged even though the charge is correct.

Low airflow over the condenser or non-condensable gasses

Give an example of a condition that can make a system appear undercharged even though the charge is correct.

Low evaporator airflow due to dirt, improper fan speed, or poor duct design

Using Figure 29-18, the suction pressure is 155 psig, the liquid pressure is 360 psig, and the ambient temperature is 95°F. What charge adjustment needs to be made?

No adjustment is necessary, the system is charged correctly.

A new split system air conditioning system is being installed. The actual line length is 48 feet. The factory charge for the condensing unit is 94 ounces. The installation instructions state that the factory charge assumes a line length of 20 feet and gives the per foot adjustment as 0.6 ounces per foot. How much refrigerant should be added to the system after the lines and coil have been evacuated?

Only the line adjustment needs to be added. The line adjustment is calculated by: (48-20) x 0.6 = 18 x 0.6 = 10.8 ounces

List the variables that affect system pressures.

Outdoor ambient temperature for air-cooled units Condenser airflow for air-cooled units Inlet water temperature for water-cooled units Condenser water flow for water-cooled units Return air wet bulb for air conditioners Evaporator airflow for air conditioners Returning water temperature for chillers Chilled water flow for chillers

How does an overcharge of refrigerant affect system efficiency?

Overcharging will cause liquid refrigerant to back up in the condenser, increasing the operating head pressure and reducing system efficiency

Using the information from question 13, what should the liquid line temperature be if the liquid pressure is 293 psig for an R-22 system?

R-22 293 psig = 129°F saturation 129°F - 4°F = 125°F liquid line temperature

Using the information in question 10, you measure a suction pressure of 130 psig for an R-410A system. What should the suction line temperature be?

R-410A 130 psig = 45°F saturation 45°F + 18°F = 63°F suction line temperature

Using the information from questions 10 & 11, what charge adjustment should be made if the suction line measures 55°F?

Recover charge, the system is overcharged.

Using the information from questions 13 and 14, what charge adjustment needs to be made if the liquid line temperature is 110°F.?

Recover refrigerant, the system is overcharged

Why must refrigerants that fractionate, such as R-410a, be charged as a liquid?

Refrigerants that fractionate, such as R-410A, must be charged as liquid to prevent the separation of the refrigerant.

How often should the refrigeration oil in a compressor be changed?

Refrigeration oil does not need regular changing. Refrigeration oil is usually only changed to clean up a contaminated system or to add a retrofit refrigerant.

The compressor in an existing split system air conditioning system is being replaced. The actual line length is 35 feet. The factory charge for the condensing unit is 100 ounces. The installation instructions state that the factory charge assumes a line length of 15 feet and gives the per foot adjustment as 0.6 ounces per foot. How much refrigerant should be added to the system after the system has been evacuated?

Since the entire charge was removed, the factory charge plus the line adjustment should be added. The line adjustment is calculated by: (35-15) x 0.6 = 20 x 0.6 = 12 ounces The total charge to be weighed in is 100 + 12 = 112 ounces

Why should a system be leak tested after it has been repaired before it is charged?

Systems with leaks cannot be properly evacuated; air will just suck in through the leak. Recovering refrigerant after it has been added adds to the time required.

Who is the most reliable source of information regarding the charge for any particular piece of equipment?

The equipment manufacturer

Outline the procedure for adding oil to a semi-hermetic compressor without opening the compressor.

The oil line must have a manual valve to control flow through the oil line. One end of the oil hose is connected to the suction service valve; the other is submerged in the oil container. Purge the air from the oil line by blowing a small amount of refrigerant vapor through it into the oil. Frontseat the suction service valve and operate the compressor long enough to create a slight vacuum. Oil is added by opening the control valve in the oil line. Close the control valve in the oil line when the correct amount of oil has been added. Backseat the suction service valve and remove the oil hose.

Why are systems with fixed restriction metering devices critical to charge?

They cannot adjust to changing conditions and they have no liquid receiver to store extra refrigerant.

What is the best method to charging a small appliance that holds less than a pound of refrigerant?

Vapor with the system not operating up to a specified static pressure at a specified ambient temperature.