Unit 29

An R-22 system operating with a liquid pressure of 226 psig (110° saturation temperature). The specified system subcooling is 5°F. The liquid line temperature should be:

105°F.

Most air cooled equipment manufactured today is designed to operate with a:

115°F condenser at 95°F.

Which of the following measures is the standard airflow across an air-conditioning coil?

400 CFM per ton

An air conditioning system charged with R-410A is operating with a suction pressure of 130 psig which equates to a 45°F saturation temperature. If the manufacturer calls for a 15°F superheat, what should the suction line temperature be?

45°F + 15°F = 60°F suction line temperature

What is the total system charge for a system with a factory charge of 56 oz of R-410A, and assumed length of 15 ft, and a 3/8 in liquid line that is 35 ft long?

68 oz total system charge

The compressor in an existing split system is being replaced. The system has a 35 foot line set. The factory charge for the condensing unit is 80 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?

80 ounces + ((35-15) × 0.6) = 92 ounces

The standard AHRI "A" design condition used for rating unitary air conditioning equipment is:

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

For air conditioners, a rise in the return air wet bulb temperature causes:

A decrease in the temperature difference across the evaporator coil.

In general, a system that is overcharged will have:

A high subcooling and a low superheat.

A thermostatic expansion valve system operating with a refrigerant overcharge will have:

A high system subcooling.

A fixed restriction system operating with a refrigerant undercharge will have:

A high system superheat.

In general, a system that is undercharged will have:

A low subcooling and a high superheat.

A thermostatic expansion valve system operating with a refrigerant undercharge will have:

A low system subcooling.

A fixed restriction system operating with a refrigerant overcharge will have:

A low system superheat.

An air conditioner with a thermostatic expansion valve is operating with a condenser saturation temperature of 95°F and a liquid line temperature of 92°F. The factory specified subcooling for the current operating condition is 10°F. The technician should:

Add charge because the system is undercharged.

Most vapor charging is done with the compressor operating while charging is taking place; however, liquid charging:

Can be done either with the system off or operating.

Returning water temperature, along with the water flow through the evaporator, are critical variables that will impact the performance of:

Chillers.

As the outdoor ambient temperature increases, the required superheat:

Decreases.

How much refrigerant does the condensing unit of a new split system air conditioning system contain?

Enough for the condensing unit, evaporator coil, and 15 to 30 feet of refrigerant line

Which of the following conditions would likely increase the system head pressure?

Increase in the outdoor ambient temperature for air cooled units

As the indoor wet-bulb temperature increases, the required superheat:

Increases.

Overcharging a system reduces its efficiency by:

Increasing the operating head pressure and compression ratio.

Even with the correct refrigerant charge, a water cooled system would appear overcharged if:

It had lower than normal water flow through the condenser.

The fastest method for charging a large system which holds 50 pounds of refrigerant is to add:

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

A dirty air filter would likely cause which of the following symptoms ?

Low suction pressure

Will a chart that only specifies the outdoor ambient temperature still read accurately if the indoor relative humidity is very far off of 50%?

No, because a central assumption of the chart would be inaccurate.

An air conditioner with a fixed restriction metering device is operating with an evaporator saturation temperature of 40°F and a suction line temperature of 42°F. The factory specified superheat for the current operating condition is 12°F. After checking the airflow and determining that it is within specifications, the technician should:

Recover charge because the system is overcharged.

An air conditioner with a thermostatic expansion valve is operating with a condenser saturation temperature of 110°F and a liquid line temperature of 85°F. The factory specified subcooling for the current operating condition is 10°F. After checking the condenser coil and airflow, the technician should:

Recover charge because the system is overcharged.

An air conditioner is operating with low system pressures, an evaporator saturation temperature of 20°F and a suction line temperature of 24°F. The suction line is frosting. The most likely cause of these symptoms is:

Reduced airflow across the evaporator.

Two variables that have a great impact on the system performance of air conditioning systems are:

Return-air wet-bulb temperature and the airflow across the evaporator.

The most reliable source of information on how to charge any particular piece of equipment is:

The equipment manufacturer.

Before checking a system's operating superheat, the technician should first check:

The evaporator airflow.

Systems with fixed restriction metering devices are critical to charge because:

The expansion device cannot adjust to changing conditions.

A split system's data plate is useful because:

The factory charge is shown on the data plate.

Why should liquid refrigerant not be introduced into an evacuated chiller?

The liquid can flash at sub-freezing temperatures and freeze the chiller bundles.

Using the liquid line ambient temperature approach method to check the charge, if the liquid line is cooler than specified:

The system is overcharged.

More charge should be added if:

The system subcooling is more than 1° F lower than specified.

Why must zeotropic refrigerant leave the cylinder as a liquid?

To avoid fractionation

When metering in zeotropes, why must the liquid flash off to vapor before entering the compressor?

To prevent too much liquid from damaging the compressor

Which of the following types of charge is usually employed when a partial charge is required?

Vapor

The most accurate method of charging a small appliance which holds less than a pound of refrigerant is to add:

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

Can zeotropes be metered in with the unit operating?

Yes, by slightly cracking open the low-side manifold valve and then closing it.

What type of refrigerant must leave the cylinder as a liquid to prevent the separation of the different components in refrigerant?

Zeotropic mixtures