HvacR Commercial Refrigeration Midterm Unit 25/26/quiz's

A ___ varies the frequency of an elctroniclly altered sine wave, which affects the speed of alternating current motors

An inverter

Large or small compressors on parallel compressor racks, which are usually dedicated to the coldest cases,are called? A. Satellite compressors B. Remote compressors C. Booster compressors D. Both A and C

D. Both Satellite and Booster compressors

What holds up the structure of a walk in cooler?

1-4 inch thick panels that are made of foam insulation sandwiched in-between galvanized sheet metal or aluminum.

What are three main reasons why automatic pumpdown systems are used refrigeration systems?

1. To rid the evaporator, suction line, and compressor of refrigerant before the off-cycle and defrost cycle to prevent migration of refrigerant back to the compressor and crankcase 2. To prevent surges of liquid refrigerant from entering the suction port of the compressor (slugging) during start up. 3. To rid the crankcase of refrigerant to prevent bearing washouts and foaming during start ups

What is the triple point of carbon dioxide

69.9 at 75psia

What is the critical point of carbon dioxide

88F at 1067psi

What will happen if the float in the oil separator collapses

A damaged, leaking or collapsed float in an oil separator will cause to orifice to stay closed and excessive amounts of oil will accumulate in the oil separator. This situation could rob the system of valuable lubricant and cause compressor damage

Define natural refrigerant

A natrually occuring refrigerant such as Co2 (R-744) Ammonia (R-717) Propane (HC-290) Butane (HC-600) and Isobutane (HC-600a)

How is defrost accomplished when the equipment room is in the back and the medium temperature fixtures are in the front?

A power supply must be supplied to the fixture and a planned of cycle defrost can be accomplished at the equipment room through a time clock and liquid line solenoid valve

Why is there a time delay in the oil safety controller

A time delay is built into the control to allow the compressor to get started and prevent unneeded cut-outs when oil pressure may vary for only a moment. This time delay is normally 90-120 seconds.

When the compressors are located in the equipment room, the piping is routed, A. In the floor chases or in plastic pipes under the floor B. In suspended ceilings C. Between the walls D. In back of the walls

A. In the floor chases or in plastic pipes under the floor

Which of the following is NOT a low ambient control on air cooled equipment A. Modulating TXV B. Fan-cycling controls C. Flooding condenser D. Fan speed control

A. Modulating TXV

The function of the evaporator pressure regulator is to A. Maintain a constant evaporator pressure B. Prevent a minimum pressure in the evaporator C. Maintain a constant evaporator temperature D. Prevent maximum pressure in the evaporator

B. Prevent a minimum pressure in the evaporator

When one large compressor is used, what desirable features should it have. A. Discharge muffler B. Suction muffler C. Cylinder un-loaders or variable speed drive D. Hot gas bypass

C. Cylinder un-loader or variable speed drive

Describe a suction accumulator

Can be located in the suction line to prevent liquid refrigerant from passing into the compressor by giving the refrigerant a place to boil off before entering the compressor.

What special precautions should be taken with drain lines in walk in coolers? A. Freeze protection B. Trapping to prevent air from being pulled in C. Downward sloping D. All of the above

D. All of the above

What special valve is used in a refrigerant air drier A. Four way valve B. Defrost valve C. Three way valve D. Hot gas bypass valve

D. Hot gas bypass valve

Exactly matching the refrigeration load with the capacity of refrigeration compressors is referred to as: A. Capacity control B. Load shedding C. Load paralleling D. Load matching

D. Load Matching

The purpose of the crankcase pressure regulating valve (CPR) is too? A. Maintain constant pressure in the crankcase B. Maintain constant pressure in the evaporator C. Prevent a compressor from going into a vacuum D. Prevent a compressor overload condition

D. Prevent a compressor overload condition

Which is not a basic component of the compression refrigeration cycle? A. Compressor B. Condenser C. Evaporator D. Suction Filter

D. Suction Filter

Describe the function of an electric evaporator pressure regulating (EEPR) valve

EEPR valves control the discharge air temperature in the refrigerated case. They regulate the saturated pressure in the evaporator in order to provide the temperature required at the discharge air sensor by way of a bi-polar step motor.

Two methods for rejecting heat from refrigerated cases are?

Evaporate condensers ad cooling towers

What is the difference between an even and uneven parallel compressor system

Even systems have compressors that are of equal size and are for refrigeration loads that stay constant while uneven systems are for systems with compressors of unequal size and varying loads

Describe a filter drier

Found anywhere on the liquid line after the king valve. Used to remove foreign material from the refrigerant such as flux, dirt, moisture etc.

The three temperature ranges for refrigeration systems are?

High (47-60), Medium (28-40) and Low (0-(-20))

Two methods for accomplishing defrost on low temperature refrigeration systems are?

Internal heat (hot or cool gas) and External heat (Electric)

Explain the main idea behind pressurized liquid systems and list their advantages.

Is a system that used a centrifugal pump to pressurize liquid refrigerant in the liquid line. By pressurizing the refrigerant the saturation temperature goes up while keeping its temperature the same. The advantage is it gives the condenser more internal volume for condensing refrigerant and in turn lowers the condensing temperatures and pressures which lower ratios and boosts efficiency

What is off-cycle defrost

It is a defrost cycle that occurs when the compressor is off. When the compressor is off, the evaporator fans will continue to run and the air in the cooler will defrost the ice from the evaporator coil due to it being at a temperature higher than freezing

Describe a low pressure switch

It is a device that provides low-charge protection and it also provides control of space temperature. For low-charge protection, the low pressure cutout will be set below the expected operating condition and above the atmospheric pressure of 0 psig. It also helps maintain the space temperature in the cooler. When the temperature drops, the pressure drops. When it gets to the cutout point, the compressor stops. The temperature will gradually rise and when it reaches the cut in pressure, it will start up again

Describe a High pressure switch

It is used to keep the compressor from operating with a high head pressure. It is usually seen on water-cooled equipment because an interruption of water is more likely than an interruption of air. It should be set above the typical high pressure that the machine would encounter. It can be automatic or manual reset.

What is the function of the microproccesor-based controller in parallel systems?

It's function is to read a voltage coming from the pressure transducer and then increase or decrease the capacity steps of the compressor to match the refrigeration load that are pre-programmed in the microprocessor

Three disadvantages of a parallel compressor system are?

Leaks that can effect an entire system, compressor burnouts that can effect the entire oil system and extensive clean up when required

Describe the function of an oil separator

Located in the discharge line to separate the oil from the refrigerant and return the oil to the compressor crankcase

Describe a receiver

Located in the liquid line and is used to store refrigerant leaving the condenser. Must be sized to handle 100% of the refrigeration charge.

Define synthetic refrigerant

Man made refrigerant suchas CFC's

Describe modulating fluid flow

Modulating fluid flow is throttling the amount of fluid through a gas line. Controlling the flow between maximum or no flow.

The two broad categories of display cases are?

Open and closed

How are conditions maintained in open display cases when there are high shelves?

Open-display cases can maintain temperature conditions because the refrigerated air is heavy as settles at the bottom. Air curtains are also applied to keep the cold air contained in the space

Define a parallel compressor system and list five advantages of of these systems.

Parallel systems apply two or more compressors to a common suction header, a common discharge header, and a common receiver. Five advantages are: 1. Load matching 2. Diversification 3. Flexibility 4. Higher efficiencies 5. Lower operating cost.

Describe a pressure tap

Pressure taps provide a method of taking pressure readings at places that do not have service ports.

Two types of off-cycle defrost are?

Random and Planned

Three advantages of unitary, stand alone refrigeration modules in supermarket refrigeration are...

Reduction in refrigeration charge by 75-80%, 75% less piping and require 75% less joints

Explain the function and advantages of secondary fluid systems

Secondary fluid refrigeration is the process of using a refrigeration system and heat exchanger to cool a secondary source of fluid to cool a space or product. This secondary fluid is usually Propylene glycol. Some advantages are large refrigerant reductions, reduced leak potential, and simpler installations

What is meant by carbon dioxide having a critical point

The critical temperature of a gas is the highest temperature a gas can have and still be condensable by the application of pressure. Above this temperature, a gas cannot be saturated regardless of the pressure applied.

Briefly explain what is meant by sub-critical refrigeration cycle and its four processes

The subcritical cycle refers to the Co2 refrigeration cycle that takes place above the refrigerants triple point and below the refrigerants critical point. It's four processes are 1. Compression 2. Condensation with subcooling 3. Expansion 4. Evaporation with superheat

What is the function of the pressure transducer in parallel compressor systems?

The transducer reads pressure in the systems suction line and converts that reading into a voltage to send back to the micro processor. This will in turn increase or decrease the capacity steps pre-programmed into the compressor

What is meant by carbon dioxide having a triple point

The triple point is where a gas can exist in all three phases at once (Liquid, Solid, Gas)

Briefly explain what is meant by surge-type receiver

This arrangement preserves liquid subcooling when free subcooling is supplied by low ambient temperatures. A solenoid valve diverts subcooled liquid from going into the receiver and directs it straight into the mechanical subcooler and then to the liquid header to feed the expansion valves.

Describe a low ambient control

This control stops and starts the condenser fan in response to head pressure. This control must be coordinated with the high pressure control to keep them from working against each other. This is done by setting the high pressure control cut out above the low ambient control setting cut in

Briefly explain what is meant by a trans-critical refrigeration cycle and list its four processes

Transcritical means that the thermodynamic refrigeration processes span above the subcritical region and into the supercritical region of pressure/enthalpy diagram. It's four processes are 1. Compression 2. Gas Cooling 3. Expansion 4. Evaporation

True or False: Two types of defrost used with low temperature fixtures are hot gas and electric.

True

Describe an oil safety switch

Used to confirm there is an adequate oil pressure 90-120 seconds after start up. Used on larger compressors of above 3hp

What is the function of the oil return hole or orifice in the suction accumulator?

Used to meter both liquid refrigerate and oil back to the compressor at an acceptable rate.

Describe a heat exchanger

Usually installed at the evaporator outlet with the heat exchange happening between the suction and liquid line. the heat exchanger improves capacity by sub cooling the liquid refrigerant entering it causing an increase in net refrigeration. It also prevent liquid refrigerant from moving out of the evaporator and into the compressor

Briefly describe the function of a non-short-cycling or "one-time" automatic pumpdown system

When components leak, the latching relay contacts (LRC) prevent the compressor from short cycling during pumpdown, which causes low-side pressures to increase and low-pressure switches to close prematurely.

What is the function of the pressure equalization orifice at the outlet of an accumulator.

When the compressor starts up, a column of liquid refrigerant would be sucked out of the accumulator and into the compressor if not for the pressure equalization orifice

Why is low-ambient control necessary?

When the condenser is located outside, the head pressure will go down in the winter to the point that the expansion device will not have enough pressure drop to feed refrigerant correctly.

What is the function of the current sensing relay in an oil safety control scheme

When the current of the compressor drops due to the motor being off, the contacts of the relay will open and take power away from the time delay heater. This prevents a tripping of the oil safety control in case the internal overload opens

What are mullion heaters

an electric heater mounted inside the mullion to prevent moisture from forming in humid weather

Two types of relief valves are?

spring loaded (will re seat) one time (will not close back up)

Explain how the oil separator, oil reservoir, pressure differential valve, and oil level regulators work together to maintain the correct oil level in the parallel compressors.

the oil separator traps the pumped oil, it is then sent to the oil reservoir, these are usually in the same tank. From the reservoir the oil passes to an oil regulator float valve, this valve senses the crankcase oil levels and open and closes as necessary to ensure an adequate oil level in each compressor crankcase. a pressure differential valve is located on the oil reservoir usually maintains a differential pressure between 5-20 psi between the oil reservoir and the common suction pressure. This valve ensures that the oil reservoir is pressurized each time the oil regulator opens