Part B Exam - Unit 9

What is the purpose of the metering device in refrigeration systems?

Constantly regulates refrigerant flow while monitoring superheat

Why does the evaporator transfer less heat than the condenser?

The evaporator has lower energy than the condenser

What process reduces the amount of flash gas produced?

Subcooling

The amount of liquid that may flash in vapour can be as high as:

30%

The coefficient of performance is the ratio of:

The heat absorbed by the refrigerant, to the energy used to drive the compressor

What's the difference between A2L and A2 refrigerants?

A2L refrigerants have a lower burning velocity

Why might refrigerants have ODP?

Because they contain chlorine

Refrigeration systems have zero GWP if they do not store which chemical?

Carbon dioxide

Why is ammonia particularly desirable as a natural refrigerant?

Because it is energy efficient and has zero ODP and GWP

What are the ideal physical and chemical properties of refrigerants? (9)

Low boiling point

Low vapour specific volume

Low condensing pressure

Low liquid density

High latent heat capacity

Inoffensive odour

Non-corrosive

Nontoxic

Non-flammable

What are the physical properties of refrigerants? (6)

Miscibility

Leakage tendency

Odour

Toxicity

Flammability

Moisture reaction

Why do ammonia systems require oil separators?

Because they are not miscible with oil

Which physical property of refrigerants affects the size of piping?

Miscibility

Why is a strong odour not ideal?

It may induce panic-like behaviour

When a torch is used for leak detection, what should be provided?

Ample ventilation

What happens when a warm moisture-saturated refrigerant is cooled?

It will produce free water

Why should moisture in the refrigerant be avoided?

It may cause ice and acid to form

What is the most common brine used for ice rinks?

Calcium chloride

What are the purposes of economizers in packaged centrifugal water chillers? (3)

Produces flash gas

Increases NRE of the evaporator

Reduces power consumption of the compressor

Only refrigerant vapour that is \_____________ goes through two compression stages.

Produced by the evaporator

Why is less compressor work required in packaged centrifugal water chillers?

Because some refrigerant is diverted to the second compression stage through motor windings

What perk do refrigeration compressors have?

They can be cooled without fins

Rotary compressors are often used in:

Compound refrigeration systems

Rotary compressors produce ___________ evaporator pressure, and discharge ___________ pressure refrigerant into the main compressor.

Very low

Intermediate

How might rotary compressors reduce the work of the main compressor?

By having the intercooler lower the density

In centrifugal compressors, high velocity vapour travels through specially shaped passages of:

Increasing cross-sectional area

The helical design is primarily used in which applications?

Medium and high-capacity refrigeration

What advantages do the flooded evaporator and liquid recirculating evaporator have over the dry evaporator?

Higher heat transfer rate and response rate

In the liquid recirculating evaporator, the refrigerant is supplied with a:

Liquid ammonia pump

What is a critical element of the liquid recirculating evaporator?

Low-pressure receiver

What materials might bare tube evaporators be constructed of?

Steel pipe or copper

If the bare tube evaporator is in flooded or liquid recirculation service, where does the outlet go?

The accumulator

Bare tube evaporators are used where temperature is maintained below \_____degC.

1

Plate surface evaporators can be easily defrosted by:

Manual scraping

Which is the most widely used type of evaporator?

Finned tube evaporator

Usually in finned tube evaporators, when the temperature decreases, the \_________ increases.

Fin spacing

In the shell-and-tube evaporator, \______% of the tubes must be submerged at all times.

75

Which type of condenser is the most environmentally friendly?

Air-cooled condensers

Why might cooling towers use water-cooled condensers?

To limit water consumption

Why is the location of the cooling tower important to consider?

Because in winter conditions, humid air leaving can cause ice buildup

Which type of condenser has the lowest efficiency?

Double tube condenser

What type of condenser is difficult to clean, but is efficient and compact?

Shell-and-coil condenser

In shell-and-tube condensers, \__________ direct cooling water through multiple passes.

Water boxes

The shell-and-tube condenser is used extensively in \_________ installations.

Ammonia

What is an advantage of evaporative condensers compared to the other two types of condensers?

Requires a smaller amount of air/water

To control compressor capacity indirectly, change the:

Evaporator capacity

In bimetal thermostats, what device indirectly controls supply to large motors?

A magnetic motor starter

To keep the compressor from short-cycling, the thermostat has a:

Differential setting

Why can a suction pressurestat not be used when an automatic expansion valve is used as a metering device?

Because AEVs maintain a constant evaporator pressure

Why can't the pressurestat be used with a capillary tube or orifice-type metering device?

Because they don't prevent liquid refrigerant flow from the high to low side

The humidistat is accompanied by:

A liquid solenoid valve with a metering device

The low chilled water temperature cut-off switch shuts down the compressor when it drops below \________degC below the set point.

2.5 to 3

The motor demand limiter overrides the water temperature sensor if:

The load reaches the maximum amperage setting

The motor demand limiter sets a permissible opening to the:

Compressor inlet vanes

Instead of demand limiters, what might some chillers use?

Soft loading strategies

When restarting an inhibit, make sure to allow the motor to cool off for at least \_____ minutes before restarting.

10

If the vane control fails to prevent overload, what does the relay do?

It operates a solenoid valve to force the motor to close the vanes

The high-pressure cut-off is often combined with the same housing as the:

Suction pressure control

How much time is the compressor given to establish correct oil pressure, before it shuts off?

30 to 120 seconds

If the bimetal becomes hot enough to bend and open the circuit, what may be a possible issue? (3)

Sump oil level may be too low

Oil is foaming

Oil pump is not working properly

The flow switch is operated by:

The force exerted on a flexible vane immersed in a flowing fluid

The flow switch is often used as \________ if fluid flow becomes insufficient

Safety lockout switches

If the oil temperature rises to above \____degC, the compressor shuts down

80

The lube oil heater starts to raise the oil temperature when it's below \_____degC.

60

The inhibitor switch keeps the compressor from starting when oil temperature is below ____degC.

35

When the temperature of the refrigerated medium rises above the set point, the thermostat of the AEV:

Restarts the compressor

Which refrigerant metering device is the most widely used?

Thermostatic expansion valve

The thermostatic expansion valve maintains a superheat of around \____degC over the evaporator saturation temperature.

6

The TEV is controlled by the interaction of these three forces:

Thermal element (opens)

Evaporator pressure (closes)

Spring (closes)

To compensate for a large evaporator pressure drop across the TEV, equip an equalizing line connecting the _______ and the ________.

Diaphragm

Evaporator outlet

What is the low-pressure float valve used for?

Maintaining a constant level of liquid refrigerant

The low-pressure float valve may be installed in an external float chamber attached to the:

Accumulator

A low-pressure float valve is common in what situation?

When rapid response to changes is necessary

What happens if the low-pressure float valve fails?

A bypass line provides cooling

In sectional evaporators, each section has a:

Refrigerant flow control valve

What is an issue with using only face dampers in evaporator dampers?

Airflow varies with cooling load

Why do variable inlet guide vanes occupy an infinite number of positions in centrifugal compressors?

So they can finely adjust centrifugal compressor capacity

What do variable position slide valves permit in screw compressors?

Low-load compressor starts

What are the different refrigeration auxiliaries? (9)

Pressure gauges

Accumulator/surge tank/surge drum

Oil separator

Suction strainer

Filter-drier

Sight glass and moisture indicators

Economizers

Distributor

Vibration absorber

Permanent pressure gauges are usually installed in \________ systems.

Ammonia

Why are pressure gauges equipped with pulsation dampeners?

To prevent flickering of the pointers, which lead to premature wear

Why do compressor service valves have back-seats and flared ends?

To permit installation of service gauge sets

Why is the oil separator installed between the compressor and condenser?

So that oil does not coat heat transfer surfaces and increase power consumption

Why must the oil separator trap be kept hotter than the condensing temperature?

To prevent HFCs and HCFCs from draining to the crankcase with the oil

Vapour bubbles show in sight glasses if:

The liquid line is restricted, or there is insufficient refrigerant

If the sight glass is used to indicate whether the system is fully charged, where should you install it?

At the receiver outlet

If the sight glass is used to indicate if the refrigerant contains flash vapour, where should you install it?

Before the expansion valve

If the sight glass is combined with a moisture indicator, where should you install it?

Between the drier and expansion valve

What are economizers used for when used for an auxiliary? (3)

Reduce temperature of refrigerant

Increase temperature of vapour

Prevent flashing of the liquid

What is common practice to do concerning economizers, when you want heat transfer to take place?

Solder the capillary tube to the suction line

When picking out materials for refrigerant piping and tubing, make sure all materials used are:

Suitable for low-temperature service

For ammonia service, materials used are:

Steel; copper and iron are prohibited

Type F is prohibited for steel, except for:

Water-based secondary coolants

DN 150 and smaller must be at least Schedule \_____.

40

DN 50 and smaller must be at least Schedule \_____.

80

Pipe to be joined with threaded connections must be at least Schedule \_____.

80

What kind of materials may HFC and HCFC service use?

Copper or steel (only type K or L)

HFC and HCFC service is limited to service above ____degC and below ____ kPa.

-20

6895

What is the most common method of joining piping?

Welding

Threaded joints must be made with:

Tapered pipe threads