ESCO Commercial Refrigeration Certification

What is a dry type or direct expansion refrigeration system and how does the boiling refrigerant affect it’s efficiency?

Superheated gas leaves the coil and the refrigerant must boil as close to the end of the coil to increase efficiency, about 8-12 F superheat

How is superheat measured?

Evaporator boiling point compared to the suction line temperature best taken under design conditions

What effects will a blocked distributor tube cause on a refrigeration system?

Insufficient cooling, low evaporator pressure and high superheat

What is the purpose of the external equalizer on a TXV?

It compensates for pressure drop on large evaporator coils that have a high pressure drop

After adjusting a TXV how much time will it take for another superheat reading?

About 15 minutes

How are low temperature systems typically defrosted?

By electric of hot gas

What metering device is used on flooded refrigeration systems?

Low side float

Which component will refrigerant exist as superheated, saturated and sub-cooled?

The condenser

Name three types of water cooled condensers

-Tube in Tube

-Shell in Tube

-Shell and Coil

When adjusting superheat on a TXV how long after the adjustment must you wait until the next reading can be taken?

10-15 minutes

What must be done periodically to keep proper air circulation on low temperature evaporator coil?

They must be defrosted every 6 to 8 hours per day

The refrigerant condensing temperature of a water cooled condenser is about _____ degrees F higher than the leaving water

10

The refrigerant in an air cooled condenser is about _____ degrees higher than the ambient air

30

Name 3 ways to control head pressure on air cooled systems during low ambient conditions

-Fan Cycling

-Dampers

-Condenser flooding

Cooling towers that pass air over the water to remove system heat are limited in capacity by:

Outdoor WB Temp

One of the main reasons that reciprocating compressors are not 100% efficient is:

The clearance volume at the head of the compressor

An open compressor is most likely to develop a leak at the ______________

Shaft seal

Name two ways that capacity control is accomplished in refrigeration

-Unloaders

-Multiple compressor operation

What can a dirty evaporator cause?

Low evaporator pressures and high compression ratios

A TXV maintains a constant _________

Superheat

An AXV maintains a constant ___________

Evaporator pressure

Some capillary tube systems use a liquid/suction heat exchanger by soldering the capillary tube to the suction line. When measuring superheat where should the superheat be taken, before or after the heat exchanger?

Before the heat exchanger

Name a type of ultra low temperature system that uses an evaporator to remove heat from a condenser

A cascade system

A defrost cycle is usually initiated by a _____________

Defrost timer

How is a defrost cycle terminated?

-Time

-Temperature

-Low side pressure

A defrost cycle is required when the evaporator operates below ________

32F

What does excessive frost cause on an evaporator?

Reduced heat transfer and a decrease of efficiency

What does an automatic pump-down system prevent?

Liquid migration during the off cycle

What are the components of an automatic pump down system?

-Tstat

-Liquid Line Solenoid

-Low Pressure Control

What is typically the cause of an automatic pump down system short cycling on the low pressure switch?

A leaking liquid line solenoid

What type of metering device is best when trying to prevent the evaporator from freezing?

An AXV

What is the purpose of a crankcase heater?

It helps fight against migration during the off cycle by heating the oil in the compressor

What compressor allows access to the internal compressor components?

A semi-hermetic

What type of maintenance does a cooling tower require?

Cleaning, water treatment, and proper airflow adjustments

High temperature refrigeration systems maintain box temperatures of _______

45 to 55 F

Medium temperature refrigeration systems maintain box temperatures of ______

30 to 42 F

Low temperature refrigeration systems maintain box temperatures of ______

0 to -20 F

What is the purpose of an oil separator?

It captures oil sends it back to the compressor and increases system efficiency by not allowing excessive oil flow through the system

How are blended refrigerants charged into an operating system?

Into the lowside as a liquid through a throttling valve

A capillary tube system is critically charged up to a ¼ ounce in order to:

Prevent liquid flood back during a light load and achieve peak efficiency and performance

What refrigerant can be released to the atmosphere and can only be used one time?

Expendable refrigerants

What is the common coil TD - temperature difference between the evaporator coil and the air when a high relative humidity is required for product storage?

10 F

Where is the refrigerant stored during a manual pump down?

In the condenser and receiver

What are the service valve positions during normal system operation?

Back seated

Which port is closed when a service valve is backseated?

The service port

What is the purpose of an accumulator?

It prevents liquid refrigerant from entering the compressor, located in the suction line

When working on a refrigeration system what position should the service valves be in to read system pressures?

The mid or cracked position

What type of oil is used with HFC refrigerants?

Polyolester

What is the low pressure switch used for?

It senses low side pressure and cycles the compressor off if the system has a low charge. It can also be used to maintain space temperatures

What valve is used to maintain different evaporator temperatures on a multiple evaporator system?

The EPR valve – Evaporator Pressure Regulator

What valve prevents the evaporator from falling below a set limit?

The EPR valve

What valve is used to prevent the compressor from overloading during a hot pull down?

The CPR valve – Crankcase Pressure Regulator

On Multiple compressor systems how are the compressors controlled?

By low pressure controls set at different operating pressures

What is an electrically controlled shut-off valve

A solenoid valve

When a TXV creates unstable suction pressures it is known to be__________

Hunting

What kind of damage can hunting cause?

Liquid slugging of the compressor

Head pressure control valves allow hot gas to flow to the __________

Receiver

What kind of damage can a coil freeze up cause?

Liquid slugging of the compressor

Where is the oil separator located?

In the discharge line

When and where should oil traps be installed?

At the bottom of a suction or discharge line riser and every 15 to 20 feet on long risers

How is the compression ratio calculated?

Discharge PSIA / Suction PSIA

Where should vibration eliminators be installed?

In the suction and discharge lines at the compressor they should run parallel to the crankshaft

With blended refrigerants what is known as the difference between boiling point and condensing point?

Temperature glide

How is the low side pressure monitored in large rack compressor systems?

By a suction transducer

Which refrigerant line is most sensitive to improper sizing?

The suction line

What is most likely the cause of the following symptom?

-The compressor shuts off on the oil safety switch

-The compressor sight glass is ½ full

-The suction pressure is 3 PSIG (R-502) oil pressure is 70 PSIG

A defective oil safety switch

What happens during the winter if the head pressure control goes bad on a system during low ambient conditions?

The condenser and evaporator pressures drop and the system will shut down on the low pressure control

A string of 5 frozen food cases are operating by on compressor. All 5 cases are iced/frosted up what is most likely the problem?

Faulty defrost timer

At what temperature does an ice maker begin to make ice?

32 F

Why would a solenoid valve be located in the oil line of an oil separator?

It isolates the high and low side during the off cycle in case the float valve is leaking

Name 2 ways that defrost is accomplished with low temp systems

Electric & hot gas

Bubbles or foaming in a compressor sight glass indicates what problem?

Refrigerant in the oil or flood back

What is the viscosity of most oils used in refrigeration systems?

100-200

What causes a symptom of a sweating liquid line?

A plugged filter dryer

If a capillary tube system is repaired and too large of a filter dryer is installed what must be done to the system?

Add more refrigerant than recommended by the manufacturer

What does excessive frost accumulation over the top third of an evaporator on a cooler indicate?

The evaporator is starved due to a partial low charge

What may cause short cycling on the low pressure control during the off cycle on an automatic pump down system?

A leaking liquid line solenoid

Why must an oil separator be kept warm?

To prevent refrigerant from condensing during the off cycle this could cause dilution of oil and foaming in the sight glass. It could cause a liquid slug on startup

What is the differential of a low pressure control that has a cut in set for 40 PSIG and a cut out set for 15 PSIG?

25 PSIG

When changing a CFC system to and HFC system what must always be done?

Extensively flush the mineral oil out of the system

What type of oils are ester based oils?

Synthetic

How is the net oil pump pressure measured on a compressor?

Oil pump pressure minus suction pressure

Draw a symbol of NO and NC timer contacts

NO: ./.

NC: ._.

What pressure opens the contacts of an oil pressure safety switch?

Crankcase pressure and oil pump discharge pressure

What are the two adjustments on a high pressure control?

Cut in and cut out

What are the temperature range of cryogenic refrigerants?

-250 t0 -450 F

What can cause binding of an auger on a flake type ice machine?

Lack of water to the evaporator

What can cause icing on the TXV and what could be the result of the ice?

Typically a low charge and the ice may cause and the expansion and contraction of the ice may cause leaks on flared fittings

If the harvest solenoid on an ice machine has a slight leak what will this cause?

Higher than normal superheat and insufficient ice production