HVAC / Refrigeration

Question 1

Define latent heat (enthalpy)

Answer 1

The heat energy required to change the state of a refrigerant from a solid to liquid or from a liquid to a vapour or gas at a constant temperature and pressure.

Question 2

Define vapour pressure

Answer 2

The pressure of a vapour in contact with its solid or liquid form

Question 3

Define saturation

Answer 3

The point at which the greatest possible amount of substance has been absorbed by a solution or vapour at a given temperature. Air is said to be saturated when it will absorb no more water vapour.

Question 4

Define superheat

Answer 4

The heat energy added to raise the temperature of a vapour or gas above the saturation temperature at which it changes from a liquid to a vapour or gas.

Question 5

Define sub cooling

Answer 5

The amount of heat energy removed below saturation temperature of the liquid refrigerant.

Question 6

Define zeotropic blend

Answer 6

A zeotrope is a blend of two, or more, gases which when mixed compose a blend that has two, or more, boiling temperatures (depending upon the number of substances). When boiling the more volatile substance evaporates first, modifying the blend composition locally.

Question 7

Define azeotropic

Answer 7

An azeotrope is a blend of two, or more, gases which when mixed compose a blend that has a common boiling temperature (independent of the boiling temperature of each component).

Question 8

Define temperature glide

Answer 8

The temperature difference of boiling temperatures of a blended substance in a zeotropic blend

Question 9

Conditions allowing legionella to proliferate

Answer 9

Air-conditioning systems have the potential for supporting colonies of bacteria particularly if they have the conditions needed to proliferate. Bacteria need the following;

Relatively warm temperatures – ideal growth is in temperatures between 35–50° C

Presence of stagnant water – contaminated water is an ideal breeding ground for bacteria

Source of nutrients – need a food supply such as rust, sludge, scale, algae, and other bacteria to survive.

Question 10

Transmission of legionella

Answer 10

Transmission can be by inhalation of infected water droplets or by ingestion of contaminated water by some means, such as water droplets settling on food or hand to mouth.

Question 11

Symptoms of legionella

Answer 11

Flu like symptoms including;

High temperature, feverishness and chills

Cough

Muscle pains

Headache

Question 12

M notice regarding legionella

Answer 12

MGN 38

Question 13

Two causes of excessive discharge pressure in a domestic refrigeration system

Answer 13

Air in system – rectified by pumping the gas into the condenser and bleeding off air

Over charge of gas – reduce charge by bleeding off excess gas into recovery bottle until correct pressure is obtained

Question 14

Two causes of low pressure at evaporator suction in a domestic refrigeration system

Answer 14

Expansion valve shut in – rectified by adjusting the expansion valve manually until suction pressure is correct

Suction filter of expansion valve choked – clean filter

Question 15

Two causes of compressor cycling in a domestic refrigeration system

Answer 15

Low pressure cut out set too high – check and reset

Expansion valve iced up due to water in the system – defrost, and replace drier

Question 16

Two causes why compressor is running for long periods but not lowering the temperature in the cold rooms

Answer 16

Cold room door not shut – shut door

Under charge of gas – check and recharge

Question 17

Two causes why the compressor is operating noisily in a domestic refrigeration system

Answer 17

Liquid refrigerant returning to compressor – adjust expansion valve to correct amount of super heat

One compressor – start standby machine and overhaul compressor

Question 18

With reference to a vapour compression refrigeration plant explain the purpose of an expansion valve

Answer 18

The expansion valve controls the flow of refrigerant to the evaporator. It uses the effect of a convergent nozzle, lowering the pressure and thus the saturation temperature of the refrigerant. As the pressure drops, the liquid is above the saturation temperature for that pressure and so the liquid starts to change to vapour. Because this change takes heat, and the source of energy is the refrigerant, the temperature of the refrigerant must fall until it reaches its saturation temperature when it will be a mixture of gas and liquid.

Question 19

Symptoms and remedy of air in a refrigeration system

Answer 19

Air will cause the compressor to overheat and a temperature and pressure in the condenser will be too high. Air being lighter than refrigerant will collect at the top of the condenser, reducing condensing capacity.

Pump the charge into the condenser with the condenser liquid stop valve shut and after waiting a few minutes remove the air and a small amount of refrigerant from the top of the condenser by pumping into a recovery bottle. Do not vent to atmosphere.

Question 20

Symptoms and remedy for water in a refrigeration system

Answer 20

Water or moisture will cause internal choking of the expansion valve with ice.

Replace drier cartridges. Warm expansion valve with hot water soaked rag.

Question 21

Symptoms and remedy for oil carryover in a refrigeration system

Answer 21

Cold room temperature not reaching desired value. Compressor cycling on low pressure switch. Apparent loss of oil from compressor.

The plant must be pumped out and shut down. The evaporator must be removed for cleaning internally, the oil may have congealed on the heat transfer surfaces. Oil separating unit must be replaced. Plant must be recommissioned.

Question 22

Symptoms and remedy for overcharge in a refrigeration system

Answer 22

High compressor discharge pressure. Full sight glass at compressor. Compressor icing on such a side.

Extract refrigerant into recovery cylinder using vacuum pump. Do not vent to atmosphere.

Question 23

Symptoms and remedy for under charge in a refrigeration system

Answer 23

Lower suction pressures, higher evaporator outlet temperature. Compressor cycling, bubbles inside glass.

Possibility of leak, investigate and rectify. Recharge system

Question 24

What is meant by secondary refrigerant

Answer 24

A primary refrigerant is one that circulates through the compressor, condenser and evaporator of a refrigeration circuit. In order to limit the amount of the primary refrigerant in circulation through large cases requiring cooling a secondary refrigerant is employed. This is cooled in the evaporator and then circulated.

Question 25

State a suitable fluid for secondary refrigerant

Answer 25

Calcium chloride and distilled water (brine)

Ethylene glycol.

Question 26

Possible consequences if the density of the brine used for secondary refrigerant is too low

Answer 26

If the density of the brine is too low there is a danger of it freezing in the system effectively blocking pipes.

Question 27

Possible consequences if the density of the brine used for secondary refrigerant is too high

Answer 27

If the density is too high there is a danger of the salt coming out of solution and solids forming and causing a blockage.

Question 28

Advantage of using fully synthetic oils in refrigeration compressers

Answer 28

Synthetic oils are stable at high temperatures with good properties at low evaporated temperatures with low wax.

Reduced oil consumption.

Well defined miscible with HFC refrigerants and they provide long compressor life to reduced component where, corrosion protection, water resistance and lubricant breakdown resistance.

Question 29

Why is under cooling of the refrigerant at the condenser outlet desirable

Answer 29

As liquid passes through the expansion valve some flash off of liquid to vapour will occur, the greater the undercooling at the condenser outlet the lower the flash off percentage and thus increase the refrigeration effect in the evaporator. This is because undercooling will bring temperature further under the saturation temperature of the gas.

Question 30

Why is superheating the refrigerant at the compressor suction desirable

Answer 30

If the gas leaving the evaporator is several degrees of superheat above its saturation temperature it will ensure the refrigerant is all gas at the compressor suction avoiding any liquid entering compressor and causing damage.

Question 31

Five thermodynamic properties of refrigerant.

Answer 31

1. Low condensing temperature – avoids heavy machinery and scantlings.
2. Low specific volume – reduces sizes and increases efficiency.
3. High coefficient of performance – High ratio of cooling effect to power consumed ,so lower power required.
4. High latent heat of vaporisation – so that less refrigerant has to be circulated for a given duty.
5. High critical temperature – so the condensation is possible at a reasonable pressure.