Description of refrigeration systems

1. Refrigeration
Systems
Elza Marisa P. de Figueiredo
quissala@quissala.com

2. Fundamentals
• Refrigeration describes the
process of removing hear from
spaces, objects or materials
• Refrigeration means an
artificial way of lowering the
temperature

3. Mechanical refrigeration
• Mechanical system or
apparatus that transfers heat
from one substance to another

4. Some questions
• Why the refrigerant changes
from liquid to vapor?
• Why it changes from vapor to
liquid?
• What happens in terms of heat
because of these changes of
state?

5. Mechanical refrigeration
(cont.)

6. Mechanical refrigeration
(cont.)
•
Compressors (generally) use motor-driven
rotating impellers to generate gas pressure.
–
•
Gaseous refrigerant enters the compressor at low
pressure and temperature and exits at high pressure and
temperature.
Inside condenser coils gaseous refrigerant
condenses to liquid state.
–
–
To facilitate phase change, the condenser dissipates
heat energy to ambient air or water.
High pressure refrigerant exits at lower temperature.

7. Mechanical refrigeration
(cont.)
• An expansion valve controls the flow of
high pressure liquid refrigerant to the
evaporator.
– As refrigerant passes through the expansion
valve it is further cooled by the Joule Thompson
effect, the scientific principle that the temperature
of a stream is reduced when forced through a
narrow nozzle and allowed to expand.

8. Mechanical refrigeration
(cont.)
• Inside the evaporator, liquid
refrigerant vaporizes into a
gaseous state.
– Vaporization requires heat
energy, which is extracted from
the industrial process load (food
items to be cooled).
– The refrigerant is returned to the
compressor to repeat the cycle.

9. How to produce a
refrigeration effect?
• The material to be cooled
needs to be exposed to a
colder object or environment
• The heat will flow in its natural
direction:
– From the warmer material to the
colder material

10. Main Parts of a
Refrigeration System

11. Main Parts of a
Refrigeration System
(cont.)

12. Expansion Valve
• It regulates the amount of
refrigerant to the cooling coil
– The amount depends on the
temperature of the space being
cooled
• The control bulb controls the
opening and closing of the
expansion valve
– The substance varies

13. Expansion Valve (cont.)
• The expansion and contraction
transmit a pressure to the
diaphragm
– Move the diaphagm
downward, opening the valve
and allowing more refrigerant to
enter the cooling coil

14. Expansion Valve (cont.)
• When the temperature at the
control bulb falls, the pressure
above the diaphragm
decreases and the valve tends
to be close
• The temperature near the
evaporator outlet controls the
operation of the expansion
valve

15. Evaporator
• It consists of a coil of cooper
• The liquid enters the tubing at a
reduced pressure and, therefore,
with a lower boiling point.
• As the refrigerant passes through
the evaporator, the heat flowing to
the coil from the surrounding air
causes the rest of the liquid
refrigerant to boil and vaporize.

16. Evaporator (cont.)
• Refrigerant temperature in an
evaporator is directly related to
refrigerant pressure.
• After the refrigerant has
absorbed its latent heat of
vaporization (that is, after it is
entirely vaporized), the
refrigerant continues to absorb
heat until it becomes
superheated

17. Evaporator (cont.)
• The amount of superheat is
determined by the amount of liquid
refrigerant admitted to the
evaporator.
• This, in turn, is controlled by the
spring adjustment of the expansion
valve.
– It increases the efficiency of the plant and
evaporates all of the liquid.
• This prevents liquid carry-over into
the compressor (flooding back).

18. Evaporator (cont.)
• Excessive circulation of the
lubricating oil will cause the
evaporator temperature to
increase.
• The main cause of slugging is
improperly adjusted thermal
expansion valve.

19. Compressor
• It is essentially a pump.
– It is used to pump heat uphill
from the cold side to the hot side
of the system.

20. Compressor (cont.)
• The heat absorbed by the
refrigerant in the evaporator
must be removed before the
refrigerant can again absorb
latent heat.
– The only way the vaporized
refrigerant can be made to give
up the latent heat of vaporization
that it absorbed in the evaporator
is by cooling and condensing it.

21. Compressor (cont.)
• Because of the relatively high
temperature of the available
cooling medium, the only way
to make the vapor condense is
to compress it.

22. Compressor (cont.)
• When we raise the
pressure, we also raise the
temperature.
• The compressor also keeps
the refrigerant circulating and
maintains the required
pressure difference between
the high-pressure and lowpressure sides of the system.

23. Compressor (cont.)
• Compressors used in
refrigeration systems may be
lubricated either by splash
lubrication or by pressure
lubrication.
– Refrigeration compressors
require a lubricant with a low
pour point, and low wax content
to keep any oil leaving the
compressor from congealing in
the evaporator.

24. Compressor (cont.)
• The sudden reduction of pressure
occurring within the crankcase of a
refrigeration compressor during
starting causes the release of
refrigerant from the oil/refrigerant
mixture.
• Foaming of the oil in a refrigeration
compressor crankcase is caused by
refrigerant boiling out of the lube oil.

25. Compressor (cont.)
• The oil in the sump of a
secured refrigeration
compressor is heated to
reduce absorption of
refrigerant by the oil.
• Excessive oil foaming in the
crankcase of a refrigeration
compressor at start up can
cause compressor damage
from improper lubrication.

26. Compressor (cont.)
• The oil level in a refrigeration
compressor, the most
accurate reading is obtained
immediately after shutdown
following a prolonged period of
operation.
• The refrigerant has had time to
separate from the oil.

27. Compressor (cont.)
• Refrigerant entering the
compressor should be
superheated vapor. Its
possible to have liquid
refrigerant returned to the
suction side of a compressor
due to a faulty or improperly
adjusted expansion valve.

28. Compressor (cont.)
• A beam valve is frequently
used in refrigeration
compressor discharge
valves, and is designed to
pass liquid slugs.

29. Capacity Control System
• Most compressors are equipped
with an oil-pressure-operated
automatic capacity control system.
• This system unloads or cuts
cylinders out of operation following
decreases in the refrigerant load
requirements of the plant.
• A cylinder is unloaded by a
mechanism that holds the suction
valve open so that no gas can be
compressed.

30. Shaft Seals
• Where the crankshaft extends
through the crankcase, a leakproof
seal must be maintained to prevent
the refrigerant and oil from escaping
and also to prevent air from
entering the crankcase when the
pressure in the crankcase is lower
than the surrounding atmospheric
pressure.
• This is accomplished by crankshaft
seal assemblies.

31. Shaft Seals (cont.)

32. Condenser
• The compressor discharges the
high-pressure, superheated
refrigerant vapor to the
condenser, where it flows around
the tubes through which water is
being pumped.
• As the vapor gives up its superheat
(sensible heat) to the seawater, the
temperature of the vapor drops to
the condensing point.

33. Condenser (cont.)
• The refrigerant, now in liquid
form, is sub-cooled slightly
below its condensing point.
– This is done at the existing
pressure to ensure that it will not
flash into vapor.

34. A water-cooled condenser

35. Condenser (cont.)
• Circulating water is obtained
through a branch connection
from the fire main or by means
of an individual pump taking
suction from the sea.

36. Receiver
• It acts as a temporary storage
space and surge tank for the
liquid refrigerant.
• It also serves as a vapor seal
to keep vapor out of the liquid
line to the expansion valve.
• A pressure drop in the liquid
line of a refrigeration system
may cause the liquid
refrigerant to flash to gas.

37. Receiver (cont.)

38. Thank you !