|
A Compressor is an equipment, which
raises the pressure of a gas from a given value to the
higher value.
A Compressor can be positive
displacement or centrifugal type.
Positive displacement Compressor can
be reciprocating or centrifugal type.
| |
 |
| What is a Tonne of Refrigeration? |
|
It is equivalent to the production of
cold at the rate at which heat is to removed from one US
tonne of water at 32 0 F to freeze it to ice
at 32 0 F in one day or 24 hours, and is
denoted as TR
Thus
1 TR = (1*2000 lb * 144 But/lb) / 24
hours
=
12,000
Btu/Hr
=
200 Btu/min
| |
|
| What is Primary Refrigerant? |
|
Primary
Refrigerant is a refrigerant, which satisfies the
requirements of the application; it also satisfies the
thermodynamic, chemical and physical properties.
Some of the
Thermodynamic properties which the refrigerant should
satisfy are Normal boiling point, condensing and
evapourating temperature, critical temperature &
pressure freezing point, volume of suction vapour per
tonne, COP, power consumption per tonne.
Some of the
chemical properties the Primary Refrigerant should
qualify are inflammability, toxicity, reaction with
water, oil etc.
Some of the
physical requirements of these include Dielectric
strength, Thermo Physical properties such as
conductivity, viscosity etc.
Some of the
popular primary refrigerants are Ammonia, Freon and
Hydrocarbons.
| |
|
| What is Secondary Refrigerant? |
|
In large refrigeration
plants, secondary coolants, such as water, brines,
glycols and sometimes even halocarbons are used for
carrying refrigeration from the plant room to the space
where it is usefully applied instead of directly
obtaining it by the evapourating refrigerant at the
place of appliance to reduce the quantity of the
refrigerant at the place of application to reduce the
quantity of the refrigeration charge in the system and
to reduce pressure losses. The desirable properties of
the secondary coolants are low freezing point, low
viscosity, non inflammability, good stability and low
vapour pressure. Chilled water is used as a secondary
refirgerant in air-conditioning applications. For
low-temperature applications, brines, glycols and
hydrocarbons are used.
| |
|
| What is Refrigerant? |
|
Refrigerant is a
medium of heat transfer, which passes through a
refrigeration cycle evapouration, condensation, and
throttling.
What is Refrigeration
effect?
Refrigeration
effect is the amount of cooling produced by a
refrigeration system.
What is a
Refrigerator?
Refrigerator is a
cabinet having a freezer and space for chilling and
storing pershiable or other commodities.
| |
|
| What is Air
conditioning? |
|
It is the maintaince of
temperature and humidity of air inside the space below
or above than that of surroundings.
The air is cooled or
heated and dehumidified or humidified before being
supplied to the conditioned space.
Different types of
compressors used in the industry are:
Reciprocating
Rotary
Scroll
Screw
Centrifugal
| |
|
| What is Evaporator? |
|
Evaporator is
the component of the
refrigeration system in which heat is removed from air,
water or any other body by the evapourating
refrigerant.
| |
|
| What is Condenser? |
|
Condenser is heat exchanger in which
vapour changes its phase into liquid due to heat
transfer to cooling medium flowing through
it.
| |
|
| What is an Expansion device? |
|
An
expansion device in a refrigeration system normally
serves
1.the thermodynamic
function of expanding the liquid refrigerant from the
condenser pressure to Evaporator pressure.
2.as the control
mechanism for the supply of the liquid to Evaporator at
the rate at which it is evapourating.
| |
|
| What is a Carnot cycle? |
|
A reversible heat engine
can be reversed in operation to work as a refrigerating
machine.
A reversible
refrigeration cycle has the maximum Coefficient of
Performance (COP).
Sadi Carnot in 1824 proposed a
reversible heat-engine cycle as a measure of maximum
possible conversion of heat into work. A reverse Carnot
cycle can, therefore, be employed as a reversible
refrigeration cycle.
The cycle consists of
two isothermals and two isoentropics as follows:
Process 1 - 2 Isoentropic compression,
s1 = s2.
Process 2 - 3 Isothermal heat rejection to
the hot reservoir at Tk = constant
.
Process 3 - 4 Isoentropic expansion,
s3=s4.
Process 4 - 1 Isothermal heat absorption from
cold reservoir at T0 = constant.
| |
|
| What are the Laws of
Thermodynamics? |
|
First law
When any closed system
is taken through a cycle ,the net work delivered to the
surroundings is proportional to the net heat taken from
the surroundings.
Second law
It is impossible to
construct a system that will operate in a cycle, extract
heat from a reservoir and do an equivalent amount of
work on the surroundings.
| |
|
| What is Vapour-Absorption Cycle? |
|
Above shows the
schematic arrangement of the actual vapour-absorption
cycle and figure below presents its thermodynamic cycle
on the hr diagram. The system consists of generator G,
analyser AN, dephlegmator D and condenser C on-the high
pressure side, and Evaporator E and absorber A on the
low pressure side. Pump P, expansion valve V I and
pressure-reducing valve V II separate the two sides. In
addition, liquid-vapour heat exchanger HE I and
liquid-liquid heat exchanger HE If are also
provided.
The vapours at 5, distilled from the
generator-analyser, enter the dephlegmator. The vapours
with higher concentration of the refrigerant at 7 then
enter the refrigerant circuit, whereas the drip at 6
returns to the generator-analyser. The vapours are
condensed to 8 in the condenser, precooled to 9 in the
liquid vapour regenerative heat exchanger and throttled
to 10 before entering the Evaporator. The state 10 is at
the same point as state 9 on the h-~ diagram as both
enthalpy and composition remain the same before and
after throttling. The refrigerant entering the
Evaporator at 10, leaving the Evaporator at 11 and the
liquid vapour heat exchanger at 12, comprises a liquid
plus vapour mixture. The refrigerant is finally absorbed
by the poor solution at 2 returning from the generator
after being cooled in the liquid-liquid heat exchanger
to 3 and throttled to 3a, whereas the rich solution from
the absorber at 4 is pumped to 4a and heated to l a
before entering the analyser. The state points 1, 2, 3
and 4 can be located -on the h-~ diagram according to
their temperatures and compositions as the enthalpy of
liquid is independent of pressure. Also, point 3a lies
at 3 only (isoenthalpic process) and point 4a lies
approximately at 4 itself as the pump work is very
small. State point 5 of the vapour is along the
isothermal tie line drawn from 1. State point 7 is on
the tie line corresponding to the dephlegmator
temperature and condenser pressure pk. Point 8 is the
saturation state at Pk and at the same
composition as 7. Point 9 after subcooling of the liquid
can be plotted according to the temperature and
composition and point 10 is at'9 itself (isoenthalpic
process). Point 11 is on the tie line corresponding to
Evaporator leaving temperature Toe =
To and pressure Po and composition
same as at 7, 8, 9 and 10. Point 12 can be similarly
located by knowing the temperature from the energy
balance of the heat exchanger.
| |
|
| What is Low Back Pressure? (Low-Pressure Applications)? |
|
The application in which
the suction pressure is in the range of 0-10 PSIG with
an equivalent evapouration of -30 to -14 C
.
Generally used for low-pressure applications like
Deep freezer.
| |
|
| What is Commercial Back Pressure (Medium-Pressure Applications)? |
|
The application in which
the suction pressure is in the range of 17 - 40 PSIG
with an equivalent evapouration of -15 to +15 C
.
Generally used for low-pressure applications like
bottle coolers.
| |
|
| What is High Back Pressure (High-Pressure Applications)? |
|
The application in which the
suction pressure is in the range of 25 - 55 PSIG in R-12
/ R- 134a and +25 to 95 in R-22 with an equivalent
evapouration of - 6.7 to -+12.8
C.
| 
