Requirements of efficient operation of refrigeration plant:-
(1) Evaporating temp. & press should be
as high as possible, such that all heat transfer surfaces are kept clean, so
that temp, difference across them is at its minimum. It is essential that evaporator
surface are supplied with liq. Refrigrant at the correct temp and in correct
condition.
(2) Condensing temperature should be as low
as possible to keep the compressure deliver) pressure
to a minimum. This means correct flow of water in the condenser and to keep air
out of the system as being non-condensible. Air will raise the pressure by
(3)Suction & Delivery valves must not leak. Correct operation of
unloading device should be ensured. Valve leakage means reexpansion of gas from
delivery to suction
pressure. And hence reduced pumping capacity.
(4)Clearance volume should be minimum otherwise volumetric
efficiency will suffer.
(5)Temp, at inlet and outlet of each side of heat exchanger must be
correct to ensure correct superheat (at evaporator) and sub-cooling temps.(at
condenser)
(6)Refrigeration Capacity is directly proportional to the weight of
refrigrant evaporated in the evaporator i.e. directly related to the weight of
vapour pumped by the compressure. This is the sum of flash gas and liquid, so
to keep the weight higher, flash gas should be minimum.
(7)Weight of refrigerant vapour pumped by a compressure depends
directly on the temperature and hence pressure difference between the
evaporator and the condenser, and this must be kept to a minimum.
(8)Density of gas at suction is higher at
higher inlet temperature, so volumetric efficiency decreases by lowering the
evaporation temperature.
Thermostatie
Expansion Valve:-
The purpose of the thermostatie expansion
valve is to control the degree of superheat at the
exit of the evaporator coil such that only vapour enters the compressor
suction, and no liquid carry over takes place into the compressor.
In one system, a bulb fitted at the exit of
the evaporator coil along
with a capillary tube filled
with the liquid refrigrant used in the main system, is acting on top of the
diaphragm and tending to open the valve when the degree of superheat increases.
Whereas saturation pressure of the liquid refrigerant at the inlet of the
evaporator acts on the lower side of the diaphragm as well as pressure exerted
by the regulating spring acting on the lower side of the diaphragm tending to
close the valve. The spring tension set by the regulating spindle, controls the
degree of superheat, a typical value is 4°C to 6°C. At any constant operating
condition, these forces are balanced.
In practice, to achieve the desired degree
of superheat at the evaporator outlet, dry expansion
evaporators require up to 20 percent of their cooling surfaces to be available
to superheat the gas, the precise area varying with demand.
Equalising Line;-
Where thermostatic expansion valves are required to operate with
evaporators which are subject to large pressure drops, additional control is
introduced by incorporating a pressure equalizing connection. In this pressure
control type expansion valve, to maintain a fixed evaporator coil pressure, a
equalizing line connection from outlet of evaporator acts inside a bellow
upwards along with the spring force. The refrigerant filled bulb fitted at the
exit of evaporator exerts pressure outside the bellow. Evaporator pressure
reduction opens valve and pressure increase closes valve and when compressor
stops, the rising pressure causes closure.
Evaporator Back Pressure Regulator Valve :-
In large
installations, electronic temperature regulators are used to provide precise
control of the air temperature in refrigerated cargo spaces or water
temperature in a water chiller, by regulating the evaporator pressure and hence
the temperature. It consists of Main Evaporating pressure regulating Back
Pressure valve, Motorised Pilot valve, electronic controller and temperature
sensor. The back pressure valve is opened and closed by the pilot valve, which
regulates the pressure on top of the piston in the back pressure valve by means
of a spring loaded diaphragm. The spring torce can be cnangea by the electric
motor which is able to move a push rod up and down via a
gear rack.
Operation :-
The temp, sensor measures the temp, of the cooled air leaving the
evaporator. The registered temp, is continuously compared in the electronic
controller with the preset reference temperature. As soon as a differential
occurs between the measured value and set value, the controller sends the error
signal with proportional plus integral control, electric impulses to the pilot
valve motor, which moves the spindle of the pilot valve up or down.
If for example, the air temp, rises, the motor will move the
spindle up,
the pressure on top of the piston of the main valve (Back Pressure valve) will
increase, and the valve will move open slightly. As a result, the evaporator
pressure and temperature will decrease, the gas flow or capacity will increase
and the air temperature will fall again.
High Pressure Cut-out :-
The discharge press acts inside the bellow
and opposed by spring setting. During normal operation switch arm is held by
the switch arm catch and holds the electrical contact in place. Excessive
pressure expands the bellow, switch arm catch goes down around its pivot and
releases the switch arm so breaking the electrical contact, and causing the
compressor to cut-out. The machine can not be restarted until the trouble has
been remedied and switch re-set by hand.
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