superheated according to the thermal expansion valve
rate of expansion of the refrigerant. The rate of
setting.
expansion of the liquid spray is dependent upon the rate
of heat absorption from the air passing over the
The temperature of the vapor in the middle of the
evaporator tubes. As the refrigerant spray changes to a
coil is 40F. By the time it reaches point C, it is 50F. It
vapor, it absorbs heat from the evaporator. The heat
has absorbed an additional 10F of superheat. The
absorbed by the vapor is called superheat. By the time
thermobulb is mounted on the evaporator at the outlet to
the refrigerant leaves the evaporator, all of it should be
the compressor. The temperature within the bulb is the
in superheated vapor form.
same as that of the evaporator at point C. The pressure
within the thermal bulb, and consequently on the
The metering valve is opened or closed by the
diaphragm within the thermal expansion valve, is 46.7
action of the diaphragm at the top of the thermostatic
psi (P1). This pressure tends to push the diaphragm
valve. A thermobulb is attached to the diaphragm
housing by a small tube. The bulb is filled with the same
down, opening the valve from the receiver. Opposing
type of refrigerant used in the system. Heat felt by the
this force is the 37 psi (P2) exerted against the bottom of
thermobulb causes the refrigerant inside it to vaporize,
the diaphragm by the vapor at the inlet of the evaporator
which increases the pressure inside the bulb. The
coil. A spring pressure of 9.7 psi (P3) added to 37 psi
pressure is transmitted through the attaching tube to the
diaphragm. The increased pressure on the diaphragm
If an increase in load occurs, the superheat in the
causes the metering valve to open. Opening the valve
suction vapor increases, which causes the thermal bulb
increases the refrigerant flow into the evaporator.
temperature and pressure to increase; a greater pressure
In figure 11-11, a thermostatic expansion valve is
on the top of the diaphragm is then exerted. This causes
shown with the evaporator for a cooling unit operating
the valve to open further, allowing an increase in the
at 37 psi suction (low-side) pressure. The refrigerant
flow of refrigerant to the evaporator to restore superheat
moving through the evaporator coil absorbs heat from
to the 10-degree setting of the valve.
the air outside the coil. At point B, it has absorbed
If a decrease in load occurs, the superheat
sufficient latent heat for complete vaporization. At this
decreases, and the pressure in the thermal bulb
point, all the liquid has vaporized. Any additional heat
decreases. Evaporator inlet pressure plus spring
absorbed from the air raises the temperature of the
pressure tends to close the valve, which reduces the
vapor; but the pressure remains at 37 psi. This is the
flow of refrigerant sufficiently to maintain the
suction pressure of the compressor. By the time the
superheat at 10F.
vapor reaches the thermal bulb, point C, it has been
P1 = 46.7 PSIG
CAPILLARY
TUBE
THERMOSTATIC
P2 = 37 PSIG
EXPANSION
VALVE
P3 = 9.7 PSIG
A
37 PSIG, 40o F
PRESSURE
PORT
INLET
37 PSIG, 40o F
FROM
RECEIVER
B
46.7 PSIG, 50 o F
ASf11011
37 PSIG, 50o F
C
OUTLET TO
COMPRESSOR
Figure 11-11.--Thermostatic expansion valve and evaporator.
11-13
