04-07-2012, 04:31 PM
refrigeration and air condition
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Introduction
When a refrigeration system is constantly
loaded, and the load balances the cold output
of the system, adjustment is unnecessary. In
practice, however, there will always be
fluctuating loads on the main components of
the refrigeration plant and the plant capacity
will often be higher than the normal cold
requirement. Therefore, a need for regulation
arises.
Regulation can be carried out manually but
that requires a large personnel and is not
particularly accurate, therefore, automatic
control is used today. To prevent injury to
personnel and damage to refrigeration
equipment, plants must be equipped with
safety controls. Danfoss regulating
equipment covers these requirements. The
following deals with those refrigeration
controls which regulate in accordance with
the on/off principle.
Thermostat design
Thermostats are temperature-controlled
electrical switches which can break or make
an electrical circuit, depending on
temperature variations at the bulb.
A thermostat, fig. 25, consists of the following
main elements:
The power element which consists of the bulb
(29), the capillary tube (28), and the bellows
element (23). The most important parts of the
mechanical section are the spindle (15), the
range spring (12), the knob (5) for adjusting
the spring pressure, and the differential
adjusting nut for adjusting the mechanical
differential. The switch (16) is a single-pole
change-over switch with momentary contact
change-over.
The power element contains a charge, the
pressure of which changes with the
temperature at the bulb. A fixed pressure
corresponds to a given bulb temperature. If
the bulb temperature increases, the pressure
in the power element increases. Hence the
spindle is moved until the pressure on the
lower side of the bellows balances the
counter-pressure of the main spring.
Special type RT thermostats
RT 4 with an electric heater in the bellows
element
As mentioned on page 13 an increased rate
of temperature change will increase the
thermal differential. To control the ambient
temperature, a small thermal differential is,
however, often necessary even if the rate of
air temperature change is high. The
application of RT 4 with an electric heater in
the bellows element is recommended for this
kind of system.
The electric heater is heating when the
refrigeration compressor is not operating,
with the effect that the bellows and the
bellows housing are always a little warmer
than the phial.
Without this arrangement, the bellows and
the bellows housing would become the
coldest parts of the power element at rapidly
increasing temperatures, since the mass of
the bellows housing is larger than that of the
phial, and the charge vapours would
condense in the bellows and thus not create
the requisite pressure for changing over the
switch.