HP Cutoff

It is a safety control used in vapour compression refrigeration plant to stop the refrigeration system when discharge pressure rises above the pre-set value of cut out pressure. This is an electrically operated switch which breaks the electrical circuit of the compressor when condensing pressure increases excessively above the normal working pressure of the system.

High pressure can be caused in a refrigeration plant due to various causes like over charge, loss of cooling water, high ambient temperature, air, or other incompressible gases in the system, and obstruction in the discharge line of the compressor. For protecting the compressor from high pressure and subsequent failure, a high pressure cut out is provided that take a pressure tapping from the discharge line and when it detects an over pressure, it stops the compressor. The HP cut out is not resettable automatically but has to be reset manually by the operator. This is because the high pressure is a serious fault and the cause must be investigated and corrected before the plant is started again.

The high pressure cut out as shown in the diagram is of a simple construction. It has a bellows that is set against a spring. The nut at the end of the spring is used to adjust the cut out pressure. When the high pressure gas enters the bellow, the bellow expands and presses the spring. At the cut off pressure the movement of the bellow against the spring releases the catch and the contact is broken and the compressor cuts off. The switch arm can be pressed and the cut out reset after the cause of the over pressure has been found and rectified.

LP Cutoff

It is also a safety control used in vapour compression refrigeration plant to stop the refrigeration system when suction pressure drops below the pre-set value of cut out pressure. This is an electrically operated switch which breaks the electrical circuit of the compressor when suction pressure reduces below the normal working pressure of the system.

To protect the compressor against low pressure in the system and to avoid the ingress of air into the system if a vacuum is generated in the lines a low pressure cut out is provided. Also when the refrigerated compartments are cut off by the solenoids and there is no return gas, the low pressure cut out is activated. When the solenoid of the refrigerated compartments open, the return gas comes in the inlet of the compressor and the suction pressure rises, and then the low pressure switch cuts in the compressor.

Unlike the high pressure cut out, the low pressure cut out is self-resettable and does not need to be reset manually.

Thermostats

Thermostats are used for controlling the temperature in cold rooms, brine tanks, freezing chambers, hardening chambers etc. It is a temperature operated electric switch making circuit when the temperature rises to a predetermined value and breaking circuit when the temperature falls to another predetermined value. It is necessary to stop the refrigeration plant at required temperature and again to start the plant at predetermined rise in the temperature. This is possible by using a thermostat which breaks and makes the electrical circuit of the compressor or solenoid valve. The different types of thermostats are available for different applications to cover wide temperature range. A schematic representation of a simple thermostat is depicted below:

Generally, thermostat should be fitted outside the cold room in which the temperature to be controlled. The phial is fixed inside a cold room by means of a clip. The capillary tube must not be passed through any room colder than the one in which the phial is fitted. The best way of passing the capillary tube through the wall of the cold room is by way of grouted-in sleeve having at least 10 mm bore. It must be seen that iron filings do not get into the instrument during installation, as such fillings will be attracted by the permanents magnet and thus lessen the proper clearance between armature and magnet.

The thermostat is adjusted by the knob (or digital interface) to interrupt the circuit at the desired temperature, which can be read on the scale on the cover of the instrument. The temperature differential desired for the starting of the plant is set by the differential adjusting nut inside the case, which is likewise provided with an adjustment scale. It is necessary to check these settings during the operation of the plant.

Humidistats

A humidistat is a device that measures the amount of moisture in the air. The device itself consists of a sensing element that is composed of a flat plate fitted with alternate metal conductors. Attached to the sensing element is a relay amplifier, which reads signals sent by the sensing elements to give accurate humidity levels.

It works by causing an electrical resistance between the conductors mounted on the sensing element based on the amount of humidity in the air. The relay amplifier measures the resistance. Based on that measurement, the humidistat turns the humidifier on if the electrical resistance shows the air is too dry or turns off the humidifier once the ideal humidity level is achieved.

A humidistat keeps the humidity at optimal levels automatically. Controlling the amount of humidity in the home is important for several reasons. One reason is economical. Keeping the humidity at an optimal level will allow to run the HVAC less. Another is comfort. The right humidity level is a key factor in making the home comfortable. Another important reason to have a humidistat is that an optimum humidity level is important to health.

Inside the home, an ideal setting on your humidistat is no higher than 58%. This accommodates the readings on the humidistat being off by as much as 10%.

You can also wire a humidistat into your thermostat. It will run your air conditioner to control the humidity level inside the house independently of temperature. The humidistat will not let the air conditioner turn on if the temperature is high but the humidity level is low. When the indoor humidity level gets into an unacceptable range, the humidistat can run the air conditioner until the humidity level is reduced. Using a humidistat this way lets you maintain the desired levels of humidity in your home while you are away without having to run the HVAC system more than is needed.