Refrigeration & Air-Conditioning

Lesson 15. Temperature and safety controls: thermostat, HP and LP cut out, overload protector

Module 4. Refrigeration plant controls

Lesson 15
TEMPERATURE AND SAFETY CONTROLS: THERMOSTAT. HP AND LP CUT OUT. OVERLOAD PRTECTOR

15.1 Introduction

It is very essential to incorporate various controls in the vapour compression refrigeration system in order to get required temperature and to control the plant operation under abnormal operating conditions. These controls help to operate the system safely and also operate the plant under optimum conditions to reduce the operating cost of the plant.

15.2 Thermostat

Thermostat is 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 thermostat is depicted in (Fig. 15.1)

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 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.

For instance, the knob on the cover is set for a scale value of –29 °C, and the differential adjusting nut for 3 °C then thermostat will start the plant at –26 °C and stop it again at –29 °C.

Thus, Starting temperature = stopping temperature + differential

15.3 High Pressure Switch (Cutout)

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.

Working of high pressure switch is indicated in . It is connected on high pressure side of the system.

The reasons of excessive high pressure are as under.

1. Inadequate or lack of cooling medium at the condenser.
2. Higher temperature of cooling medium used at the condenser.
3. Poor rate of heat transfer at the condenser
4. Presence of non-condensable gases in the refrigeration system.

This switch may be fully automatic (stars and stops plant) or semi-automatic (stops the plant automatically but re-starting manually) depending upon the wiring connection of the switch with the system.

15.4 Low Pressure Switch (Cutout)

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.

Working of low pressure switch is indicated in . It is connected on suction side of the compressor.

The reasons of excessive low pressure are as under.

1. Very low load at the evaporator.
2. Defective expansion valve/ blockage of expansion valve.

This switch may be fully automatic (stars and stops plant) or semi-automatic (stops the plant automatically but re-starting manually) depending upon the wiring connection of the switch with the system.

15.5 Over Load Protector

The National Electric Code (NEC) defines Motor Overload Protection as that device which is intended to protect motors, motor-control apparatus, and motor branch-circuit conductors against excessive heating due to motor overloads and failure of the motor to start. Overload protection for large three-phase motors is provided by Thermal Overload Relays which are connected to Current Transformers (CT’s). But in new installations utilize microprocessor-based motor protective relays which can be programmed to provide both overload and short-circuit protection. These protective relays often also accept inputs from Resistance Temperature Devices (RTD’s) imbedded in the motor windings (usually two per phase) and the relays are capable of displaying the winding and motor bearing temperatures, and provide both alarm and trip capability. The over Load protector is operated by a snap action of bimetal disc and is sensitive to both temperature and current. When properly connected shuts off the motor when temperature exceeds maximum safe level due to an overload condition.