Refrigeration is removals of heat. Used for food service at homes, hotels, restaurants, health care facilities, air-conditioning, industrial works etc. cooling is done by removing heat from any objects.
Before dealing with refrigeration process some related scientific terms are being discussed below.
Heat and temperature:
It is necessary to distinguish between the above terms. Heat is a form energy while temperature is a measure of intensity of heat. Quantity of heat is measured in heat units like British thermal units (BTU) and also centesimal heat units (CHU). Temperatures measured as degree centigrade, Fahrenheit etc.
Sensible heat and latent heat:
When a substance does not change its physical state but there is change in its temperature due addition or removal of heat, in that case the heat absorbed or rejected is called ‘sensible heat’.
Latent heat is the heat that is absorbed or given off a substance while changing its physical state with no change in its temperature.
So, sensible heat is the heat that affects the temperature of a substance and latent heat the physical state but not temperature.
Boiling point of liquid:
It is the temperature of a liquid at which the liquid starts vaporizing at certain pressure. At higher and higher pressure the boiling point of the liquid will be higher and higher.
Mainly two systems. Vapor absorbing and vapor compression. The later type more efficient with lower operating cost. This type is mostly used nowadays. But then maintenance cost rather on higher side. Normal interior temperature inside refrigerator about 40of while that of freezer compartment is kept at about 0of, components of compression type consists of vapor compressor, condenser, expansion valve, evaporator, different compartments, shelves, crisper at bottom, freezer at top etc.
Various compartments and shelves accommodate different food materials. Crisper is used for vegetables and fruits etc. Freezer for making ice, ice-cream and also for keeping fish, meat etc in condition for considerable time.
In refrigerators different refrigerants like ammonia, carbon-di-oxide, Freon gas etc. utilized. Most commonly Freon gas used in vapor compression refrigerators.
Regarding vapor absorption cycle the insulated box inside of the refrigerators is more or less the same for storage of food materials. It differs in the matter of refrigeration process. The main parts are- “generator” where dissolved ammonia solution is heated, “condenser” where ammonia gas get condensed giving out heat, “evaporator” where liquid ammonia boiled producing cooling effect in refrigerator, the gaseous ammonia then flows down to the “absorber” where water absorbs the gaseous ammonia and then drains to the “generator” and the cycle repeats.
Care and maintenance of refrigerators:
In refrigeration cycle the refrigerant undergo number of changes in its state to produce refrigerating effect. Such changes constitute the refrigeration process. For carrying out the process the system is provided with components and control devices for efficiency of the machine. These controls and components are to be checked and repaired as and when necessary. Similarly any leek in the system to be taken note and rectified accordingly.
There are safety devices like high and low pressure cut outs, motor overload switch, relief valve, oil safety switch etc. which are also to be taken care of. The valves and safety devices constitute the controls.
The refrigerants are actually the heat-carrying medium. Refrigerants absorb heat from place at low temperature level and reject it at place having higher temperature. Such heat rejection is managed at the expense of heat energy.
Desirable properties of refrigerants are- non-explosive, non-toxic, non-flammable, non-corrosive, odorless, non-injuring to human being, food, fabrics etc. Must posses suitable boiling point and high latent heat.
Simple method to do it is to shut down the refrigerator until all frost in it melted and then cleaning done by wiping. But this takes long time.
Other methods like: (a) pressure defrosting, (b) temperature defrosting, (c) electrical (d) automatic defrosting are there. The most favorable one is the automatic type.
Whether summer or winter, the following factors matter so far as physical human comfort is concerned-
Heat production in human beings,
Heat and moisture losses from human body,
Air humidity (relative humidity).
It is defined as the ratio of actual weight of water vapor in a given volume of air, to the weight of water vapor contained in the same volume of air if the air is saturated with the water vapor at the same temperature.
Humidification and de-humidification:
Humidification of air means introduction of moisture into it. The reverse is what is called de- Humidification.
Dew point Control:
In practice dew point temperature taken as the temperature at which the condensation of water vapor will begin when the moist air under consideration is cooled at constant pressure. So controlling of temperature (dew point) of the air vapor mixture shall determine the change of state of air vapor.
Air conditioning systems are employed to tackle with above factors to bring about desired comfort conditions.
Unit of air conditioning is “Tons”.
Window type air conditioner:
Window or room air conditioners are designed to air condition separate rooms of a house or a building.
Separate temperature control exists for each room unit.
Ducts are not required,
No plumbing necessary.
Window space needed,
Fixed air quantity,
Wall appearance effected.
The equipment for such air-conditioning are centrally located in a suitable place of the building. The units are combinations for both cooling and heating with desired objective of making comfort conditions.
These have following advantages:
Comfort condition for whole building from a central unit.
Very efficient and less noise, being located at suitable distance from living area.
Automatic central control.
Easy maintenance as cooling and heating equipments in one place.
Better air distribution.
For summer, air pass through filter, refrigeration cooling coils, and then dehumidified and blown through ducts to different rooms. Temperature and humidity of air kept at certain level for human comforts. For winter after filter air pass through hot furnace then Humidifier or dehumidifier as necessary and then through ducts to rooms.
Disadvantage mainly in higher cost for equipments and installation.
This is important for any type of air-conditioning units. Points to be noted for such maintenance are as below;
filter and condenser coil cleaning, periodically. Also changed when necessary.
fan motor lubrication by say SAE-20 oil at bearings in the beginning of each operating season.
manufacturer’s service instruction to be followed properly.
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