Refrigeration basics
Refrigeration
The mechanism used for lowering or producing low temperature in a body or a space, whose temperature is already below the temperature of its surrounding, is called the refrigeration system.
The transfer of heat from a low-temperature region to a high-temperature one requires special devices called refrigerators.
Refrigerators are cyclic device, and the working fluids used in the refrigeration cycles are called refrigerants. Here the heat is being generally pumped from low level to the higher one & is rejected at high temp.
Another device that transfers heat from a low-temperature medium to a high-temperature one is the heat pump. Refrigerators and heat pumps are essentially the same devices; they differ in their objectives only.
The term refrigeration may be defined as the process of removing heat from a substance under controlled conditions. It also includes the process of reducing heat & maintaining the temperature of a body below the general temperature of its surroundings.
In other words the refrigeration means a continued extraction of heat from a body whose temp is already below the temperature of its surroundings. A refrigerator is a reversed heat engine or a heat pump which takes out heat from a cold body & delivers it to a hot body. The refrigerant is a heat carrying medium which during their cycle in a refrigeration system absorbs heat from a low temperature system & delivers it to a higher temperature system.
Its advantages in comparison with other types of refrigeration are the compact design of the system’s elements; high coefficient of performance (COP); reliability, safety, and flexibility in operation; relative simplicity of its maintenance; and reasonable price. This rejection of heat from low level to higher level of temperature can only be performed with the help of external work according to second law of thermodynamics.
Refrigeration Terms:
Refrigeration Terms:
Heat Exchanger -
A device for the transfer of heat energy from the source to the conveying medium, with the latter often is being air or water. Most common combinations are: Refrigerant to air or Refrigerant to water, Water to air, Steam to air, Steam to water.
Humidity -
The total amount of moisture in air. Relative humidity (RH), is the amount of moisture in air, relative to its total capability based upon its temperature (dew point). Moisture will condense on surfaces which are below this dew point.
Absorbent -
Substance with ability to take-up, or absorb another substance.
Absorber -
A solution or surface that is capable of soaking up (taking in) another substance or energy form.
Absorption Chiller -
A chiller that uses a brine solution and water to provide refrigeration without the aid of a compressor.
Absorption Refrigerator -
Refrigerator which creates low temperatures by using the cooling effect formed when a refrigerant is absorbed by chemical substance.
Heat -
Invisible energy (except high intensity infra-red) caused by the motion of molecules within any substance or matter. Will always travel from warm/hot to cold, via either or a combination of conduction, convection or radiation. Materials which resist flow or transfer of heat are called insulators, or insulation.
Temperature -
Degree of hotness or coldness as measured by a thermometer; measurement of speed of motion of molecules.
Accumulator -
Storage tank which receives liquid refrigerant from evaporator and prevents it from flowing into suction line.
Adiabatic Compression -
Compressing refrigerant gas without removing or adding heat.
Wet Bulb -
Device used in measurement of relative humidity. Evaporation of moisture lowers temperature of wet bulb compared to dry bulb temperature in same area.
Sub Cooling -
Process whereas additional sensible heat (as opposed to latent heat) is removed from condensed refrigerant liquid prior to the metering device. The proper method for charging a system utilizing a TXV.
TON -
A unit of measurement used for determining cooling capacity. One ton is the equivalent of 3024 kcal per hour.
Adsorbent -
Substance which has property to hold molecules of fluids without causing a chemical or physical change.
Air -
Invisible, odorless, and tasteless mixture of gases (consisting mostly of Nitrogen, Oxygen and Carbon Dioxide) which form earth's atmosphere.
Calorimeter -
Device used to measure quantities of heat or determine specific heats.
CFM -
A standard of airflow measurement. Cubic feet per minute.
Compressor -
The heart or "pump" within an air conditioning or heat pump system. The compressor maintains adequate pressure to cause refrigerant to condense and flow in sufficient quantities to meet the cooling requirements of the system.
Economizer - A mechanism that removes flash gas from the evaporator.
Fan - A radial or axial flow device used for moving or producing artificial currents of air.
Filter - Device for removing small particles from a fluid.
Gas - Vapor phase or state of a substance.
Thermostat - A temperature control device. Typically mounted in conditioned space.
Absolute Humidity - Amount of moisture in the air, indicated in kg water/kg dry air.
Absolute Pressure - Gauge pressure plus atmospheric pressure.
Absolute Temperature - Temperature measured from absolute zero.
Absolute Zero Temperature - Temperature at which molecular motion ceases.
Insulation - Any material or substance which has the ability to retard the flow or transfer of heat.
Latent Heat -
Heat energy absorbed in process of changing form of substance (melting, vaporization, fusion) without change in temperature or pressure. Also referred to as "hidden" heat.
Manometer -
Instrument to measuring pressure of gases and vapors. Gas pressure is balanced against column of liquid such as mercury, in U-shaped tube.
Metering Device -
TXV, capillary tube assembly, constant pressure expansion valve or bullet type piston orifice designed to regulate flow of liquid refrigerant entering the evaporator. Creates pressure drop to allow liquid refrigerant to boil and absorb latent heat. Separates high side of system from low side.
Refrigerant -
A substance produces a refrigerating or cooling (heat absorbing) effect while expanding or vaporizing.
Refrigeration -
The moving of heat from an undesirable location, to that of a location where its presence is less undesirable.
Safety Control -
Device used to electrically shut down a refrigerating unit when unsafe pressures and/or temperatures exist.
Saturation Temperature -
The temperature where a refrigerant exists in both liquid and vapor form relative to its measured pressure.
Super Heat -
The temperature rise within an evaporator/suction line assembly from the evaporator's saturation temperature.
TXV - Thermostatic Expansion Valve-
A metering valve which acts as a superheat controller. Most are mechanically operated, and utilize a remote sensing bulb attached to the outlet of the evaporator assembly to regulate flow of sub-cooled liquid refrigerant at the evaporator inlet.
Valve, Solenoid - Valve actuated by magnetic action by means of an electrically energized coil.
Pascal's Law - A pressure imposed upon a fluid is transmitted equally in all directions.
Psychrometer -
Either a sling type, or electronic. Instrument used to determine wet bulb temperatures and relative humidity. Combining RH with dry bulb temperature will yield total heat.
Range - Pressure or temperature settings of a control; change within limits.
Water -Cooled Condenser -
Condensing unit which is cooled through use of water.
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