The variance of the operational condenses of the system and discusses the different energy in which it operates under.
An evaporator is any heat transfer surface in which a liquid is vaporized for the purpose of removing heat from a refrigerated space or product.
Heat is a form of energy.
Energy is defined as the capacity for doing work.
(First Law of Thermodynamics) The conservation of energy law states that energy can neither be created nor destroyed, but it can change forms.
Heat or energy can only be moved, but based on the law for any system, open or closed; there must be an energy balance. Air conditioning and refrigeration systems are balanced systems. The total heat or energy absorbed by the evaporator and suction line, plus the heat or energy that the compressor imparts in to the refrigerant, must be rejected out of the condenser in order to maintain a balanced system.
If the evaporator cannot absorb heat or if the condenser cannot reject heat, the system will not be balanced and a loss of efficiency and capacity occurs.
(Second Law of Thermodynamics) The Second Law of Thermodynamics states that heat flows from a warmer substance to a cooler substance.
The relative temperature of the substance determines the direction of heat flow. The speed of the heat flow is determined by the difference between those temperatures and the insulation value of the substance, causing heat to be transmitted.
The amount of heat transmitted by a material divided by the difference in temperature of the surfaces of the material is called Thermal conductance.
The heat that flows across a surface per unit area per unit time, divided by the negative of the rate of change of temperature with distance in direction perpendicular to the surface called Thermal conductivity.
Given the above definitions of what energy is and that, it moves in one direction, irreversibility must be considered.
Irreversibility can be defined as the difference in temperature between the condenser and evaporator.
For example, the larger the irreversibility in a refrigeration cycle, operating with a given refrigeration load between two fixed temperature levels, would result in a larger the amount of energy being required to operate the refrigerant cycle.
To improve the cycle performance a total reduction of the irreversibility in the refrigerant cycle, must occur.
The capacity of any evaporator or cooling coil is defined by the rate at which heat will pass through the evaporator walls from the refrigerated space or product to the liquid refrigerant that is vaporizing. This is usually expressed in watts or in BTUH (British Thermal Unit per hour). BTU is a measurement of a quantity of energy.
This quantity of energy will raise one pound of water one degree. To meet the specified design conditions of a system BTUH requires the correct selection of an evaporator. An evaporator selected for any specific application must have sufficient heat transfer capacity to allow the vaporizing refrigerant to absorb heat at the rate necessary to produce the required cooling and dehumidification.
Heat transfer is an important component in the evaporation process.
Heat reaches the evaporator by three methods of heat transfer:
1. Thermal Conduction: is the flow of thermal energy through a substance molecule to molecule from a higher to a lower temperature region.
2. Thermal Convection: is the transfer of thermal energy by actual physical movement from one location to anther of a substance such as air or water which thermal energy is stored.
3. Thermal Radiation: is the energy radiated by solids, liquid and gas in the form of electromagnetic waves, which transfer energy because of their temperature.
This energy transfer heat through a space without heating the space but is absorbed by objects that it reaches.
Most of the heat in air-cooling applications is carried to the evaporator by Thermal convection currents. The Thermal convection currents set up in the refrigerant space either by action of a fan or by gravity circulation resulting from the difference in temperature between the evaporator and the space. In addition, some heat is Thermal radiated directly to the evaporator from the product and from the walls of the space. When the product is in thermal contact with the outer surface of the evaporator, heat is transferred from the product to the evaporator by direct Thermal conduction.
For a liquid cooling application, where the liquid is being cooled, there must always be contact with the evaporator surface and some circulation of the cooled fluid either by gravity or by action of a pump.
Regardless of how the heat reaches the outside surface of the evaporator, it must pass through the wall of the evaporator to the refrigerant inside by conduction. Therefore, the capacity of the evaporator (the rate at which heat passes through the walls) is determined by the same factors that governing the rate of heat flow by Thermal conduction through any heat transfer surface.
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