Characteristics of Evaporators

Roger D. Holder, CM, MSME

Heat or Energy

In this paper, we will discuss the characteristics of an evaporator coil.  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.

 Evaporative Coil

 The evaporator coil is a critical component of an air conditioning and refrigeration system. The refrigerant enters the evaporator as a boiling low-pressure liquid. At about 70% to 80% liquid and 20% to 30% vapor.  To find the exact liquid vapor mixture, the refrigerant cycle must be plotted on a pressure-enthalpy diagram. The lower the vapor percentage, the more liquid the evaporator has to vaporize in the vaporization cooling process.  The process of vaporizing a liquid at low boiling point so that heat can be carried away is known as Vaporization cooling.  The vapor released in the expansion device is called flash gas or flash vaporization.  Flash gas is the liquid refrigerant that flashes (changing of densities) to a vapor because of the presser drop in an expansion device. Energy exchange does not remove heat or gain heat.  This energy exchange drops the temperature of the liquid refrigerant to the evaporator temperature (an exchange of density).  The vapor and liquid mixture has the same total energy.  By exchanging liquid for vapor, a density change always changes the temperature, but not the total energy.  The greater the temperatures change between the high-pressure liquid refrigerant and the low-pressure liquid vapor refrigerant, the greater the densities change and a decrease of efficiency and capacity.  The decrease in efficiency and capacity can be overcome by subcooling of the refrigerant in the liquid line.  For every 1ºF of subcooling at the same condensing pressure an increase of .5% system efficiency is achieved.  Increasing the subcooling with an increase of compression ratio decreases system efficiency and capacity. 

 When a condenser is dirty or unable to reject heat or energy, heat increases both the temperature and pressure of the refrigerant liquid line. The higher total energy increases the flash gas that decreases system efficiency and capacity.

 The liquid refrigerant continues to boil at one temperature as long as the pressure remains the same.  Latent heat of the refrigerant in an evaporator accounts for 96% to 97% of the total heat that the refrigerant adsorbs in an evaporator.

 Evaporators are designed to have a 2-Fahrenheit degrees change in the evaporator.  2-Fahrenheit degrees change is a pressure change from the entering of the evaporator to the leaving of the evaporator. (See Table 1)  Pressure is read at the leaving of the evaporator because it is the closest to the end of saturation.  At saturation, refrigerant is absorbing latent heat, is the change of state heat. The refrigerant changes state at one temperature (for any one pressure) from the beginning of the evaporator until the liquid refrigerant has become a vapor. The only variable that can change a temperature is a pressure change.  If a temperature change occurs a pressure change occurs.

                                                                                        Table 1

Pressure drop, Psig at 2F degrees

(Table 1)

(Table 1B)

In a latent heat state, a continuous liquid and vapor mixture is being fed through an evaporator.  The liquid and vapor are at the same temperature due to equilibrium contact.  Equilibrium contact is the process in which liquid and vapor are in contact with each other and are at the same temperature and pressure.  In a latent heat state of the evaporator, the liquid is vaporized by the heat the evaporator is absorbing.   The vapor will not absorb heat it will only pass heat to the liquid until all the liquid is vaporized.  A continuous feed of refrigerant into the evaporator and the continuous withdrawal of superheated refrigerant gas by the compressor maintain the pressure.  The temperature and pressure will change with fluctuations of heat that the refrigerant is continuously absorbing.  When heat is added to the gas, past saturation pressure-temperature, it is called superheat. (See Table 2)

 Table 2
Refrigerant –22

 (Table 2) Psig is at 29.92”

 Bernoulli Law of Velocity: states that with an increase of velocity there will be a decrease in pressure.  In the center of a refrigerant pipe with an increase of velocity, the vapor pressures drop.  Vapor pressure is less in the center of the refrigerant pipe because of a higher velocity.  At the same time, there is a higher pressure and lower velocity on the walls of the refrigerant piping due to friction loss.  Liquid may be in the center of the refrigerant piping at a lower pressure and a higher velocity.  With the pressure drop in the center of the refrigerant pipe, liquid can be present until .5ºF to 2ºF of superheat is attained.

Superheat is the measurement of how full the evaporator is of liquid refrigerant.  High superheat means the evaporator is empty.  Low superheat means the evaporator is full.  Superheat should never fall below 4° degrees or a compressor failure will occur.  Never add refrigerant when the compressor is at full load amp (FLA) or running load amp (RLA).  High superheat at (FLA) or (RLA) means high heat load on the evaporator or condenser.

 To measure evaporator superheat:

1.      Take a pressure reading of the suction line leaving the evaporator to get refrigerant saturation pressure-temperature.

2.      Convert pressure to temperature with a pressure temperature chart.

3.      Take a temperature reading at the leaving suction line of the evaporator.

4.      Subtracting one from the other, the difference is the amount the refrigerant gas has heated past saturated pressure temperature. 

 If a pressure reading is obtained at the compressor, and not at the evaporator leaving line, 2-Fahrenheit degrees in pressure may have to be added to the gauge pressure.  This pressure change is to compensate the gauge pressure for the pressure drop in the suction line.  A degree change is pressure changes due to pressure drop. (Refer back to Table 1)  Superheat is calculations of the heat add to the refrigeration once the liquid has vaporized.  All pressure drops in a system suction line must be in the calculation of superheat.

 The Superheat of Metering Devices

 The two most common metering devices used in air conditioning and refrigeration systems are the thermostatic expansion valve and fixed orifice.  Each of these metering devices is charged differently.  The thermostatic expansion valve will open and close with the superheat of the evaporator and the fixed orifice will never change in size by itself. 

 The superheat of a thermostatic expansion valve is normally set between 8º to 12º and can vary with the design of a system.  To measure superheat for a thermostatic expansion valve, the measurement must be obtained at the leaving refrigerant line of the evaporator, where the thermostatic expansion valve bulb is located.

 An air conditioning and refrigeration system with a fixed orifice is charged to the superheat of the suction line leaving the evaporator. A fix orifice system has a critical charge.  The smaller the system is the more critical is the charge of refrigerant.  A fix orifice itself has no means to adjust the flow of refrigerant to maintain superheat.  The only adjustment for superheat is the charge of refrigerant.  There are two reasons that cause a change in superheat in fix orifice system: 1) the amount of refrigerant entering the evaporator and 2) the total heat of the air entering the evaporator. Force and load set the superheat.

 Force is the pressure of the high side, forcing the refrigerant into the fix orifice so it can be measured by the outside air temperature at the condenser.  Load is the total heat of the air entering the evaporator and can be measured by the wet bulb temperature.  Wet bulb temperature is an indication of the total enthalpy of the air.  When the outside air temperature changes or the heat (enthalpy) of entering air to the evaporator change, the superheat changes.  Superheat is the gas temperature above the saturated temperature.

 Superheat can be split into two types of heat:

1.      Superheat of the evaporators; and

2.      Total superheat entering the compressor. 

The evaporators superheat must be figured at the evaporator outlet not at the compressor inlet (See Table 3).  Total superheat is figured at the compressor inlet (See Table 4).

 Table 3

Superheat for A/C with fixed Orifice R-22

Evaporator Inlet Air Temperature Fahrenheit Wet Bulb (enthalpy)