Evaporator - Types Of Evaporators

The evaporator is kind of heat transfer apparatuses where the heat transfer is done by forced convection or natural convection. And it’s an important component of refrigeration system and air conditioning system.

Evaporation process is rejection of water  (or other liquids) by concentrating the solution. The required time for this process can by shortened by increasing the surface area, the solution is exposed to it, or by exposing  the solution to heating to a higher  temperature.

How does evaporator work ?

For a basic refrigerant cycle shown in figure 1, which is used in many forms in all common air conditioning, refrigeration, heat pump applications. It uses a proper refrigerant that has the capability to change phase from  liquid to gas and from gas to liquid.

For a home air conditioning example, the refrigerant enters the compressor is a low pressure cool gas, where it compressed to a high pressure hot gas, then passes to the condenser. In the condenser coil, the refrigerant phase changes from  hot gas at high pressure to hot liquid at high pressure. This condensation causes a large heat rejection to surrounding.

The liquid refrigerant is then passed through a fixed nozzle or expansion device to exit as a cold liquid at a low pressure. Finally refrigerant enters  the evaporator at about 70% to 80% liquid and 20% to 30% vapor.

In the evaporator a very large amount of heat is absorbed from surrounding (the indoor air) causing the liquid refrigerant to boil and turn into a gas phase, which is passed to compressor to repeat the cycle again untile the indoor air be comfortable and sufficiently cool.
The evaporator works at a constant temperature as long as the pressure remains constant.

Types Of Evaporators

The evaporators may be classified into a forced convection type or natural convection type, depending on whether  the substance to be cooled lows naturally by difference in density  through the heat transfer surfaces of the evaporator  or forced by pump or fan.

In some kinds of evaporator, the refrigerant flows in the tubes and substance to be cooled surrounding it. But in other cases, substance to be cooled in the tubes and the refrigerant is in the shell.

Evaporator are also classified into flooded type and dry expansion type, depending on whether  the refrigerant covers all the surface of heat transfer or some portion of heat transfer surface is having gas being superheated.

• Flooded evaporators 

A flooded evaporator type with float control valve shown in fig. the liquid flow on low passages passes the tubes upwords, and boils due to heat adsorption from the warmer substance, which is cooled. The resulted vapor so formed on boiling bubbles up in flash chamber, where separates liquid from vapor.

Separated vapor  passes to compressor, and liquid flows back to the evaporator. The flash chamber collects the vapor formed by liquid refrigerant boiling in the evaporator, and  vapor obtained in the expansion device.

In a flooded type evaporator refrigerant liquid level is maintained. Float valve is used as throttling device.

The heat transfer efficiency increases because the entire surface is in contact with the liquid refrigerant. But the refrigerant charge is relatively large as compared to dry expansion type.

The accumulator or flash chamber is used to prevent liquid Cray over to compressor.  The evaporator coil is contacted to accumulator and the liquid flow from the accumulator to the evaporator coil is generally by gravity. The vapor formed by the vaporizing of the liquid in the coil being lighter rises up and passes on to the top of the accumulator from where it enters the suction line. 

In some cases liquid eliminators are provided in the accumulator top to prevent the possible carry over of liquid to suction line. Also a liquid suction heat exchanger is used on the suction line to superheat the suction vapor.

• liquid chiller

Two types of liquid chillers are shown in fig.  where the  refrigerant in the shell and liquid to be chilled in the tubes,  whereas the latter has liquid to be chilled in the shell and  the refrigerant in the tubes. When the refrigerant is in the shell, the refrigerant liquid level is so kept that there is enough space on the top portion of the shell for the liquid and vapor to separate.

Vapors are drawn from the top portion by the compressor. Liquid level must be maintained constant as the chilled tubes are also immersed in the rerigerant liquid. Thus  tloat control is preferred. However thermostatic expansion valve is preferred , when the refrigerant is in the tubes and the liquid to be chilled is in the shell .

The refrigerant gets superheat in the last portion of the set of tubes and is collected in the end chamber from where it is sucked by the compressor. In order to facilitate proper contact of water with the rerigerant tubes, baffles are provided to ensure larger circuit up and down for the water, leading to increased turbulence and hence better overall heat transfer co-efficient. Thus, the first fig. can be termed as looded  whereas second evaporator to be dry.

• Direct Expansion Coil Evaporator

Unlike the liquid chiller which the chilled liquid is fed to the coils, which are used for cooling air, the evaporator is called direct expansion evaporator  if the coils of the evaporator with refrigerant passing through them are used directly to cool air by natural or forced convection.

To improve the lubricating oil return to the compressor,  the refrigerant feed comes through the thermostatic expansion valve more often located at the top particularly for F12, F22. Air is blown over the outside of the finned tubes.

The direct expansion coil is preferred For air conditioning purposes,  where the evaporator is very near to the compressors. This  is direct method of cooling the substance and, therefore quite eficient. it is preferred to chill the water and pump it to the air-cooling coil, when the coil has to be located very far away from the compressor.

There is possibility of refrigerant leakage for long distances, and the cost of the refrigerant would be high. Also the pressure drop in the line would decrease evaporator coefficient of performance and efficiency.

The expansion valve controls the rate of the refrigerant to evaporator in such a way that all the liquid is vaporized and the vapor is also superheated to a limited extent. The inside of evaporator is far from dry but wetted with liquid.

All the same, this type is called dry expansion to distinguish it from flooded system and also probably because by the time the refrigerant reaches the evaporator outlet it from flooded system and also probably because by the refrigerant reaches the evaporator outlet it is no more wet but dry superheated vapor.

Related Articles:

What is Absorption Chiller ?

Read more »

What Is An Absorption Chiller ?

What is An Absorption Chiller ?

An Absorption Chiller is a machine uses a heat source to generate chilled water rather than electrical source that is used in vapor compression cycle.

It’s an attractive idea to produce chilled water with heat.

As the same in vapor compression cycle, absorption cycle achieve removing heat using the evaporation of a refrigerant at low pressure and the ejection of heat using the condensation the refrigerant at high pressure.

The major difference between  a vapor compression system and an absorption chiller is that a vapor compression system using a compressor to make the pressure difference required to circulate the refrigerant while the absorption chiller using heat.

The absorption chiller is very attractive because using energy in the form of heat, such as solar or waste heat, makes it using a little work input and saving in money. Also it can works with industrial waste heat streams.

Why Do We Use Absorption Chiller ?

Vapor compression systems use a high grade of energy input to drive a compressor, so it consumes a lot of electricity. This also leads to more releasing of CO2 emissions. While the absorption chillers use a low grade of energy to generate a cooling effect. So we don’t have to use electricity but we can use any other heat source.

Absorption chiller also have other advantages like silent operation which is not compared with other systems.

Comparison Between  Absorption Refrigeration Cycle and Vapor Compression Cycle:

The vapor compression cycle basically consists of an evaporator, condenser, throttling valve, and a compressor. Figure below illustrates the components and flow arrangement for it.

In the shown cycle, refrigerant in the form of a cool mixture of liquid and vapor centers the evaporator at low pressure (4). Then using relatively warm air or water, the liquid refrigerant will boil. The resulted vapor (1) is pumped by the compressor, which raises refrigerant vapor pressure and temperature.

This hot refrigerant vapor at high pressure (2) enters the condenser, where heat is released to a lower temperature ambient air or water, and the refrigerant vapor turns into  a liquid, then passes to the expansion device (3), where the pressure of the refrigerant is reduced to the pressure of the evaporator, and a little amount of the refrigerant boils, cooling the staying liquid refrigerant to the wanted evaporator temperature. Then mixture of the cool liquid and vapor refrigerant (4) passes to the evaporator and repeats the cycle.

The absorption cycle is similar to the vapor compression cycle in many components, but it uses a different compression method and different refrigerants unlike the vapor compression cycle.The absorption cycle replaces the compressor with a generator, an absorber, and pump.

As we said, a refrigerant enters the evaporator at low pressure in the form of a cool mixture of vapor and liquid (4). Then the liquid refrigerant boils due to heat transferred from a relatively warm water to it. The function of the absorber is similar the suction side of the compressor, which the vapor refrigerant enters it and mixes with the absorbent.

The pump make the same process of the compressor, which pumps the mixture of absorbent and refrigerant to the generator at high pressure. In the generator, the refrigerant vapor is extracted and delivered to the condenser (2), where the refrigerant vapor condenses into a liquid due to heat transferred from it to lower temperature water.

Then the liquid refrigerant passes to the expansion device (3), where the pressure of the refrigerant is reduced to the pressure of the evaporator, and a little amount of the refrigerant boils, cooling the staying liquid refrigerant to the wanted evaporator temperature. Then mixture of the cool liquid and vapor refrigerant (4) passes to the evaporator and repeats the cycle.

• Similarities between basic Vapor compression and Vapor absorption cycles:

1. Both transfer heat between two fluids by circulating a refrigerant inside the chiller.

2. Both have an expansion device to ensure the internal pressure difference, and a device to increase the refrigerant pressure.

3. Condenser condenses the refrigerant vapor at low pressure and temperature by rejecting heat to the surroundings.

4. By absorbing heat from the chilled water, the refrigerant vapor is evaporated at low temperature and pressure.

• Differences between Vapor compression and Vapor absorption cycles

1. Unlike the vapor compression system, the absorption system achieves the cooling effect using heat energy in form of waste heat, steam, or direct fuel firing.

2. Vapor compression cycle uses compressor, but the absorption chiller system uses a liquid pump to produce the pressure difference between condenser and evaporator.  A liquid pump is much easier in work and cheaper than compressing a gas, so that it takes less work input. However, the absorption chiller system replaces the compressor work input with a heat pump in the generator.

3. The absorption systems use different refrigerants that haven’t bad effect on environment, ozone depletion .. etc. It uses –for example- lithium bromide as absorbent and water as the refrigerant.

4. The absorption chillers have very few moving parts compared to compression system, so they are less vibration and noise, requiring little maintenance. Also they are compact in large capacities.

5. The absorption chillers don’t depend very much on evaporator superheat, and are not affected by load variations.

6. The absorption chillers have a low operating cost due to using a low grade waste heat. But the coefficient of performance (cop=load/input) is lower compared to vapor compression system.

Summary of The Comparison:

Read more »