Hello Readers! Any idea about what is a condenser? In this article, we will learn all about condensers along with basics, definition, meaning, purpose, types, air-cooled and water-cooled condenser, advantages, disadvantages, applications, etc.
What is Condenser? Definition. Meaning
Basics of Condensers
When milk is boiled and then it is cooled, we generally cover the whole utensil with some plate. After some time when milk gets cooled, we see the droplets of water accumulated on the covering plate.
So where this water comes from?
- This is a result of the phenomenon of condensation.
- When the water vapour of milk is stopped from flowing out and get trapped by this plate whose temperature is below that of vapour.
So these water vapors transfer heat to this plate as we know heat travels from high temperature to lower temperature.
So as a result due to this heat exchange, the water vapors temperature got reduced and it changed its phase from steam to liquid. This process is known as condensation. So now we know in laymen’s terms what condensation is?
Definition of Condenser
A condenser is a device that works with this same principle. In fact, the evaporator also works on this principle.
- Basically, both are heat exchangers but they differ on the basis of their function.
- Heat is rejected by the condenser while it is absorbed by the evaporator.
- The condenser is an important component of steam generation, electricity generation, and refrigeration plants.
- It is also used in chemical industries as a separator and as a heat recovery system.
Therefore understanding the condenser and its functioning becomes very important to achieve good performance of a system,
Condenser in Refrigeration & Air Conditioning AC Units
The refrigerant rejects heat out of the system. The outside fluid acts as a sink. This happens due of the temperature difference is maintained. A temperature gradient is a change in temperature divided by distance. So due to high-temperature gradient heat transfer takes place.
Condenser reduces the temperature of the refrigerant in 3 phases – De-superheating, change of state and sub-cooling.
The above T.S diagram will explain this in a simple manner.
In a refrigeration cycle, there are 4 important components. Compressor, condenser, expansion device and evaporator. The compressor compresses refrigerant, in this process it gets heat up due to increasing pressure and it gets superheated so it has to be cooled by passing through the condenser. Condenser first de-superheats, changes it’s medium and then sub-cools it as per need. Then expansion device slowly cools the system by reducing its flow and lastly evaporator absorbs outside heat.
1-2 process show compression. While 2-3’-3-4 shows condensation. Process 4-5 is the expansion that is the work of the capillary tube process while 5-6 is outside heat is getting absorbed by the evaporator.
For us 2-3’-3-4 is very important. At point 2 the refrigerant is superheated vapour. It has to be cooled as the system demands the same. When it passes through the condenser it rejects its heat to the outside fluid. It may be water or air or any fluid depending on the type of condenser. As there is a temperature difference the refrigerant starts to cool down by rejecting heat. Between process 2-3’ it is going through the de-superheating phase. That means the refrigerant is not totally in a vapour state or totally in liquid phase. So it’s more of a mixture. Its sensible heat is getting removed. At point 3 the refrigerant is not superheated anymore, Vapours and liquid mixture continues to coexist. The process 3’-3 is where actual condensation takes place as latent heat is getting removed while at this process temperature remains constant but the phase change in the refrigerant is taking place. This process continues till point 3 where the refrigerant is totally liquid. Between processes 3-4 refrigerant gets sub-cooled. At this point, the refrigerant is totally in liquid phase and at a very low temperature.
Condenser in Steam Plants
As we energy neither gets created nor destroyed, it travels from one form to another. So it becomes very important to harness this energy without getting wasted.
- In steam generation plant when steam gets created in boiler and then allowed to expand in turbine, the turbine rotates and creates electricity but the outlet of turbine also gives out steam which still has lot of heat energy.
- So if this steam is removed from the system then that heat energy will get lost.
- To harness this energy condenser is used so energy will be saved and the efficiency of the system increases.
Steam from the outlet of the turbine will be passed through the condenser. There are many types. Consider shell and tube condenser in this case, so when steam enters into this device, it is carrying heat.
When it travels through tubes that heat is transferred to the cold liquid moving in cylinder/shell. As there is no direct contact between steam and cold liquid, heat transfer by conduction takes place. The cold liquid will get heat up and steam will cool down as condensate. The liquid in the shell gets hot and will be used as feed water in boiler. As it is already pre heated, it will require less energy to get converted into steam.
Types of Condensers – Air Cooled, Water Cooled, Evaporative
- Air cooled condenser
- Water cooled condenser
- Evaporative condenser
As we discussed earlier, the condenser rejects heat outside the system. In this case that external fluid is air itself.
It may be of 2 types-
- Natural convection type or
- Forced convection type
As its name suggest the natural air condition is used, that is buoyancy in the air happening due to the natural convection and radiation, As heated air is lighter and cold air is heavy so using this natural effect flow of air over fins or tubes is obtained. As the flow rate of air is very small hence it becomes necessary to have a big surface area of contact to transfer heat. Hence such types are used in small scale systems such as refrigerators. These condensers can be of 2 types – 1. Plate surface type 2. Finned tube type
In plate surface type the whole outside body of the refrigerator acts like a fin. The refrigerant carrying tube is mounted on walls from outside. The insulation is given between outside tube and inner tube. Due to this outside surface is always little hot. This is generally used in small freezers.
In a finned tube type condenser, the fins are mounted on the backside. Fins are spaced so air should pass and the buoyancy effect is fastened. The tube is connected to these fins. Such a refrigerator must be kept away from the wall so as to allow the flow of air.
Forced convection type
As the name suggests, air circulation is forced one. The circulation is forced by blower or fan. Such condensers are used in water coolers, air conditioning systems and window air conditioners, construction varies as per need. It is mounted over chassis- the compressor, induction motor, condenser with condenser fan, accumulator, HP/LP cut-out switch and pressure gauges as an overall system are known as condensing unit of rated capacity.
Water Cooled Condenser
In these types, water acts as an external fluid. Based on its construction it has the following types
- Tube in tube type
- Shell and coil type
- Shell and tube type
Tube in tube type
It is also known as a double pipe type condenser. Normally used up to 10TR capacity. Cold water is passed through the inner tube while the refrigerant is passed from outside tubes. Headers are used to separate at entry and exit. Both liquids flow in opposite directions which gives better heat transfer. Heat transfer is low due to long flow and coiled tubes.
Shell and coil type
They are generally used up to 50 TR capacity. Depending upon the capacity flow of refrigerant is determined, whether coil or shell. In small capacity condensers, refrigerant is passed through coils and water through shell. Multiple coils can also be placed in the shell. While in the large-capacity condenser, refrigerant is passed through shell and coolant through coils. To clean coils, chemical agents are needed to reduce fouling.
Shell and tube type
Horizontal shell and tube is a most common type of condenser. It can be used from 2TR to thousands of TR. Refrigerant is passed through shell and coolant is passed from tubes. These tubes are in multiple to maximise contact.
As coolant enters from one side it flows to next end of the shell where all coolant gets mixed. From that back end again it separates by passage through tubes and moves out. Refrigerant is in contact with tubes for maximum time. The entry and exit of coolant and refrigerant are always opposite such that entry of chilled coolant and exit of refrigerant is on the same side, this is done to achieve the sub-cooled phase. The refrigerant also rejects heat outside the system through the shell.
This type uses both air and water to condense refrigerants. In a way it is like a combination of a cooling tower and a water-cooled condenser. In this system, water is sprayed from the top in atomised jet form while the air is blown from downward to upwards. The blower is used at the bottom. Between these both ends there is a refrigerant tube. Hot refrigerant comes and after multiple turns, cold refrigerant is obtained. This whole system functions as a closed-loop system. Evaporative cooling takes place and hence heat transfer is very high.
It is used in a big system where a high load is there. It is also used in places which has water scarcity. In terms of cost it is cheaper than water cooled condenser. As water needed is low and losses are low is generally most favoured choice. But whole system has to be kept outdoors, and it requires a greater amount of refrigerant and longer length of tubing. Outside environment affects its functioning. If the outside temperature is very low it might freeze the water and hence water supply can be switched and only air can be blown over refrigerant tubes. So system can be turn into air-cooled system. Air is used to speed up the evaporation rate.
There is one more subtype to this condenser called as atmospheric condenser. Whole principle of working is same just air blown over the tube is not forced but guided by vents. Though these types of condensers are cheap they are not used because the air flow is not always maintained as per need and the tubes face the formation of algae. It is efficient in one way as power is saved.
Advantages and Limitations of Condenser
- They are simple in construction.
- Air circulation is easy as no specific pipes are needed for circulation.
- Warm air disposal is not issue here.
- Fouling issue is minimum and hence maintenance cost is low.
- Heat transfer is minimum
- Useful for low load system- household appliances.
- Cost is more compared to water cooled condenser.
- Heat transfer rate is more.
- Rigid construction and hence cost is less.
- Water can’t be disposed directly in water body, it may harm marine ecosystem.
- Cooling tower becomes necessary to treat water before disposal.
- Water demand is more.
- Fouling, algae formation, rusting issues and hence Maintenance cost is more.
- Bulky in size.
Applications of Condenser
- To extract heat energy from steam in electricity generation plants. Mainly evaporative condenser are used.
- To cool the refrigerant in household appliances. Air cooled condensers are preferred.
- In ice factory, ammonia refrigeration system, mainly atmospheric condensers are used due to the cost constraint.
- In dairy industry, shell and tube condensers are used. As big tanks are to be cooled, refrigerant is passed through tubes and milk is chilled in shell.
- Chemical industry, distillation plants also used condenser to cool machinery.
- HVAC industry. Air conditioning in big malls.
What is fouling in Condenser?
There are impurities present in water. When water is allowed to pass through intricate tubes it may lead to the formation of blockage which affects the flow of water. It naturally reduces the efficiency of the system, therefore an area of tubes has to be increased for compensating for future fouling of the system as it reduces heat transfer. Higher is the fouling factor the lesser is the heat transfer. Being a closed loop system is becomes very hard to remove such problems at a later stage so systems are generally designed considering the future of the system.
Formula Used for Condenser
Heat rejected Qc is given by
Qc = m(h2-h4) = mext Cpextt (Text.o-Text.i)
- Qc is heat rejected,
- m is mass flow rate of refrigerant
- h2 h4 are inlet and exit enthalpy
- mext mass flow rate of external fluid
- Cpextt specific heat of external fluid
- (Text.o-Text.i) input output temperature of external fluid
Selection Criteria of Condenser
- Heat rejection calculations.
- Water inlet and outlet temperature.
- Availability of water.
- Placement- indoors or outdoors.
- Maintenance cost.
- Fouling factor and other hygienic constraints.
- Flow of water and type of water.
- Type of industry.