Refrigeration & Air-Conditioning

Module 3. Refrigeration plant components

Lesson 12
CONDENSER; TYPES, CONSTRUCTION, WORKING AND MAINTENANCE

12.1 Introduction

Condenser is a heat exchanger in which heat transfer from refrigerant to a cooling medium takes place. The heat from the system is rejected either to atmosphere air or to the water used as cooling medium. The water used as cooling medium which in term rejects the heat to the atmosphere. For the steady state operation, heat rejected in the condenser is the sum of heat absorbed by the evaporator and heat equivalent of work supplied to the compressor. On account of heat transfer in the condenser refrigerant passing through it is first de superheated and then condensed and may be little sub-cooled. Thus, the function of the condenser is to convert superheated refrigerant vapour into liquid refrigerant. The cooling medium used may be air, water or combination of air and water depending on the type of condenser employed in the refrigeration system.

12.2 Condenser Load

The amount of heat rejected or transfer by the condenser is termed as condenser load or condenser capacity.

Condenser capacity = Mass flow rate of refrigerant, kg/s × Enthalpy change of refrigerant while passing through the condenser, kJ/kg (kJ/s)

Based on the amount of heat to be rejected at the condenser, the flow rate of cooling medium required can be estimated as under.

The condensing pressure of the refrigeration system mainly depends on the temperature of cooling used in the condenser. It is desirable to use low temperature cooling medium in the condenser to get lower condensing pressure. This is desirable to get better COP of the refrigeration plant. Where the entering temperature of the condensing medium is relatively high, larger surface area of the condensers and higher flow rates are required to provide lower condensing temperatures than where the entering temperature of the condensing medium is lower.

12.3 Types of Condenser

There are three types of condensers commonly used in vapour compression refrigeration system.

1. Air cooled condenser

2. water cooled condenser

3. Evaporative condenser

12.3.1 Air cooled condensers (Fig. 12.1)

The atmospheric air is used as a medium of heat transfer in air cooled condenser. The heat rejected by the refrigerant is received by the air. The air circulation over an air cooled condenser may be either natural convection or by the action of blower or fan. Accordingly, they are classified as natural draft or mechanical draft condenser. The air cooled condenser consists of finned tubing of copper or other suitable metal in which the vapour of the refrigerant enters from the top and the liquid refrigerant leaves from the bottom of the condenser. The heat transfer area, temperature of the air, velocity of the air, overall heat transfer co-efficient etc. are important parameters affecting the performance of the condenser. This type of condenser is used for relatively small capacity system as heat rejection rate per unit area of the tube is less as compared to other type of condensers. The air velocities normally employed are 2 m/s to 6 m/s.

In case of natural convection air cooled condenser, the air quantity circulated over the condenser is low and hence condensing surface required is relatively larger as compared to mechanical draft air cooled condenser. Generally, forced convection type air cooled condenser requires 10-15 m² surface area per ton of refrigeration considering 2-5 m/s face velocity of air over the coil. The air is blown or drawn through the condenser by a propeller type fan. Natural air cooled condensates are either plate surface or finned tubing or copper or steel or other metal depending on the refrigerant used. The condenser should be located in a well ventilated and cool space where sufficient quantity of air is available. The sketch of mechanical draft air cooled condenser and the photograph of a mechanical draft air cooled condenser are shown in Fig. 3.2 and 3.2 respectively.

The design of the condenser is relatively simple, it’s operating and maintenance cost is also low. It is necessary to remove dust, lint etc. settled on the condenser by using a portable blower to maintain its heat transfer performance

Air cooled condensers are used in small capacity refrigeration systems such as window air conditioners, water coolers, split air conditioners etc. as the overall heat transfer co-efficient is low as compared to other types of condensers. The capacity of an air cooled condenser depends on heat transfer area, temperature difference, air velocity and overall heat transfer co-efficient between refrigerant and cooling air.

• Air cooled condensers are generally designed for condensing temperatures of 15 ºC to 20 ºC above the atmospheric temperature and the air quantity of 20 to 30 m3 /ton.
• The condenser is mounted at a higher level than the compressor and provided with oil loop at the compressor discharge.
• Air cooled condenser are rarely made in size over 5 TR due to higher discharge pressure, higher power consumption and excessive fan noise.

So far as maintenance is concerned, removal of dust, dirt, lint etc. settled on the surface of the condenser is to be removed periodically by using air blower in order to maintain better heat transfer rate. The bearing of the blower fan requires lubrication as per the recommendation of the blower.

12.3.2 Water cooled condensers

In water cooled condenser, heat is rejected in the water which in tern cooled in the cooling tower and the same water is circulated in the water cooled condenser. Based on the constriction, water cooled condenser are of three types viz. shell and tube type, double pipe and shell and coil type.

In double pipe arrangement, the refrigerant condenses in the outer pipe and the cooling water flows through the inner pipe in counter current direction. The shell and coil type condenser consists of shell in which water coil is placed for the circulation of water. Both of these types are not commonly used on account of difficulty of cleaning the water side surface of the pipe or coil.

Shell and tube type water cooled condensers are widely used in commercial refrigeration plant. It consists of a cylindrical shell in which a number of tubes are arranged in parallel and held in place at the ends by tube sheets. The condensing water is circulated through the tubes and the refrigerant is contained in the shell. The end plates being baffled to act as manifolds to guide the water flow through the tubes. The arrangement of the end plate determines the number of passes the water makes through the condenser before leaving the condenser. The shell diameter range from 100 mm to 1500 mm, whereas length varies from 1000 mm to 6500 mm. The number and the diameter of the tube depend on the diameter of the shell. The tube diameter varies from 16 mm to 50 mm. A schematic diagram of two pass shell and tube type water cooled condenser is shown in

Single pass vertical shell and tube type condenser may be employed in large capacity plant. The condensing water flows in the tubes by gravity through distributor installed at the top of each tube which imparts a swirling motion to the water. The hot refrigerant vapour usually enters at the side of the shell near the middle of the condenser and the liquid refrigerant leaves the condenser at the side of the shell near the bottom.

Water cooled condensers are designed for condensing temperature of about 10-12 ºC above the entering water temperature. The heat transfer rate in the condenser depends on the temperature of water, velocity of water, scale deposit, properties of refrigerant, surface area of the condenser etc. It is very important to maintain high overall heat transfer co-efficient for efficient condensation of the refrigerant. The temperature rise of the condenser water can be estimated based on the total heat rejected at the condenser and the mass flow rate of the water in the condenser.

It is necessary to use soft water for the water cooled condenser. The regular cleaning of the condenser tubes is necessary to maintain optimum heat transfer rate.

12.3.2.1 Heat transfer in water cooled condenser

The first step is the heat transfer taking place from refrigerant vapour to the tube through the film of condensed refrigerant liquid on the outside of the tube. The second step is the heat transferred from outside surface to the inside surface of the tube. The third step is the heat transferred through the layer of scale formed inside the tube. Finally, the heat is transferred from the boundary layer film to the stream of water flowing in side the tubes. Heat transfer steps involved are shown in

Thus,

Q = U × A × Δt

Where,

Q = Heat transfer rate, kJ/s

U = overall heat transfer co-efficient, kJ/kg K

A = Condensing area of heat transfer, m²

Δt = Temperature difference between refrigerant and cooling water, ºC

12.3.3 Evaporative condenser

The evaporative condenser uses both, water and air for the condensation of the refrigerant. The evaporative condenser consists of a coil in which the refrigerant is flowing and condensing inside and its outer surface is wetted with water and exposed to stream of air to which heat is rejected principally by evaporation of water. The water is spayed over the pipes carrying hot refrigerant vapour and movement of air over the wet tubes creates evaporation of water resulting into cooling of the refrigerant. The heat lost from the refrigerant is carried by the air-water mixture leaving the condenser. These types of condensers are mainly classified in two groups.

1. Natural draft evaporative condenser

2. Mechanical draft evaporative condenser

The natural draft evaporative condenser is installed on open space or on the terrace of the building so that sufficient air at maximum velocity is available. The water is pumped from the sump prepared below the condenser and it is spayed on the condenser tubes. The performance of natural draft evaporative condenser varies considerably due to variation of air velocity over the condenser. The space requirement is also more as compared to mechanical draft evaporative condenser. Therefore, mechanical draft evaporative condenser is a better choice to achieve consistent performance of the condenser. The mechanical draft evaporative condenser consists of fan or blower which creates air draft over the wetted tubes. The schematic diagram of an induced draft evaporative condenser is shown in Fig. 12.3 It is necessary to supply make up water in the sump to compensate the water loss due to evaporation .

The induced draft fan is more desirable than forced draught one as it has following advantages.

• It provides even air distribution over the coil.
• It eliminates the chance of re-circulation of the same air.

Most of the heat given by the refrigerant vapour is carried by the air in the form of sensible and latent heat, hence the effectiveness of this type of condenser depends upon the wet bulb temperature of incoming air. Lower wet bulb temperature of incoming air is desirable to achieve better performance of the condenser. The quantity of water circulated through the condenser should be just sufficient to keep the condenser coil thoroughly wetted.

The removal of scale deposited over the surface of the condenser coil and cleaning of water sump at regular interval are necessary to maintain the rate of heat transfer.