MODULE 5

CONDENSERS FUNCTION

• Condensers are used to convert a vapor back to a liquid. Condensers are found on larger turbines used for power generation or process systems

TYPICAL CONDENSER

• Made up of four main parts1. The shell2. The tube sheets3. The water boxes4. The tubes. The shell is the main part of the condenser’s structure.Condensers are used to convert vapor back into liquid.

The flow path of water is referred to as the tube side of the condenser. The path of the steam on the outside of the tubes is called the shell side

PARTS OF THE CONDENSING UNIT

1.Condensing unit fan motor BLADE ASSEMBLY. 2.Condenser FAN MOTOR. 3.Fan motor MOUNTING BRACKET. 4.CAPACITOR to assist motors in running more efficiently. 5.CONTACTOR (the relay that allows the 230 VAC to turn on the motors. 6.CONDENSER COILS (refrigerant runs through the tubes and ambient air is pulled through the fins to cool the hot gas). 7.SUCTION LINE VALVE that closes the refrigerant flow and/or gives a port in which to attach the pressure gages and hoses. 8.LIQUID LINE VALVE to close off the high pressure and/or provides an access port for pressure measurement. 9.COMPRESSOR MOUNTING BASE. 10.COMPRESSOR motor and pump. The newest compressor is the SCROLL, a giant step forward in efficiency.

• As, the hot vapor passes over the tubes, it gives up heat and condenses back into liquid, which collects at the bottom of the shell side area. This area is called the hotwell.

OPERATION:

• Steam flows down through the shell of the condenser over the bundle of tubes.

• The steam’s heat is given up to the cold water flowing through the tubes. The steam gives up its heat until it reaches its saturation temperature – the point at which it condenses into water. Droplets of liquid then form on the tubes and fall down through the drain at the bottom of the condenser and into the hotwell at the bottom of the shell.

• The main condenser for a larger steam turbine is typically installed suspended directly below the turbine

Low level condenser

Barometric condensor

Jet condensor

• Steam exhausts from the turbine straight done into the condenser shell and passes over the tube bundle.

• When the water reaches the far end of the water box, it flows up and back through the upper tube bundle to the outlet water box. The water flows out from there, and eventually returns to the body of water from which it came.

TYPES OF CONDENSERS

Condensers are classified as1. SURFACE CONDENSER2. JET CONDENSER In surface condenser, the steam to be

condensed is usually passed over a large number of tubes through which cooling water is passing. The steam is condensed on the surface of the tubes as it gives up its enthalpy to the cooling water passing through the tubes

• In jet condenser, the steam to be condensed comes into direct contact with the cooling water which is usually introduced in the form of a spray from a jet.

• Both types of condenser may be operated as either a wet or dry condenser.

• In wet condenser, any gas which does not dissolve in the condenser is removed by the same pump which is dealing with the condensate.

• In dry condenser, the free gas and the condensate are removed separately.

• A further division in condensers is possible being a function of the relative directions of flow of the condensing steam and the cooling water

• The three possibilities are:1. Transverse flow in which the steam flows across

the path of the cooling water.2. Parallel flow in which the steam flow is in the

same direction as the cool water.

3. Counter flow (contra flow), in which the steam flows in the opposite direction to the cooling water.

Still further division of the condensers is possible :

1.The barometric condenser: the principle of operation is illustrated in the fig

• The condenser here is mounted on along pipe which is usually at least 10.34m long .The pipe, called the barometric leg, acts in every similar way to a barometer. If the water was used in a barometer then the barometer height would be about 10.34m.

• If however, instead of a Toricellian vacuum existing on the top of the water in the pipe, there exists some positive pressure, less than atmospheric such as in the case of a condenser, then the height of water column will be less than 10.34m, being a function of the degree of vacuum which exists.

• This is illustrated as h in the fig. Using this atmospheric leg it is possible, in the condenser, for the condensate to drain away by gravity into the atmospheric tank at the bottom. The atmospheric leg dips deeply into the water in the atmospheric tank. The discharge from the atmospheric tank is from a stand pipe, whose entry is high up maintaining a constant high level discharge. In this way there is no possibility of breaking the vacuum in the condenser.

2.THE LOW LEVEL CONDENSER• The condensate is removed by means of pump. Its

installation is appropriate when there is not enough height available for the installation of a barometric condenser.

• Figure illustrates a transverse flow surface condenser. Steam is admitted to the top of the condenser and is removed as condensate from the bottom having been condensed at the surface of the water tubes. Cooling water flows in at the bottom and out at the top of the condenser.

• Inlet and exit in this case are both at the same end of the condenser and hence the water makes two passes through the condenser.

• The condenser illustrated is a dry condenser since air and condensate are extracted separately.

JET CONDENSER• It consists of a tall cylinder into which are

introduced perforated baffle plates. They are fixed alternatively on either side of the cylinder and cover just over half the cross-sectional area of the cylinder. Cooling water is introduced, in the form of a spray at the top of the condenser. Steam is introduced at the bottom of the condenser.

• Due to its low density it will begin to rise up in the condenser and the spray and condensate will fall together to the bottom of the condenser and will be extracted. Air entering the condenser will be warm and hence will rise to the top and from here it is extracted.

• This condenser is of the dry type since both air and condensate are extracted separately. The steam and cooling water move in opposite directions through the condenser and hence this is a counter flow condenser

SAFETY PRECAUTIONS• Safety precautions are taken to prevent potential

harm to the maintenance personnel.• The first of these is to shutdown the cooling water

system of which the condenser is a part. Most condensers have their water sides divided into two separate sections. Each half can be taken out of service, one at a time, with the turbine still in operation at reduced capacity. The cooling water to one half will be shut off while the other half is still running.

• The valves and pumps are then tagged out of service.

• The next step is to open the condenser and perform an air quality check. The air in the condenser is checked to make sure that there is enough oxygen to support life and that there are no poisonous or explosive gases present.

• Low voltage safety lighting is provided to prevent the hazard of shock to personnel. As in any confined space, the two-man rule is followed- a helper keeps watch from outside to ensure the safety of the worker who goes inside

SERVICING CONDENSERS• There are three routine maintenance

procedures that are usually done:1.The water boxes are cleaned out and the

tube sheets scraped or brushed clean.2.The tubes are cleaned of any blockage or

buildup, and3.Any leaking tubes are found and plugged.Clearing the tubes is often done by a method

called shooting them.

• Shooting the tubes involved the use of special water gun. This procedure may be done with high pressure water, water and compressed air, rubber squeezees, brushes or metal bladed scrapers. When the tubes are cleared, it is easier to detect leaking tubes that must be plugged.

• To seal leaks while the condenser is on line, any of several substances can be introduced into the cooling water before it goes through the tubes. The substances must be bio degradable to prevent any danger to the environment when water flows back out.

• One of the substances used is sawdust. The use of sawdust to plug leaks is one example of an ingenious stop-gap.

CONDENSATION PROCESS

• The vapor starts condensing on a surface when the vapor saturation temperature is more than the surface temperature. The temperature of the condensate formed on the surface is less than its saturation temperature and it becomes sub-cooled, more vapor will condensate on the exposed surface or on the previously formed condensate as the temperature of the previous condensate is less than the saturation temperature of vapor

TYPES OF CONDENSATION PROCESSES

• Film condensation -the condensate tends to wet the surface , forming a liquid film. The heat from the vapor to the cooling medium is transferred through the film condensate formed on the surface.

• Dropwise condensation- the condensate forms droplets on the surface and every time fresh surface is exposed to the vapor.

• In dropwise condensation only a part of the surface is covered with condensate. Very high heat transfer rates are reported due to good contact between the vapor and surface.

THEORY OF LAMINAR FILM CONDENSATION

• Most condensation encountered in actual equipment are film wise in character.

• The physical nature of the laminar film condensation process is well understood. Nusselt described the process in 1916 and analyzed condensation process occuring under several different geometric and vapor velocity conditions.

NUSSELT’S THEORY OF LAMINAR FILM CONDENSATION

• The following assumptions are made for derivation.• The liquid film is in good thermal contact with the

cooling surface and therefore it is assumed that the inside film face has the temperature of the cooling surface.

• The flow of liquid film formed on the surface remains in laminar region.

• Viscous shear of vapor on the liquid film is negligible at the boundary.

• Linear temperature distribution exists between wall and vapor

• The properties of liquid film as ρ, K μ, and Cp are constant throughout the liquid film

• The latent heat of condensation at the liquid and vapor interface is carried by conduction through the liquid film along y- direction.

• The velocity of liquid film formed along y-direction is zero.

• Assuming the vapour is condensing on a flat vertical wall as

• Ts= saturation temperature of the steam• Tw = temperature of the wall • ρ=density of liquid film• μ=absolute viscosity of condensing liquid.• hfg= latent heat of vapour

• K=conductivity of liquid

X=length measured from the starting point of the filmg=acceleration due to gravity.If the vertical height of the plate is L, then the average heat transfer coefficient is given by,

Drop wise condensation:when saturated pure vapor comes into contact with a cold

surface such as tube, it condensate and form liquid droplet on the surface of the tube, they fall from it leaving bare metal on

which successive droplet of condensate may form. When condensate occur by this mechanism ,it is called drop wise

condensation.A considerable amount of attention has been given to the drop wise condensation process because it gives heat transfer rate

ten to twenty times as great as that for film wise condensation. Film wise condensation is one in which condensation will occur

in the form of a thin film

Effect of air(or other gases) in condensor

A very small air leak into the condensing system of steam power plant will have a far more effect on efficiency , than that cause by partial pressure of the air. In fact all condensing vapor heat

exchanger perform much better when all non condensable gases are eliminated.