1,Only latent heat should be removed as this increases thermal efficiency
Even when the steam is expanded to vacuum conditions some 60% of the initial enthalpy at boiler conditions is thrown away in the condenser
3, Air must be removed from the condenser because;
The stm quantity reduces and hence it is responsible for less of the total pressure. Hence it is at a lower pressure ,has lower saturation temperature and so is undercooled with respect to the actual pressure within the condenser ( that is to say the condensate should be at a higher temperature equal to the saturation temperature at the pressure measured in the condenser.
Daltons law of partial pressure
Each constituent of a gas mix exerts a partial pressure equivalent to that if it occupied the space alone.
Condensate falling through the lower cooler regions containing the high air content is further cooled and re absorbs gases.
Must have large surface area available for cooling . Hence large number of small diameter tubes.
Cross flow is adopted for ease of manufacture, this allied to the change of state gives a cooling efficiency approaching that of counterflow
Taking into account tube material ,
max sea water flow rate should be maintained so as to;
maintain a sufficient steam/ coolant tempo difference across the material along the tube length
Circulating system should offer no undue resistance to flow and supply water equally to all tubes.
The tube batches should be so arranged so as to provide no resistance to the flow of steam. There is normally a narrowing inlet space within or surrounding the bank so as the passage area remains constant as the steam condensers.<
Failure to provide even flow leads to ;
pockets on non-condensable gasses being formed in the tube banks.
Allowance in the design should be made for some expanding arrangement.
PROTECTION OF CONDENSERS
Avoid low water speeds which causes silting.
Too high a speed leads to erosion.
Cathodic protection for plates and tubes by using soft iron / mild steel anodes.
>The effect can be increased with the use of impressed current using anodes of larger size and different material.
Alternately coating of the tubes with a 10% ferrous sulphate solution.
Rubber bonding of water boxes.
Marine growth prevention
When laying up the following procedures should be carried out to prevent damage;
Before draining ensure no special chocking arrangements are necessary to prevent loading on springs or damage to the LP exhaust inlet gasket.
The indications that a leak is in existence is that of high salinity measured in the condensate and boiler combined with a rapid drop in pH.
The first aid should be the injection of sawdust followed by a shut down at the soonest possible time.
There are three methods for leak detection;
Ultrasonic-Here, electric tone speakers are fitted in the steam space, and a microphone passed down the tubes. Alternately, instead of speakers a vacuum can be drawn with the microphone picking up air leakage.
Fluorescent-The water side is cleaned and dried, chocks are fitted and the steam side filled with water containing a quantity of flourescene. A UV lamp is then used on the water side.
Vacuum test- Draw a vacuum and cover the tube plate with plastic or use the ultrasound microphone.
With the regenerative effect the water is heated to within one degree of the sat temperature so releasing dissolved gases which may have been re-absorbed as the drops where falling.
The dissolved oxygen content should be less than 0.02 ml/litre.
At the air ejector take off for the gasses, a cooling space is so arranged so a to ensure that there is no reheating of the gasses which would lead to expansion and reduce the efficiency of the process.
Drains which are led to the condenser are led to the top so the water is reheated/dearated before extraction.
The increasing use of scoops has led to the single pass condensers with SW velocities of 2 - 4 m/s being the ideal with minimum's to prevent silting of 1m/s.
Material of tubes
Cheap aluminium brass has a low allowable flow speed of 5 m/s; cupro-nickel has a higher flow of 10 m/s but is dearer and a poorer conductor of heat.
This is by expanding and bell mouthing or with by ferrules and alternately fibre and metallic packing at the other end,
Tube stays cannot be used where the tubes have been expanded at both ends, the tubes must support themselves.
Advantages of the different designs of condenser
This provides a short path for both the ahead and astern steam, however the steam must turn through 90o before entering and hence there is a possibility of increased windage losses Windage loss is where the ahead steam is picked up by the astern turbine which recompresses it and heats it up leading to a loss of efficiency, reduced vacuum and heating up of the astern turbine.
The length is reduced but at the cost of increased height. The fitted support springs under the condenser carry two thirds of the weight and allow for expansion.
Axial plane( radial flow )
Reduced height at the expense of length. The ahead and astern both exhaust in the same direction so reducing the chance of windage.As the turbines are situate lower and inline the alignment to the tailshaft is simpler.The big disadvantage is that the condensate level is critical and must not be allowed to spill out.
Axial plane condenser as fitted to the Stal-Laval AP Plant
The tube pitch at the bottom of the tube stack is increased by leaving out alternate tubes over the final three rows. This helps to encourage the regenerating effect. A cooling pack of coarse pitched tubes is fitted within the bellmouth for the air extraction to prevent reexpansion of any gases and removal of vapour
The condenser is a dry bottom type with a low water level in the hot well being maintained by the super cavitating pumps
This single plane design of condenser is of the single pass type and is well suited to use with scoop systems. This is were cooling water flow to the condenser is supplied from an angled inlet pipe on the ships side. For this to operate the engine has to be travelling at a certain speed to give the correct flow of water. Below this speed the scoop must be shut off and a centrifugal main circulating pump in use. The advantage of this system is that the main circ can be of a much smaller size than would be required if it had to supply cooling water requirements for full engine load conditions. In this case it would be normal to fit to pumps of 50% capacity.
Scoop System layout
Although electrically powered Steam Ejector units offer the advantage of ease of installation and slightly improved operating efficiency their maintenance requirements has ensured that the most common type on larger installations are steam powered
Their primary function is to remove non- condensable gases from the condenser
After passing through the nozzle the high velocity stream jet entrains air and vapour , compresses it, and the mixture passes to a condenser section were it is cooled. The air with any uncondensed steam and vapour passing to the second stage were further compression of the air takes place.
Depending upon the number of stages of the air ejector, the air is now discharged to atmosphere or to a final stage and then to atmosphere.
The condensers are of the surface type and are cooled by condensate, in this way acting as a feed heater.
Either, two complete units or two ejectors mounted on one condenser are used , nozzle diameters are very small typically 1.2 to 4.7 mm and are liable to wear, abrasion and blockage.
When manoeuvring or at rest provision must be made to ensure that there is adequate flow of condensate through the condenser to provide cooling . This is achieved by means of a recirculating v/v which leads condensate from the outlet of the air ejector condenser outlet ( and other low pressure feed heaters such as an evaporator ) back to the main condenser. The opening of this v/v should be limited as it leads to a loss of plant efficiency
Two single stage Air Ejectors mounted on a common condensate cooled condenser. A vent is typically fitted on the condenser where the none condensable gasses are released. To improve efficiency an electric fan may be fitted to draw the gasses off.
Electric Driven Air Ejectors