Special features
● Specially developed water cooled condenser presentsan almost isothermal surface to the steam.
● Bench top, compact and portable unit requiring onlya 3 kW electrical supply. cooling water and a drain.
● Self-contained - incorporates its own steam generatorand air extraction system.
● Stabilises very quickly -many different conditionsmay be investigated in a normal laboratory period.
● Heat fluxes approaching 106wm-2 are possible.
● Thermocouple attachment technique allows measurement of mean surface temperature withoutinterfering with the surface properties.
● Easily controlled.
Experimental capabilities
● Visual observation of filmwise and dropwise condensation, and of nucleate boiling.
● Measurement of heat flux and surface heat transfercoefficient in both filmwise and dropwise condensation at pressures up to atmospheric.
● Investigation of the saturation pressure/temperaturerelationship for H20 between about 20°C and 100°C.
● Demonstration and investigation of the effect of airin condensers.
● Demonstration of Daltons Law.
Introduction
The use of steam both for power production and toconvey heat has a long history and its use in these fieldsis likely to continue into the foreseeable future.
In all applications, the steam must be condensed as ittransfers heat to a cooling medium which could be thecold water in the condensers of a generating station, thehot water in a heating calorifier, a sugar solution in asugar refinery, etc. During condensation very high heatfluxes are possible and provided the heat can be quicklytransferred from the condensing surface into the coolingmedium, steam using heat exchangers can be compactand effective.
Steam may condense onto a surface in two distinctmodes, known as Filmwise and Dropwise. For the same temperature difference between the steam and thesurface, dropwise condensation is several times moreeffective than filmwise, and for this reason the former isdesirable although in practical plants it seldom occursfor prolonged periods.
Filmwise Condensation
Unless specially treated, most materials used in theconstruction of heat exchangers are "wettable"" andduring condensation a film of condensate spreads overthe surface. More vapour condenses onto the outside ofthis film, increasing its thickness and causing it to flowdownward and drip from the lowest points.
The heat given up by the vapour during condensation isconducted through the film to the metal beneath andfrom this, to the coolant. The liquid is a comparativelypoor conductor of heat, and although the film may berelatively thin, its thermal resistance is appreciable. It isthis resistance which accounts for the large differencebetween the effectiveness of filmwise and dropwisecondensation.
Dropwise Condensation
By specially treating the condensing surface the surfacebecomes "non-wettable"" and as the steam condenses,alarge number of generally spherical beads form on itssurface.These beads become larger,coalesce, and thentrickle downwards.The moving bead gathers all thestatic beads along its downward path,becomes larger,accelerates and leaves a virtually bare surface in its trail.The ""bare"" surface offers very little resistance to thetransfer of heat and very high heat fluxes are thereforepossible.
Unfortunately, as stated earlier, due to the nature of thematerials normally used in the construction of condensing heat exchangers, filmwise condensation isnormal, but the desirability of dropwise condensationhas led to many investigations into methods which willpromote and maintain it in practical plants.
Avariety of compounds (usually fatty acids) which maybe injected into the steam and then onto the surface, aswell as a variety of direct surface treatments have beentried with various degrees of success.
The considerable practical advantages of dropwise overfilmwise condensation are
(i) A smaller heat transfer surface for a giventemperature difference and heat transfer rate.
(ii) A smaller temperature difference for a given areaand heat transfer rate.
(li) A larger heat transfer rate for a given area and temperature difference.
lt is important that all engineers and technologists shouldbe aware of the above and should understand the featuresof filmwise and dropwise condensation.
The upper cover houses two water cooled coppercondensers,one of which is gold plated to promotedropwise condensation,and the other is in its naturalstate to give filmwise condensation.The cover is alsofitted with a pressure relief valve and has a connection toa pressure gauge, a pressure switch and to an air
Description
Steam generation and both types of condensationtake place in the same thick walled glass cylinder withmetal cover plates.
The lower cover houses an electric heating element, athermocouple to measure the saturation temperature ofthe H-0 and a combined filling and draining valve. Theheat input to the element may be varied from 0.4 kW to3 kW by a manual triac control.
The upper cover houses two water cooled coppercondensers,one of which is gold plated to promotedropwise condensation,and the other is in its naturalstate to give filmwise condensation.The cover is alsofitted with a pressure relief valve and has a connection toa pressure gauge, a pressure switch and to an air extraction system.
Considerable effort has been devoted to the design of the condensers, so that, although water cooled, there islittle variation of the surface temperature and also thatthe mean metal temperature is obtained without the useof external thermocouples.The water flow rate throughthe condenser is measured by flow meters which arefitted with control valves.
A water jet vacuum pump is used intermittently to drawair (plus a certain amount of steam) from the chamber.Before entering the pump, the mixture is allowed tocool slightly so that some of the steam condenses.Thecondensate is removed by a separator and returned tothe chamber, while the saturated air passes to thevacuum pump.
the condensers, so that, although water cooled, there islittle variation of the surface temperature and also thatthe mean metal temperature is obtained without the useof external thermocouples.The water flow rate throughthe condenser is measured by flow meters which arefitted with control valves.
A water jet vacuum pump is used intermittently to drawair (plus a certain amount of steam) from the chamber.Before entering the pump, the mixture is allowed tocool slightly so that some of the steam condenses.Thecondensate is removed by a separator and returned tothe chamber, while the saturated air passes to thevacuum pump.
A multi point electronic thermometer indicates all importanttemperatures and the rate of heat transfer is calculated
from the water flow rate and its temperature rise.
