Specifying Steam Surface Condensers Cooling Water Entering the Condenser by Elliot Spencer and the Quantity of Water Available
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Specifying Steam Surface Condensers cooling water entering the condenser by Elliot Spencer and the quantity of water available. Graham Mfg.Co., Inc., Great Neck, Long Island, N.Y. The steam condensing temperature can get down to as low as 5F above Factors affecting condenser design and selection. the entering cooling water temperature, A typical air conditioning surface condenser specification is included. but in most air conditioning installations the differential is 20F, sometimes 10F, but seldom smaller, as from the turbine manufacturer. the closer approach would require a costly amount of condenser surface. THE FORMULA for determining the Generally speaking, on systems where In practice, cooling tower water is first amount of surface required in a surface the motive steam pressure is used in the refrigerant condenser, condenser is as follows: approximately 150 lb and the turbine is where it rises about 10F and is then Q = UA (LMTD) (1) discharging to a vacuum of 26.inches available for steam condensing. The where of mercury (4 in. Hg absolute) , a steam great bulk of steam-driven cen-trifugal Q = Heat to be absorbed, Btuh = rate of 14-16 lb per hr per bhp is refrigeration systems operate with a pounds per hour of steam times 950 economically feasible. Although steam condenser vacuum of Btu per pound; greater efficiency is available at approximately 4 in. Hg abs., which is U = Overall heat transfer rate, Btu per increased cost, utility costs for this equivalent to a condensing (hr) (sq ft) (F); type of installation seldom justify temperature of 125.4F. Thus, referring A = Surface required, sq ft; reducing the steam consumption by to Fig 1 and presuming that 85F cooling water is supplied to the system LMTD = Loge mean temperature means of higher ef-ficiency turbines or difference between condensing lower condensing pressures. When from the cooling towers, water temperature and cooling water. special conditions are present, the temperature into the steam condenser No attempt will be made to go through specifying engineer may consider is approximately 95F. Assuming a a formal condenser selection. The either of the above modifications to turbine steam rate of 14-16 lb per hr per procedure for condenser rating is improve performance. bhp (bhp per ton of refrigeration is amply covered in the publication of approximately 1) and a condensing Heat Exchange Institute entitled, Condenser Vacuum water circulation rate of 3 gpm per ton "Standards for Steam Surface The vacuum produced in a steam of refrigeration, water rise on the steam Condensers."* Rather we shall attempt condenser is that pressure condenser is also about 10F. Thus, the to discuss qualitatively the factors corresponding to the temperature at leaving water temperature approach is affecting selections and their effects which the steam condenses. This 20F(125F minus 105F). on size and price. steam condensing temperature, in turn, is dependent upon the temperature of Turbine Steam Rate Turbine steam rate is usually expressed in pounds per hour of steam per brake horsepower and is commonly called the water rate. This water rate is a function of motive steam pressure and temperature, condensing temperature, or equivalent vacuum, and turbine efficiency. Specific information for a given turbine application can be obtained _____________________________ _ This Standard is currently out of print, but a new edition is in preparation and will be available in the future from Heat Exchange Institute, 122 E. 42nd St., 'New York, N.Y. 10017. If, on the other hand, we designed the be specified. This temperature is accessory items have built-in excess steam condenser for 3 in. Hg. abs., determined by the water source and capacity), no statement of excess which is equivalent to a condensing varies with the location of the capacity need be made. temperature of 115F for the same installation. Water flow rate to the approximate water temperature rise, the condenser is usually determined by Water Pressure Drop approach would be 10F (115F minus the refrigeration manufacturer and is Water side pressure drop is a function 105F). Thus from Eq (1), because the generally set at approximately 3 gpm of tube velocity, condenser length and log mean temperature difference is per ton of refrigeration when cooling the number of passes the water has to greater, we see that the condenser towers are used. However, when river make in a given condenser. Air designed for 4 in. Hg abs. requires less water or water at relatively low conditioning practice permits surface than the one designed for 3 in temperature is available, water flow velocities as high as 10 fps in the tube Hg abs. rate varies and specifications should side of refrigerant condensers and, in The foregoing example is an take these variations into account. recent years, this has been extended to oversimplification. In practice, the A recent trend in large multi-story apply to steam surface condensers for steam rate of a turbine discharging at 4 office buildings has been to use 2 gpm air conditioning installations. in. Hg. abs. is somewhat higher than per ton of refrigeration. Although this It has been common practice to limit one discharging at 3 in. Hg. abs. affects condenser, and turbine water pressure drop to 10 ft. However, Choice of a lower absolute pressure adversely due to higher condensing if this limitation is rigidly enforced, might permit using a turbine with one temperatures, there is a saving in velocities must be reduced to comply less wheel, which would result in condenser water piping. with this pressure drop requirement, reduced turbine cost. This reduced Too often, specifications state that the resulting in lower heat transfer rates turbine cost might or might not be surface condenser shall be capable of and higher condenser costs. Generally, offset by the increased condenser handling turbine exhaust while water pressure drops of approximately cost. (See "Is There a Steam Turbine in maintaining the desired vacuum with a 15 ft can be attained in most single- Your Future," ACHV, April, 1968, pp 5% increase in normal steam load. pass condensers operating at 4 in. Hg 43-47.) Similarly, condenser cost could Compliance with this requirement is abs. Most two-pass condensers, be reduced if the condenser were almost impossible. Specifications usually those running at 3 in. Hg abs., operated at 5 in. Hg. abs. (133.8F seldom provide the 5% greater water require pressure drops as high as 20-30 condensing). However, turbine cost quantity needed to balance this 5% ft. might increase more rapidly if the loading factor. Further, usually no Specification of water velocity and steam rate had to be held to a provision is made for the extra load on materials of construction for tubes and previously set value, which, as the cooling tower and the change in tube sheets where river water or indicated, is generally in the range of steam rate of the turbine when brackish water is involved must be 14-16 lb per hr per bhp. overload hand valves are used. carefully evaluated. Information Thus, the specification should simply Furthermore, the refrigeration machine should be obtained on the experience state the number of pounds per hour is operating at a different point on its of other users in the area having of steam to be used and the maximum curve and may be rejecting more heat similar design conditions before a temperature and quantity of available which, in turn, means hotter water to specific material is selected. Generally cooling water, omitting the precise the steam condenser. The net result speaking, for brackish water operating vacuum to be maintained by under the 5% excess load condition is applications, velocities in excess of 6.5 the surface condenser. This permits that the steam condenser receives fps can seriously reduce tube life, but the manufacturer to offer the best cooling water that is more than 5% for cooling tower applications a 10 fps combination of condenser, turbine and above the initial design temperature velocity is acceptable. refrigeration unit to comply with the and the resulting excess heat imparted limitations on steam consumption to the water overloads the cooling Fouling Factor specified. tower, further aggravating the Most surface condensers for air Other considerations, such as length situation. conditioning and power plant limitations and turbine vapor exhaust If it has been decided that the surface applications are designed with a size, very often have a determining condenser should have additional cleanliness factor of 85%. This means affect on condenser cost. surface in it, the most practical that the heat transfer rate used in approach would be to state that the designing the condenser is 85% of the unit shall have 5% excess surface or Water Temperature clean heat transfer coefficient. For 10% or whatever is needed. In general, As cooling water temperature those installations where cooling water a surface condenser will handle the determines the vacuum that can be sources produce rapid fouling of produced, it is imperative that the entire excess load at a slight increase maximum anticipated water temperature in back pressure. If this is permissible (and it must happen unless all the tubes, a higher factor must be field for proper vapor velocities within leakage quantities based on various used. This holds true for both the the shell. team quantities entering the refrigerant condenser and the steam Calculation of condenser heat load is condenser. Specification that the condenser. based on the fact that steam entering ejector shall be designed for the For most applications using clean the condenser has a value of quantities of air leakage as designated cooling tower water, refrigeration approximately 950 Btu per lb. More in HEI Standards is sufficient. condensers should be specified with a precise calculations can be made if the fouling factor of 0.0005 and steam specific turbine characteristics are Codes surface condensers designed for an available, but this figure is sufficiently 85% clean tube coefficient.