Document 5LpEVEx6496eo6xQQBzYRmYY8

1 Rotary pump hos eccentric rotor that creoles space for gas entering suction ot A At low.condenser.-jsteom flows the oh*: top while compressing previous charge to discharge pressure and forcing it out m pump copaclty detcermines Ihe vocuumRotary or Steam-Jet Air Pumps? During the past year several large surface condensers have been installed with rotary-type vacuum pumps instead of ateam-jet air ejectors. The decision on the proper vacuum pump is boat made after comparing the merits of the two types. Here we report certain items to consider in preparing such a survey, and discuss the calculation of air-pump capacity. For a number of years, ever since it supplanted reciprocating and hydraulictype vacuum pumps, the steam-jet air ejector has been standard equipment for surface condensers. Compared with a mechanical rotary pump the ateem-jet air ejector has the following advantages: (1) lower first cost (2) no moving parts (3) greater air-removal capacity. Several factors, however, favor the rotary air pump and become Increasingly important with the trend to higher operating pressures and temperatures. These are: (l) Am monia contamination, believed to be introduced by returning air-ejector drains to the system, is a problem in some plants. (2) Steam jets normally operate at reduced pressure and some times excessive maintenance of reducing devices is encountered. The rotary pump performs both the operations of normal air removal and of pumping down the unit when start ing up. Large capacity "hogging jets" must supplement air ejectors to start Here', a comparison of cost and operation of two types efj surface-condenser air-removal equipment, made by W C Woorfj man, mechanical engineer, Stone & Webster Engineering Corp/j up large condensers in a reasonably short time. This duplication of jets, however, is not o serious operating 'dis advantage. Rotary Pump. One type of rotary vacuum pump* is shown in Fig. 1. An eccentric rotor displaces air and vapor ahead of It, drawing these gases into a suction port end. discharging through the outlet No sliding vane or metalto-metal contact exists between rotor and casing. A viscous fluid maintains the seal, tri-eresyl phosphate being em ployed for. condenser-vacuum-pump service. The air-vapor mixture together with some of the sealing fluid dis charges through a baffled tank in which the sealing fluid aeparatea out and returns to the pump. In con denser-vacuum-pump service an addi tional whirl-type separoting chamber completes the separation of air and moisture. This pump type has served for some years in evacuating electric light bulbs and (n hydrogen-cooled generator-seal oil-conditioning systems. MloUrr-*uni *hrdrflkt Ifi this inlet* r* hurt During the war they served as "roegM lafi pumps" In certain high-vacmxoJj proceases. These pumps can rcdoce, pressures below 0.001 in. Hg abs, wefr beyond power-plant requirements. The largest available sire of tbit] pump bas been customarily applied to) surface condensers. This unit has 3 pumping capacity of about 700 cfm stj atmospheric pressure and about cfm at 29 iu. Hg vacuum. A 40-bl motor is usually provided with thk size, although continuous operatise needs at 29-in. Hg vacuum are erally less then 25 hp. Maximum pof demand occurs at about 22 in. Hi COMPARISON OP AIR PUMPS In selecting the proper type of sk pump, the following factors requir* comparison and study: first Cost. In an installation of so] appreciable site and importance t*s rotary pumps should be used for *3 liability. Normally one pump in eration should be sufficient, with second held in standby reserve. Condenser manufacturers often l> 81 (470) POWER Augut* service costa. Piping installation is less water is taken from a river or other extensive for the rotary pump, but cost source contaminated with sewage, or of electric-power supply must be added. it may be caused by circulating-water Installation of steam and mechanical leakage in the condenser. types often cost about the same. Ammonia vapor is least soluble under Capability. One of the most impor condenser pressure and temperature tant items to consider is the pump ca conditions, eo that the air-vapor mix pacity to remove from the condenser ture withdrawn by the air pump con- an air-vapor mixture resulting from free taina some free ammonia vapor. A air leakage into turbine and condenser. rotary pump will permanently remove Capability of the two equipment types this vapor by discharging it to the at differs considerably. It depends on a mosphere. In a ateam-jet air ejector the variety of conditions, including vacuum, ammonia will recondense and redls* temperature of air-vapor mixture et solve )n the air-ejector draina and re air offtake, air-leakage rate and the turn to the plant cycle. Some planta volumetric pumping rate of the air- are now wasting air-ejector condensate remorel equipment. rather than risk reoontamination. Given two oxamples of air pumps Maintenance. Presently air-ejec and assuming one baa a capacity just tor installations usually require reduc equal to inward rate of air leakage, this ing steom to 200-300 paig at the jets. 3Typieol 2-stoge stom-jet air ejec tor performance ot various suctions pump would maintain a vacuum equal to that which the condenser can pro duce. If the second unit had a smaller Maintenance on high-pressure reducing valves has been excessive and_aome have been undeeirobly noisy. Recently capability, the condenser vacuum would there has been a tendency to omit the nish s twin-element 2-atage air ejector with each element having enough ca pacity to handle normal air leakage. But these twin elements are mounted on a single inter- and after-condenser. To insure continuous service and pro vide the same degree of reliability as with two rotary pumps, the specifica tions given the condenser manufacturer should require furnishing of two banks of inter and aftertondentera, each with its own 2-stage steam jet These ejec tor units should be piped in parallel In the condensate line, and each should have capacity to handle maximum ex pected normal sir leakage. It is also accessary to furnish d high-capacity auxiliary hogging jet for starting. Here's the first cost comparison: Two roury vacuum pumps complete with motor drives, baseplates sod individual sepantors, but with common whirl cham ber, capacity of each equivalent to 4-cfm bee air leakage at 1.0 In. Hg abs and 7-3-F temperature depression-- 811,500 Two single-elemeat 2-stage steam-jet air rieciora, each with its own later- and her<ondener, capacity of each, 10l.23.li) sir-vapor mixture, equivalent to .73-cfm free air leakage at 14) in. H8 abs and 7.5-F temperature depreaslon ad one bogging jet-- $6700 fait of! until, under the increased abso lute pressure, the specific volume of the air-vapor mixture would bring the pump capability at tho decreased vacuum in equilibrium with the inward air-leakoge rate. Air-removal equipment must have the capacity to deal with the maximum ex- . pected air leakage at the vacuum the condenser must maintain. Capability under varying conditions will be diecussed later. Storting Capacity. Characteristics of the rotary vacuum pump. Fig. 3, enable it to remove large quantities of air when starting up and to pump down the turbine casing and condenser shell to a partial vacuum, as well aa to op erate under running condition!. Initial pump-down time can be shortened by running both rotary pumps in parallel during the starting-up period, and later shutting down one and allowing the remaining pump to handle normal con tinuous leakage. Steam-jet air ejectors have insuf ficient capacity for inltlol pump-down and must be supplemented by en auxil iary high-capacity hogging jet No great difference in pump-down time rosuits from the two methods, and either type of apparatus should be able to automatic pressure reducing volve end substitute a plain manually operated valve of the ported type. After ad justing the valve by means of a pres sure gage on the downstream eide the valve is locked in position. Rotary pumps will necessarily, in lime, involve some maintenance, even where contact of internal parts has been eliminated. Operatg Com. Comparative figures,, based on coal-at 17.50 per ton, 13,750 Btu per lb and 7000-Kr-per-year opera tion, indicate nearly, equal operating costa for the equipment sires selected. Steam consumption of an air ejector U very nearly proportional to Its nominal rating and if an ejector with a smaller air-handling capacity were selected the annual operating cost would be pro portionally less. Operating' cost of i single 2-tUge 6.75- cfm sir ejector that is using 405-lb-per-br of steam, with .correction for ihe conden sate heating effect in the air-ejector condenser-- 1550 per yr Operating cost of a rotary pump that is using 25 hp-- 1500 par yr Port-Lood Operation. At low loads the air-removal equipment, rather then the condenser, controls the vacuum. Highercapacity air pumps .will produce a reason for choosing a' 6.75-cfm air reduce condenser pressure to 10-In. slight improvement in vacuum.-. II ejector is explained later. Ejector Hg obs within IS minutes. light-load performance is important, Prices are proportional to capacity and Ammonia Contamination. The plant the improvement in economy will have !* * 4-cfm unit were selected, it would designer must judge whether ammonia some effect on the selection of elr* lower first cost. These air- contamination Is likely to be present removal apparatus. ejector prices assume purchase as part This has been a serious problem In Theoretically a demand charge 0 condenser since prices of auxiliaries some plants, attributable to contaminat ahould be essessed against the air ejec i,e ',**her when bought separately. ing tho sir-ejector condensate with am tor for the boiler-plant capacity re "telloHdn Costs. It is necessary to monia end its return to the system. quired and against the rotary pump P*re piping and electric-power This sometimes occurs when makeup for the boiler and generating capacity. FOWgg August 1948 (47/1 M