Major Components of Steam Jet System

Major Components of Steam Jet System Points : Major Components of Steam Jet System, Primary Ejector and Condenser, Secondary Ejector and Condenser, The Flash Tank, Open and Closed System, The Condenser, Condenser Water Requirements, Primary Ejector and Condenser Reciprocating compressor is used for removing and compressing flash vapor. The volume of steam at low pressure is so large as to require ‘impractically large cylinder and prohibitively high compressor speeds. The two methods commonly for compressing flash vapors, are the steam jet ejector or booster, and the centrifugal compressor, which will be described later.

The steam jet ejector illustrated utilizes the energy of a swiftly moving jet of steam to entrain the vapor from the flash tank and compress it. High pressure steam is delivered to the nozzle of ejector. The steam expands which flows through the nozzle. Due to the great drop in pressure of steam its velocity increases enormously. While flowing through the nozzle, leaving at a speed of about 4000 ft /sec. which is over 2700 miles / hr. due to this high velocity flash vapor from the tank is continuously aspirated into the swiftly moving team. The mixture of steam and flash vapour then enters the diffuse section where a velocity is gradually reduced, because of the increasing cross-sectional area.

The energy of high velocity steam compresses the vapor during its passage through the diffuser. The pressure of steam will have been increased (corresponding to 45 F) to about 0.95 psia at the condenser (corresponding to a condensing temperature of 100 F).

The mixture of ‘high pressure steam and flash vapor, after having been compressed to as absolute pressure about 0.95 psia by the ejector will then liquefy at the primary condenser at a temperature of about 100 F. The latent heat or condensation is transferred to the condenser water which is available at about 80 F. The condensate is removed from the condenser by means of a centrifugal pump and is then returned to the boiler to be vaporized again at high pressure, In order to obtain economy and flexibility in the operation of large units; several boosters are frequently installed on one flash tank. As many boosters as are required by the cooling load can be supplied with steam at any one time. This is discussed in more detail later. Although only one steam nozzle is shown per booster in the illustration, some units are built with several small steam nozzles in each booster, all of which must simultaneously be supplied with steam. Some ejectors require more steam for starting than is needed during operation. Some ejectors have separate starting steam nozzles which are shut off immediately after the unit is placed in operation. Secondary Ejector and Condenser Some air is always found in a solution of water. This is particularly tree if the water is sprayed into the air being cooled as in an air washer. In addition slight amount of leakage through the pump packing or pipe flanges is unavoidable. Due to the sudden lowering of the pressure any air is dissolved in the water liberated in the flash tank. This occurs in the same manner as gas escaping from carbonated water when it is suddenly exposed to atmospheric pressure. This air is carried along with the flash vapor by the booster steam and delivered to the condenser. Although the amount of air dissolved in the water is small, it must be removed or it would accumulate in the condenser and eventually increase the pressure to a point where a booster ceases to function. As air cannot be condensed it must be removed from the primary condenser by means of a compressor: The pressure in the condenser is about 0.95 psia. Therefore the air in the condenser must be compressed from this low pressure to a point slightly above atmospheric pressure before it can be discharged to the atmosphere.

Because of the large pressure different between the condenser and the atmosphere the air is compressed in steps or stages. The secondary ejector group in the figure is provided in order to compress and ‘discharge the air from the condenser. This secondary group usually consists of these ejectors which draw the air form the primary condenser at about 28” vacuum (0.95 psia), and compress it to’ about 22” of vacuum (3.9). The mixture of steam and air is delivered by the first of the secondary ejectors into the’ inter-condenser. There the steam supply to this booster is condensed. The second of the secondary ejectors now withdraws the air and compresses it to slight above atmospheric pressure. The steam supplied to the final booster is condensed in the after-condenser while the air escapes through a vent-hole in the shell. The quantity of steam required for the secondary group is comparatively small, usually amounting to less than 10% of the total steam used by the, steam. The condensate from the inter- condenser usually discharges through a water seal into the well of primary condenser, but the small of condensate from the after condenser is usually discharged.

The secondary group will not operate with low pressure steam. The minimum pressure at which steam can be used in secondary group varies form 30 to 80 psia among the different manufactures. When the initial pressure of the available steam is low it is necessary to substitute an electrically driven vacuum pump for either the final secondary ejector on for the entire secondary group. Sometimes instead of the vacuum pump if water is available at high pressure water operated separator is used to remove the air thereby being accomplished in three stages. Low pressure steam is sometimes used for the primary booster even steam at high pressure is available. This occurs usual when low pressure steam is obtained from the exhaust of an engine or turbine. In such a case the secondary group alone may be supplied with high pressure steam thus avoiding the use of an electrically driven vacuum pump. The Flash Tank Evaporation of the water takes place in the flash tank. The relatively warm water returned to the flash tank for cooling is broken up in some manner so that a large area of water surface is exposed as it drops to the bottom of the tank. This is usually accomplished by spray nozzles. The flash tank should be thoroughly insulated as any heat conduced through the walls of tank to the water also vaporizes the water, and thus adds to the weight of one vapor which the primary booster must compress and deliver to the condenser.

As previously stated, as a result of the cooling of the return water in the flash tank, a portion of it vaporizes. This flash vapor is withdrawn continually from the flash tank by the primary ejector and delivered to the condenser. As is shown later, this loss of water from the flash tank amounts to about 13.3 pounds of water per hour per ton or refrigerating capacity, and it must be replaced or the flash tank would soon be emptied. Open and Closed System The term open and closed systems have no references to the steam ejector itself. They refer only to the manner in which the chilled water is utilized ft)r cooling the air in the cooled system, illustrated in the figure the chilled water withdrawn from the flash tank is pumped through the coil used to cool the air, and then returned to the flash tank. The water is never brought into direct contact with the air in open system illustrated in the figure, the chilled water is prayed directly into the air. The water is then collected in the tank of the air washer and returned to the flash tank, there to be cooled again.

In the closed system of figure just as much water is returned to the flash tank as is removed by the pump. There is no loss or gain or water in the coil. The only major loss is due to the vaporization of water inside the flash tank. When the closed system is used to cool air, the makeup water pipe is connected directly into the flash tank. A float valve actuated by the level of the water stored in the tank controls the supply of the water.

In the open system, the loss of Water from the flash tank due to vaporization also amounts to about 11.3 pounds per hour per ton. .But in this system, the moisture condensed from the air in the air washer mixes with the chilled water and also falls into the tank of air washer. Because of this moisture condensed from the air, slightly more water falls into the tank of the washer than is ‘supplied to the sprays by the pump. In order to control the supply of make up water in the open system, two floats valves are needed One float ‘valve is installed on the one pipe line leading from the air washer tank to the sprays of the flash tank. This valve of the flash tank this valve is actuated by the level of the water in the flash tank. A second float valve is installed in the air washer tank itself and controls the supply of makeup water. The amount of water lost by vaporization in the flash tank is always larger than the amount gained by condensation of moisture from the air because the tonnage needed dehumidification alone is always less than the total tonnage. The Condenser The primary condenser must be properly designed and constructed if the high vacuum is to be maintained. Either of two types of condensers may be used. The surface type is more common in air—conditioning installations because the jet type has more disadvantages which disqualify it except under certain conditions.

In principle and construction the surface condenser is similar to the shell and tube, condenser used in ordinary refrigerating work., The water flows through the tubes while the steam condenses on the outside. As a rule, these condensers are constructed with either two or four water passes.

The jet condenser: In this condenser the steam is condensed be mixed directly with the condenser water, no tube surface is provided.

The jet condenser is of two different types. One type is known as barometric and other as low-level. In operating principle, the two types of condensers arc similar. In the barometric type, the condenser is elevated to a distance of 34 feet above the level of water in the hot well. Because of high vacuum carried in the condenser (about 28 inch of Hg) a tail pipe of about this length is needed if the condenser water and condensers are to be drained by gravity. In the low-level jet type, the tail’ pipe is eliminated; the, condenser water and condensate are removed by a pump. The pump delivered the water from the condenser, under a 28 inch of vacuum, to the hot well which is under the atmospheric pressure: whereas only the condensate must be pumped from the low-level jet condenser. For this reason, the cost of pumping water from the low-level jet condenser is much greater than the cost of pumping it from a surface condenser. In order to eliminate this expanse, jet condensers when they are. used. are invariably of the barometric type. In addition, the barometric condensers less than either low-level or surface type.

One advantage of using jet type condensers is that dirty water can be used for condensering purposes. and all, the condenser water and condensate wasted. Dirt is no handicap here as there is no metal heat transfer surface. However, when condensate is wasted to the sewer fresh water must be supplied to the boiler. In order to prevent the formation if scale deposits on the heat transfer surface of the boiler, the boiler—fixed water must be treated. This is true even though clean condensing water is used and the makeup water for the boiler is unbroken from the hot well. In this case the condensate is diluted with such a large amount of condensers water treatment is still required.

When surface condensers arc used, the water is continually returned to the boiler, just as is ordinary heating systems. As the water is not contaminated or diluted, no water treatment is required. However, the tubes of the surface condensers must be kept clean for satisfactory operation. Because feed water treatment is required when jet condensers are used, and also because they must be installed either 34 feet above the hot well or else has the entire condenser water pump from them, jet condensers are little used for comfort air conditioning. The surface condensers are used almost altogether for this work. However, barometric condenses are often used in industrial work, where their ability to operate with dirty water may be a decided asset. Steam Requirements The quantity of stern required to operate a steam ejector system depends to a marked extent upon the initial pressure of the steam. With steam pressures below 100psig, the quantity of steam required increases rapidly as the pressure drops. With pressures above 100 pig, decreases the weight of steam required are negligible. For this reason, little operating economy is gained by increasing the term pressure above 100 psig. As discussed, the’ steam required for the secondary ejector is only a small fraction of the amount required for the primary boosters. Usually this amounts to less than 10 percent of the total steam used by the unit.

The quantity of steam used by the secondary ejectors depends on the amount of’ air to be removed from the primary condensers. As previously stated, more air is dissolved in chilled water when using a open system than when using a closed system. Therefore slightly more steam is required for the secondary group when the open system of water circulation is used then when the closed system is used although the difference is not an important factor because the Steam by the secondary group is only a small fraction of the total. About 1.5 Pounds of steam per hour, at a pressure of 100 psig will be required per toil of refrigerating capacity for secondary group. When using the closed system, the about 2.5 pounds per hour when using the open system. However, a minimum of from 50 to: about 150 pounds of steam per hour depending upon the manufactures, is required for even the smallest units. Condenser Water Requirements The absolute pressure of vacuum can be maintained in a condenser depends upon the initial temperature and quantity of the condenser water available. As the pressure to be carried in the condenser rises, the steam consumption of’ the primary booster also increases. However the higher the condensing pressure, the higher will be available temperature rises of the condenser water, and therefore the smaller ‘viii be the quantity needed.