The principal of operation is very simple. For each given pressure, water has a corresponding boiling point or temperature of vaporization. For instance, at 114.7 psia pressure, water will vaporize at 170 °C. At atmospheric pressure, 14.7 psia , water will vaporize at 100 °C. When contained in a vessel and subjected to a vacuum of 0.1475 psia ( 7.6 Torr ) , water will boil or vaporize at 7.2 °C. Hence, the steam vacuum refrigeration unit produces a pre-determined vacuum that corresponds to the chilled water temperature required, and thus flashes off water vapour until the balance of the water that does not flash is chilled to this required temperature.

Each pound of water that changes its state and flashes into a vapour takes with it approximately 1068 btu's. Therefore, if we flash off 11.25 # / hr we have extracted 12,000 btu / hr, or one ton of refrigeration.

In the steam jet refrigeration system, the pressure inside the chill tank compartments will be reduced by a series of steam jet boosters,  to a pressure corresponding to the saturation temperature of the chilled water required and steam jet boosters are sized accordingly, to remove the calculated amount of water vapour evaporated for the required tonnage of refrigeration.

The booster compresses the vapour by the help of a motivating steam to the desired discharge pressure. Where it can be easily condensed in the direct contact type condenser with 32 °C cooling water. The non-condensable gases dissolved in the chilled water and system  air leakages, are handled  by the vent group ejectors.

The steam jet refrigeration system can be designed by using either direct contact  condensers or surface condensers and it can be barometric or ground level installation as per the site requirements. System consists of barometric condensers require a barometric leg of 34' vertical to remove the water by gravity from chill tank and condensers. Alternatively a high NPSH pump can be used where height is a constraint. Condensate can be recovered, if required, by using surface condensers.

The steam consumption can be reduced by shutting off one or more boosters  when operating load reduces or the chilled water inlet temperature. Chilled water inlet temperature may decrease in winter or when there is low demand in the plant. The temperature sensor will actuate the motive steam control valve of the booster to cut off the steam during this condition.

It must be recognized that 35 °F is just about the limit for chilled water, and if possible, consideration should be given to producing the same tonnage of refrigeration at a higher temperature, say 45 °F or 50 °F, since the higher the temperature of the chilled water the lower the first cost and operating cost.

ADVANTAGES OF STEAM JET REFRIGERATION SYSTEM

·        Steam jet refrigeration equipment features unequaled reliability since there are no moving parts. Mechanical systems rely on complex high speed reciprocating or centrifugal compressors for operation. Very fine clearance, high speed, requirement of very effective lubrication need, wear and tear leads to the requirement for stand by system i.e. simply more capital cost. 

·        Reliability enables steam jet refrigeration systems to be operated and maintained by plant personnel with minimum supervision. Most mechanical units require highly skilled factory technicians for routine operation, maintenance and simple repairs.

·        Highly corrosive and toxic refrigerants make mechanical and absorption equipment potentially dangerous if they are not properly maintained and operated. Absorption unit's main disadvantage is the difficulty in maintaining “tight” system with the highly corrosive Lithium Bromide and an operating pressure of 5 torr in the absorber and evaporator.

·        Due to potential hazards like irreversible damage to the human lung tissue caused by this colour less and odour less toxic gas - Lithium Bromide, many states in U.S.A. now require  licensed engineers to supervise the operation round the clock.

·        Steam jet refrigeration system can be installed indoors or outdoors and do not require special building or weather protection like mechanical systems. Special foundations or mounting structures are not required since steam jet refrigeration system exhibit little or no vibration. Floor space requirements are typically half of what is required for mechanical or absorption units. Integral hotwells can also be supplied to minimize piping and installation cost.

·        Mechanical and absorption refrigeration units require many tubes that are easily fouled and require periodic cleaning, therefore, they are not suitable for locations with hard industrial water. Steam jet refrigeration systems fitted with a barometric condenser, can typically tolerate up to an inch of scale build-up without reducing efficiency. Hard industrial water can be used for the condensers. Our many installations near by the sea, uses directly sea water into the direct contact condenser to cut down the capital cost on cooling towers. 

·        The amount of maintenance, down-time and spare parts associated with mechanical units can be very expensive, while for steam jet refrigeration records from many installations show that the cost for repairs or replacement parts average less than Rs. 500 per year. Obviously maintenance is not a factor here. Many of these machines have been in continuous service for 20 to 30 years. Water - the only medium. Operation is maintained through a continuous supply of water. No out-offs due to lack of refrigerants, no cost for replacing expensive refrigerants through normal losses , and no toxic gases.

·        Steam jet refrigeration systems do not require complex control equipment; the instrumentation and controls are similar to those used throughout most plants. Mechanical and absorption units require a multiplicity of controls that increase first cost, installation cost and maintenance cost.

·        Low pressure steam , exhaust or waste can be used in the boosters as a motive medium. The booster stages may be operated on down to 2 psig, whereas the secondary ejectors should not run on pressure below 25 psig. If pressure 25 psig or higher are not available, then other means of evacuating the main condenser may be supplied.

·        Chilled water produced by this system is deaerated, resulting in increased solubility for any gas, for an example, Steam jet refrigeration units are used for chilling and deaerating water to absorb the chlorine dioxide used in paper bleaching operations. Due to the greater solubility of Chlorine Dioxide in cold deaerated water, the bleaching agent is more efficiently absorbed, hence  being advantageous to the Paper industry.

·        Oxygen, Carbon dioxide, Nitrogen and many other gases dissolved in the chilled water will be liberated out inside the chill tank during deaeration process and as a result this deaerated chilled water will minimise the corrosion of the jacketed reactors, vessels with limped coils, condensers, coolers and chilled water pipelines.  

·        If the refrigeration requirement is for a process plant and must go on for 24 hours a day, then steam jet refrigeration would be the answer. The system does its job smoothly and freely without the constant attention of an operator or multiplicity of controls.

·        Steam jet refrigeration system offer simplicity in design, lowest first cost, reduced maintenance, unequaled reliability and safe operation with minimum or no supervision. These factors must be considered when purchasing refrigeration equipment.