10-08-2013, 12:54 PM
Study of Steam Jet Refrigeration
Introduction
The steam jet refrigeration system (also known as ejector system refrigeration system) is one of
the oldest methods of producing refrigeration effect. The basic components of this system are an
evaporator, a compressor device, a condenser and a refrigerant control device. This system
employs a steam injector or booster (instead of mechanical compressor) to compress the
refrigerant to the required condenser pressure level. In this system, water is used as the
refrigerant. Since the freezing point of water is 0°C, therefore, it cannot be used for applications
below 0°C. The steam jet refrigeration system is widely used in food processing plants for pre-
cooling of vegetables and concentrating fruit juices, gas plants, paper mills, breweries etc.
Principle of steam jet refrigeration system
The boiling point of a liquid changes with change in external pressure. In normal conditions,
pressure exerted on the surface of a liquid is the atmospheric pressure. If this atmospheric
pressure is reduced on the surface of a liquid, by some means, then the liquid will start boiling at
lower temperature, because of reduced pressure. This basic principal of boiling of liquid at lower
temperature by reducing the pressure on its surface is used in steam jet refrigeration system.
The boiling point of pure water at standard atmospheric pressure of 760 mm of Hg is 1 00°C. It
may be noted that water boils at 12′C if the pressure on the surface of water is kept at 0.014 bar
and at 7′C if the pressure on the surface of water is 0.01 bar. The reduced pressure on the surface
of water is maintained by throttling the steam through the jets or nozzles.
Evaporator
The evaporator or flash chamber is a large vessel and is heavily insulated to avoid rise in
temperature of water due to high ambient temperature. It is fitted with perforated pipes for
spraying water. The warm water coming out of refrigerated space is sprayed into the flash
chamber where some of which is converted into vapours after absorbing the latent heat, thereby
cooling the rest water.
Steam Ejector
The steam ejector is one of the important components of a steam jet refrigeration system. It is
used to compress the water vapours coming out of the flash chamber. It uses the energy of fast
moving jet of steam to entrain the vapours from the flash chamber and then compress it. The
high pressure steam from the boiler expands while flowing through the convergent divergent
nozzle. The expansion causes a very low pressure and increases steam velocity. The steam
attains very high velocities in the range of 1000 m/s to 1350 m/s. The nozzles are designed for
lowest operating pressure ratio between nozzle throat and exit. The nozzle pressure ratio of less
than 200 is undesirable because of poor ejector efficiency when operating at low steam pressure.
The water vapour from the flash chamber are entrained by the high velocity steam and both are
mixed in the mixing section at constant pressure. The mean velocity of the mixture will be
supersonic, after the mixing is complete. This supersonic steam gets a normal shock, in the
constant area throat of the diffuser. This results in the rise of pressure and subsonic flow. The
function of the diverging portion of the diffuser is to recover the velocity head as pressure head
by gradually reducing the velocity.
Multi-Nozzle Ejectors
A multiple nozzle ejector usually has one nozzle on centre while remaining of the nozzles
equally spaced peripherally around it. In most of the cases, multi nozzle ejectors designed for the
same conditions usually reduce steam consumption by 10% to 15% as compared to Single
Nozzle Ejector. The advantage of multiple nozzle ejectors is its much higher efficiency
compared to single nozzle ejectors. It is also considerably shorter in length than an equally rated
single nozzle ejector.
Conclusion
In this system there is no problem with load variation quickly adjusted. It is widely used in
process industry, steel plant, petroleum plants and thermal power plants. But this system is
having very low C.O.P. of the order of about 0.4 to 0.6. By increasing the condition of water
vapour in the evaporator or flash chamber then C.O.P. increases. In this system water vapour not
freezes below 4°C temperature, then this is the best system for any varying cooling load.