Steam jet refrigeration

Introduction
• Steam jet refrigeration system using a high vacuum
in evaporator and steam ejector as compressor.
• The major difficulties in development of this system
for the use of industrial purpose were
1. Very high vacuum required to be maintained in the
evaporator.
2. Enormous volume of vapor to be compressed.
3. High freezing point of water.
• This system cannot be used below 0⁰C temperature.
• Steam jet units are gaining importance especially
for industrial uses with the development in design
and construction of the ejectors.
• It is commonly used for precooling of vegetables
concentrating fruit juices.
Steam jet refrigeration cycle
• In this system water is boiled in an evaporator and
converted into vapor at a very low pressure.
• The vapor is then entrained by jet pump composed to a
high pressure and condensed in the condenser.
• The boiling or vaporization process in the vacuum
chamber requires a substantial amount of heat.
• This heat must come from water itself, hence
refrigeration is accomplished.
• By maintaining the proper vacuum by continuous
removal of vapor, water can be passed continuously to
the vacuum chamber and cooled to a desired
temperature.
• The jet pump uses steam at high pressure to pump
the water vapor directly.
• This motive steam must also be condensed
together with the water used as refrigerant.
• Refrigeration by direct evaporation is possible
because the latent heat of vaporization of water is
high compared to the specific heat of water and its
heat transfer coefficient is much larger than that
of other refrigerants.
• The steam jet refrigeration unit consists of an
evaporator, one or more booster ejectors a surface
type or barometric condenser and two stage
ejector air pump.
• High pressure steam is supplied to the nozzle inlet
and it is expanded.
• Here the water vapor originated from the flash-
chamber is entrained with the high velocity steam
jet and it is further compressed in the thermo-
compressor.
• The K.E of mixture is converted into static pressure
and mass is discharged to the condenser.
• The condensate is usually returned to the boiler.
• Generally 1% evaporation of water in the
evaporator is sufficient to decrease the temperature
of chilled water to 6⁰C.
• The chilled water in the evaporator is circulated by a
pump
• The warm water from load is returned to the
flash tank.
• The water is sprayed through the nozzle to
provide maximum surface for evaporation.
• The water which is flashed in the evaporator and
any loss of cold water in the load must be
replaced by make up water added to cold water
circulating system.
• This is accomplished by float valve that allows
addition of sufficient water to maintain a
constant water level in the evaporator.
 Analysis of steam jet refrigeration system
Mass of steam required per ton of refrigeration
Pb = pressure of steam supplied from boiler
Pc = pressure in the condenser
Pe = pressure in evaporator or in flash chamber
ab = Isentropic expansion of steam through nozzle
ab′ = Actual expansion of steam through nozzle
a = condition of steam supplied
c = condition of water vapor formed in flash chamber
b′ = condition of steam coming out of nozzle
d = condition of steam just before mixing
with water vapor
e = condition of mixture of steam at d and water vapor at c
after mixing and just before starting the
compression in the booster ejector
𝑓 ′ = condition of mixture entering into the condenser
 𝑒𝑓 = 𝑖𝑠𝑒𝑛𝑡𝑟𝑜𝑝𝑖𝑐 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑖𝑜𝑛 𝑖𝑛 𝑏𝑜𝑜𝑠𝑡𝑒𝑟 𝑒𝑗𝑒𝑐𝑡𝑜𝑟
𝑒𝑓 ′ = 𝐴𝑐𝑡𝑢𝑎𝑙 𝑐𝑜𝑚𝑝𝑟𝑒𝑠𝑠𝑖𝑜𝑛 𝑖𝑛 𝑏𝑜𝑜𝑠𝑡𝑒𝑟 𝑒𝑗𝑒𝑐𝑡𝑜𝑟
• The actual expansion through nozzle does not follow
isentropic process, so that the actual drop is taken into
account by nozzle efficiency.
Actual enthalpy drop ha − hb′
ηn = =
Isentropic enthalpy drop ha − hb
• The water vapor formed in flash chamber has negligible
velocity compared with velocity of steam coming out of
nozzle which is equivalent to
2gJ ha − hb′ m/sec
• The quantity ha − hb′ is equivalent to KE of motive
steam, available for entrainment of vapor in flash
chamber.
• The process of giving the momentum of water vapor
formed in flash-chamber by high steam velocity is
known as entrainment of vapor.
• During the entrainment, steam will lose some energy.
• This process of entrainment is very inefficient and part
of original motive force available for compression is
reduced and it is taken into account by a factor known
as entrainment efficiency.
ha − hd
ηe =
ha − hb′
• The actual compression of mixture does not allow the
isentropic compression.
hf − he
ηc =
hf′ − he
• Required work of compression in system = Available
energy for compression.
• Assuming 𝑚𝑠 kg of steam is required per kg of vapor
in flash chamber.
Available energy for compression = ms ha − hd
required energy for compression
= ms + 1 hf′ − he
ms + 1 hf′ − he = ms ha − hd
hf − he
hf′ − he =
ηc
ha − hd = ηe × ha − hb′

ha − hb′ = ηn × ha − hb
 ha − hd = ηe × ηn × ha − hb

ms + 1 hf′ − he = ms ha − hd

hf − he
ms + 1 = ms × ηe × ηn × ha − hb
ηc

ms × ηe × ηn × ηc × ha − hb
ms + 1 =
hf − he

hf − he
ms =
ηe ηn ηc ha − hb − hf − he
• The returned water enters the flash chamber at
condition h and leaves flash chamber in form of
vapor at condition c.
• So that each kg of vapor formed carries ℎ𝑐 − ℎ𝑔
quantity of heat from flash chamber.
heat of water at g = heat of water at h
• The required quantity of vapor formed in flash
chamber to take T tons of refrigeration load must
be equal to
3.5𝑇
=
ℎ𝑐 − ℎ𝑔
Components of the plant
Steam ejector
• It is also known as jet pump or thermo-compressor.
• High pressure steam from boiler is admitted to the
steam chest and expanded in a convergent nozzle to a
very low pressure and attains supersonic velocities in
range of 1000m/s to 1350m/s
• The flash chamber is connected to the region of low
pressure of ejector.
• The water vapor from the flash chamber are entrained
in high velocity jet of stream.
• The momentum transfer takes place and when the
mixture is further passed through a convergent-
divergent nozzle the pressure is raised and finally it is
condensed in the condenser.
Convergent-Divergent Nozzle
• The super saturation effects have to be
considered to avoid unstable operation of
nozzle.
• The nozzles are designed for lowest operating
pressure ratio between nozzle throat and exit
• The losses mostly take place in divergent
portion and it should be as short as possible.
Mixing section
• The mixing of stream and vapor from flash
chamber takes place in the mixing length and
then it is diffused in the diffuser.
• In practice if D is diameter of throat section, then the
length of constant area section is kept below 7(L/D)
where L is total length.
• The converging angle should not be more than 4⁰.
• The entrance to the mixing portion is as large as 30⁰
angle to prevent separation of secondary steam.
Diverging diffuser
• The diverging portion function is to recover velocity
head as pressure head by gradually reducing the
velocity.
• The area at exit should be twice the throat area.
• To optimize the friction and flow separation from the
walls , the inclined angle should lie between 6 – 12⁰