STEAM JET REFRIGERATION CLASS NOTES FOR MECHANICAL ENGINEERING

STEAM JET REFRIGERATION

CLASS NOTES FOR

MECHANICAL ENGINEERING

 

This refrigeration means refrigeration from

heat. It uses a steam nozzle to create

vacuum in the flash chamber. High

vacuum causes water to evaporate and

evacuate. Water vapors mix

with high velocity steam jet. These are

compressed in the diffuser.

Mixture condenses in the condenser for

recycling. In this, it is able to create a

temperature of 5⁰C in the flash chamber. It

is a vapor compression refrigeration.

STEAM-JET WATER VAPOR REFRIGERATION

OR

EJECTOR REFRIGERATION

OR

THERMAL COMPRESSION SYSTEM

Type of the system-Vapor compression system

Means of compression

Kinetic energy converts into pressure energy

Motive force for the vapor compression–Kinetic energy of the steam

Source of steam-From boiler or waste steam from a process

Need of the system

1. Refrigeration with waste steam.

2.  Ejector compression system can handle very large volume of refrigerant (water).

   More economical than the centrifugal refrigeration.

Highlight of the system

Cooling from heat. Summer requires more cooling. Using solar collectors

 solar energy produces steam .  Heat produces cooling.

Principle of jet refrigeration

(i)  Principle of jet compression produces compression. Kinetic energy converts into pressure energy.

(ii)  Boiling point decreases with the decrease of pressure.  Water boils at low temperature under vacuum.  Example

Sr. No.        Pressure, bar             Boiling temperature

1.               1.01300                        100⁰ C

2.               0.05940                        36⁰ C

3.               0.01072                        8⁰ C

4.               0.00872                        5⁰ _C

Water vapor refrigeration

The system uses water as a refrigerant. Principle of jet compression produces compression. It employs kinetic energy of steam as the motive power for vapor compression. It is steam-jet water vapor refrigeration Or steam refrigeration.

Applications

(i)  Used in industries where process steam is already available.

(ii)  Hospitals use it where steam is available.

(iii) Can be used in cold countries like USA, Canada & Europe. Use of boiler is quite common for heating of  living and work spaces.

(iii) Employed in concentration of fruit juices

(iv) Applied in drying of foods and chemicals

Fig. Line diagram of Steam-Jet Water Vapor Refrigeration

Construction

It contains the followings:

Boiler,

convergent divergent nozzle,

diffuser,

condenser,

pump for feed water to the boiler,

flash chamber,

A centrifugal pump circulates chilled water around hot products.

Float valve in the flash chamber for make-up water.

 convergence angle is about 21⁰

angle of divergence is 10 to 12⁰.

Length of convergent = 7 d,

 throat length= 3 d,

 divergent length= 5 d, where d is the diameter of the throat portion.

Refer to Schematic diagram and temperature entropy chart

Point 1 pressure and temperature of steam entering the nozzle,

 2 is the condition at throat after actual expansion

2′ is the condition at throat after theoretical expansion

1-2’ theoretical expansion of steam in the nozzle

Process 1-2 actual expansion of steam in the nozzle,

Point 3 is the condition of stem after some loss in the entrainment of water vapor from the flash chamber

 4 Condition of vapor from the flash chamber,

 5 Condition of mixture of steam and water vapor

 5-6’ is the theoretical compression in the diffuser,

 5-6 is the actual compression in the diffuser

7 steam and water vapors condense in the condenser,

 8 condition of make-up water after the float valve in the flash chamber

  Use of nozzle in steam jet refrigeration system

 Nozzle expands steam and produces supersonic velocity. It decreases pressure of steam in the flash chamber to very high vacuum. This high vacuum enables water to flash vapor in the flash chamber at 5⁰ C temperature. This low temperature water produce refrigeration.

WORKING

1. Steam at 7 bars is fed to the convergent divergent nozzle. After passing through the convergent portion, it acquires sonic velocity.  Pressure lowers to 6 mm of mercury (vacuum). At this pressure, water starts flashing into vapors at 5⁰C. Water becomes chilled water. Thus flash chamber becomes the evaporator of a refrigeration system.

2. Pressure reduction ratio in the convergent portion is 800. It is not achievable by a number of expansion devices.

3. The compression ratio of the ejector system is nearly 8 = p _condenser /p _{flash chamber}

4. High velocity steam sucks water vapors. Steam entrains these vapors in the mixing chamber near to throat. Vapors get some velocity. Velocity of steam decreases. The enthalpy of steam rises due to decrease in velocity.

5. The mixture now passes through the divergent portion in the diffuser. The pressure of the mixture increases nearly 8 to 10 times. Diffuser becomes THERMAL COMPRESSOR.

6.  Water cooled condenser condenses the mixture. Pressure in the condenser is about 50 to 60 mm of mercury (still vacuum).

    7. A pump sends the condensate in two different streams. One, going to flash chamber as make-up water for the vapor flashed. A float valve controls the level of water in the flash chamber. The second line goes to boiler as feed water.

8. Pump circulates chilled water for cooling the space. Chilled water cools the space. Water heats up. It returns to the flash chamber. Nozzles sprays hot water into the flash chamber. The cycle repeats time and again.

9. The steam jet refrigeration cycle removes heat through  evaporation of a refrigerant at a low pressure (vacuum).   The condenser rejects heat.

10. Thus steam jet refrigeration is another type of vapor compression refrigeration system.

11.  Evaporation of nearly 2 % of water present  in the flash chamber takes place. It decreases the temperature of the remaining water to 5⁰ C.

EXAMPLE

1. A flash chamber contains 100 kg of water.

2. Reduce Pressure to vacuum by throttling of steam through nozzles.

3. Say 1 kg of water vaporizes under vacuum. Water left in the flash chamber is 99 kg.

4. It removes 2500 k J of heat from the water. (Latent heat of evaporation of water at 5⁰C)

5. The temperature of the remaining water decreases in the flash chamber . Calculate the fall in temperature of the remaining water as given below:

6. Latent heat = Q = m Cp dT

7. Fall of temperature = dT = 2500 / 99 x 4.187=6⁰ C

8. Evaporating 1 kg of water reduces the remaining water temperature by 6 ⁰C. Thus by continuing this process, the temperature of remaining water temperature lowers. But practically lowering water temperature limit is 5⁰C.

Analysis

P Pressure

Firstly η_n   nozzle efficiency

Secondly η_e entrainment efficiency

Thirdly η_c Compression efficiency

W_s = mass of steam required to eject 1 kg of water vapor

W_w = mass of water vapor for 1 ton of cooling per hour

N  = Refrigeration effect or cooling effect

CALCULATION OF Ws/kg, Ws/ton/h, W_w

1. P₂= P₂’ = P₃= P₄ = P₅

2. Nozzle efficiency          η_n= h₁–h’₂)/(h₁ –h₂)

3. Entrainment efficiency η_e= (h₁–h₃)/ (h₁ –h₂)

4. Compression efficiency η_c= (h₆–h₅)/ (h’₆ –h₅)

5. v₂= 2gJ η_n(h₁—h’₂)

6. Equating momentum W_sv₂/g = (W_s+ 1)v₅/g

7. Equating enthalpies W_s h3 + 1 h₄= (W_s+ 1) (h5 + v₅²/2gJ)

8. W _s= (h6 –h5)/((h₁–h’₂) η_n η_e η_c –(h_6’ –h₅))

9. N = h₄– h₈

10. W _w/ton/h = 211 x 60/(h₄– h₈)

11. W _s /ton/h = (w _s from point 8) x (211 x 60)/(h₄– h₈)

Advantages

1.  It is flexible in operation; cooling capacity can be easily and quickly changed by varying the flow rate and pressure of the steam.

2.  It has no moving parts.  It is vibration free.

3. No noise and maintenance is low.

4. The system is very reliable.

5. The operating cost is less than that for vapor compression systems.

6. The weight of the system per ton of refrigerating capacity is less than that for vapor compression system.

7.   Outside installation is possible.

8. The system is applicable to the processing of cold water used in rubber mills and distilleries. Used in paper mills, food processing plants and refineries.

9. This system suits air-conditioning. There is a complete safety because water as refrigerant. System adjusts quickly to load variations.

10. There is no hazard from the leakage of the refrigerant.

11. A single Unit of 300 ton of refrigeration is possible.

12. Cost of this system with 75 ton of refrigeration equals to that of vapor compression system. But for capacity greater than 75 tons, steam jet system is economical.

13. Pump requires electricity. So overall electric consumption is less. 

Disadvantages

1. Direct evaporation produces chilled water. Steam jet handles tremendous volume of vapor. It handles 665 m³/h/ton as compared to 7m³/h/ton for R-12. Centrifugal compressor if used will require a very high speed. The steam jet is most economical.

2.  The size of condenser is big in this system. In this, heat removed in the condenser is twice than in the vapor compression refrigeration system per ton of refrigeration.

3. The system is useful for comfort air-conditioning. It is not practically feasible for temperature below 5⁰C.  The freezing point of water is 0⁰C.

4. COP is less i.e. 0.4 to 0.6.

 

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