14. REFRIGERATION
Refrigeration
Refrigeration
• A process where heat is transferred from a lower-to a highertemperature level by doing work on a system.
Carnot Cycle
Temperature-entropy diagram of the Carnot cycle
Q high= Q low + W
T [K]
Thigh
3
2
W
Tlow
4
Q low = Tlow+ dels
1
s[kj/kgK
Cycle consists of:
• (1-2) - adiabatic compression
• (2-3) - isothermal rejection of heat
• (3-4) - adiabatic expansion
• (4-1) - isothermal addition of heat
Refrigeration
Basic Refrigeration Methods
There are many means to obtain refrigeration effect, but only the 3
basic methods are introduced here:
1. Mechanical refrigeration (vapor compression system)
2. Absorption refrigeration system
3. Steam-jet (ejector) system
All 3 methods use similar processes for obtaining refrigeration effect:
• Evaporation in the evaporator
• Condensation in the condenser where heat is rejected to the
environment
• Expansion in a flow restrictor
Refrigeration
Main differences:
Is in the way compression [-method of transforming low pressure vapor
into high pressure vapor in refrigeration system] is being done:
• Mechanical refrigeration (Vapor compression system)
- Compressor is used
• Absorption refrigeration system
- Absorb vapor in liquid while removing heat
- Elevate pressure of liquid with pump
- Release vapor by applying heat
• Steam jet (ejector) system
- ejector
Refrigeration
Compression
1. Mechanical (Vapor compression)
- Compressor
2. Absorption
- Absorb vapor in liquid
while removing heat
- Elevate pressure of liquid with
pump
- Release vapor by applying heat
3. Steam jet (ejector)
- ejector
high pressure vapor
condenser
expansion valve
evaporator
low pressure vapor
1. Mechanical Refrigeration (vapor compression system)
1.
•
•
Mechanical Refrigeration (Vapor-Compression System)
Most widely used refrigeration principle.
Used widely in oil/gas industry [and probably elsewhere]
heat
high pressure vapor
condenser
2
3
expansion valve
compressor
4
1
evaporator
low pressure vapor
work
heat
Mechanical Refrigeration – (vapor compression system)
Equipment
1.
Compressors
This could be classified by one criteria (the way the increase in
pressure is obtain):
1. positive displacement (reciprocating and rotary)
2. dynamic (centrifugal)
Using other criteria compressors are classified as:
1. Open
Characterized by shaft extension out of compressor where it is
coupled to the driver
2. Semi-hermetic
Electric motor is in the same housing with compressor
mechanism, but
housing is tightened by screw
3. Hermetic
Same as semi-hermetic but have welded housing (enclosure)
Mechanical Refrigeration – (vapor compression system)
Equipment (con’t)
2.
Condensers
These are heat exchangers that convert refrigerant vapor to liquid.
Heat is tranferred in 3 main phases:
1. desuperheating
2. condensing
3. subcooling
The 3 main types of refrigeration condensers are:
1. air cooled
2. water cooled
3. evaporative – with dedicated cooling tower
Mechanical Refrigeration – (vapor compression system)
Equipment
3.
Evaporators
These are heat exchangers where refrigerant is
evaporated while cooling the product, fluid or body.
Refrigeration evaporators could be classified according to the
method of feed as direct (dry expansion) or recirculating ( flooded
/liquid overflow) system.
2. Absorption Refrigeration
solution
heat
heat
high pressure vapor
generator
condenser
throttling valve
expansion valve
heat
heat
evaporator
absorber
low pressure vapor
pump
Absorption Refrigeration System
Two main absorption systems are used in industrial application:
1.
Lithium bromide-water
- limited to evaporation temperatures above freezing because
water is used as the refrigerant.
2.
Ammonia-water
- Because ammonia is the refrigerant it can be applied for low
temperature requirements.
Ammonia – Water Absorption System
ammonia vapor + small amt of water
cooling fluid
analyzer
water rich liquid
rectifier
condenser
heat source
generator
ammonia vapor + solution
heat exchanger
cooling fluid
COOLED FLUID
pump
absorber
evaporator
Absorption Refrigeration
Absorption Refrigeration System
Ammonia-Water Absorption System
• Refrigerant is ammonia
• Absorbent is dilute aqueous solution of ammonia
• Water (the absorbent) is also volatile, so the regeneration of weak
water solution to strong water solution is fractional distillation
• Ammonia vapor from the evaporator and the weak water solution
from the generator are producing strong water solution in the
absorber.
• Strong water solution is separated in the rectifier producing:
(1) ammonia with some water vapor content
(2) very strong water solution at the bottom, in the generator
Absorption Refrigeration System
Ammonia-Water Absorption System (con’t)
• Heat in the in the generator vaporizes ammonia and the weak
solution returns to absorber.
• Other stream, mostly ammonia vapor but with some water vapor
content flows to the condenser.
• To remove water as much as possible, stream passes through
analyzer where it is additionally cooled.
• The remaining water escaped from the analyzer pass as liquid
through the condenser and the evaporator to absorber.
• Operation pressure about 1100 – 2100 kpa absolute in condenser.
3. Ejector (Steam Jet) Refrigeration
heat
high pressure vapor
condenser
diffuser
low pressure vapor
ejector
evaporator
boiler
heat
pump
heat
Steam Jet (Ejector) Refrigeration System
Steam nozzle
1
Secondary ejectors
Primary ejector
Boiler
Primary condenser
2
Air
vent
cw
Make up H2O
Heat load
3
Flash tank-evaporator
Intercondenser
Aftercondenser
Cold water circ. pump
Condensate pump
drain
Steam (Jet) Ejector System
• This system substitute an ejector for the mechanical compressor in
a vapor compression system.
• Since refrigerant is water, maintaining temperatures lower than
environment requires that the pressure of water in the evaporator
must be below atmospheric.
• Main components:
1. Primary steam ejector
The high velocity steam flowing through the nozzle of the
ejector causes flash vapor from the tank to continually aspired into
the moving steam.
The mixture of steam and flash vapor enters the diffuser section
where the velocity is gradually reduced because increasing crosssectional area.
Steam (Jet) Ejector System
2. Condenser
The component of the system where the vapor condenses and
where heat is rejected
3. Flash tank
This is the evaporator of the ejector system, Since refrigerant is
water, maintaining temperature lower than the environment
requires that the pressure of water in the evaporator must
below atmospheric. Warm water returning from the process is
sprayed into the flash chamber though nozzles.