vapor compression

VAPOR COMPRESSION REFRIGERATION

C/W

Process Fluid

C/W Sub Cooler

The vapor from the compressor discharge flows through a condenser where it liquefies. Cooling water or air are used as coolant

In the chiller, the liquid refrigerant removes heat from the process fluid flowing through the tubes. As the refrigerant removes heat, it boils.The vapors flow out of the top of the chiller to the compressor

VI. MECHANISM OF COMPRESSION REFRIGERATION

Refrigerant Vapor

Superheated Vapor

Warm StreamCold Stream

Refrigerant Liquid

RefrigerantLiquid + Vapor

EXPANSIONVALVE

EVAPORATOR

COMPRESOR

CONDENSOR

Cooling WaterIn the compressor, the refrigerant pressure is raised, consequently the temperature also is raised. The pressure is raised sufficiently to be able to condense at approx. ambient temperature

Liquid from the condenser/ surge tank flows through an expansion valvewhere its pressure is reduced and its temperature is lowered

Process stream


CONTROL OF REFRIGERATION

• The function of the refrigeration system is to chill / cool a process fluid to a certain temperature

• The temperature of the process fluid is controlled by regulation of the backpressure of the refrigerant vapour on the chiller

• The chiller pressure determines the boiling temperature of the refrigerant

• Compressor circulates refrigerant

• The purpose of the compressor control system is to match the compressor capacity exactly to the amount of refrigerant leaving the chiller

CONTROL OF REFRIGERATIONCHILLERThe chiller pressure can be controlled in one of two ways:

1) Controlling compressor capacity (for variable compressor speed speed)

2) Use of a control valve in the refrigerant vapour line leaving the chiller (for constant speed)

• By maintaining constant pressure on the chiller the boiling temperature of the refrigerant remains constant. This means that the outlet process gas temperature will be essentially constant.

• The flow rate of refrigerant entering the chiller is controlled by the level control valve on the chiller (JT-valve is also act as chiller level control).

• If the chiller duty changes due to a change in the gas flow rate, the level control valve will change the refrigerant rate.

• By maintaining a constant level in the chiller, the chiller level control valve acts like a flow controller. As the level of refrigerant tries to change due to an increase or decrease in the chillerduty, the level control valve opens or closes to control the refrigerant rate into the chiller.

• So, the temperature of the gas leaving the chiller is controlled by manipulating the boiling pressure of the refrigerant.

• The refrigerant circulation rate is controlled by the level controller on the chiller.

CONTROL OF REFRIGERATION

COMPRESSOR• The capacity of a centrifugal compressor depends only

upon two things:- The speed of the compressor, and - The differential pressure across the compressor

(P discharge – P suction)• The purpose of the compressor control system is to

match the compressor capacity exactly to the amount of refrigerant leaving the chiller.

• The capacity of a centrifugal compressor is always expressed in terms of actual m3/s [actual ft3/min].

• Therefore, as the suction pressure and temperature change, the capacity of the compressor in kg/s [lb/min] changes.

CONTROL OF REFRIGERATIONCONDENSER

– The condensers are controlled to hold a constant pressure at the compressor discharge. A pure component, like propane, boils and condenses at a constant pressure and temperature. So, we actually control the condensing pressure by controlling the condensing temperature.

– The rate may be controlled with louvers, variable pitch for blades, variable speed motors or a combination of the three. Louvers are the simplest, but are not very energy efficient. They may also be unsatisfactory at low flow rates because their response is non-linear.

– Variable pitch fan blades and variable speed motors are more expensive but give excellent control and save fan power at low flow rates. Many times, louvers are employed on one bay and variable pitch fan blades on the remaining bays. The variable pitch fan can be used for base loading and the louvers for trim control.

SINGLE STAGE EXPANSION

MULTI STAGES EXPANSION

PROPANE REFRIGERANT SYSTEM

First stage suction

Third stage suctionSecond stage suction

Discharge

Medium pressure

High pressure

4C-6

2nd

Suctdrum

4PV-8

4PV-9

4PV-8Low

pressure 4C-19

1st

Suctdrum

PROPANE REFRIGERATION SYSTEM

4C-5

3C-10 High pressure

Low pressure

Medium pressure

4EDPV-3

4EDPV-2

PSS

4ESDV-6

4ESDV-9

4ESDV-4

PSS PSS

4ESDV-11

4ESDV-10

PROPANE PROPERTIES

T: -40°F, P: 15.7 Psia, Hf : 105 Btu/lb Hg : 310 Btu/lb

P-H Diagram of Propane

Back to P.5

Evaporation

Cooling

-40°F

-40°F

40°F

-40°F

100°F

260°FSub cooling

100°F

100°F40°F

40°F

EVAPORATOR

COMPRES SOR

CONDENSOR

Cooling Water

EXPANSIONVALVE Com

pres

sion

Expansion

Condensation


PROPANE PROPERTIES

T: -40°F, P: 15.7 Psia, Hf : 105 Btu/lb Hg : 310 Btu/lb

P-H Diagram of Propane

Back to P.5

Evaporation

Cooling

-40°F

-40°F

40°F

-40°F

100°F

260°F

100°F

100°F40°F

40°F

EVAPORATOR

COMPRES SOR

CONDENSOR

Cooling Water

EXPANSIONVALVE

Compr

essio

nExpansion

Condensation


EVAPORATOR

Evaporation

vapor compression

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