RG00A502.pdf

Automation of

Commercial Refrigeration Plant

REFRIGERATION AND

AIR CONDITIONING

Automation of commercial

Refrigeration Plant

The purpose of this manual is to show some

examples of the use of Danfoss automatic

controls for commercial refrigeration plants.

A simple, hand-regulated plant is used as the

starting point of a step-by-step automation and a

short description of the function of each control is

given.

For additional training material we refer to

http://rc.danfoss.com/SW/RC_Training/En/Index.htm

Contents

Page

Hand-regulated refrigeration plant.....................................................................................................................................................................................................2

Refrigeration plant with thermostatic expansion valve and air-cooled condenser..........................................................................................................3

Refrigeration with Finned evaporator................................................................................................................................................................................................4

Thermostatic expansion valve.......................................................................................................................................................................................................5

Thermostatic expansion valve with distributor.......................................................................................................................................................................5

Expansion valves.................................................................................................................................................................................................................................6

Thermostatic expansion valve, method of operation...........................................................................................................................................................7

Thermostatic expansion valve with MOP charge....................................................................................................................................................................8

Combined high and low pressure control.................................................................................................................................................................................9

Low-pressure and High-pressure control..................................................................................................................................................................................9

High-pressure control, method of operation.........................................................................................................................................................................10

Thermostat..........................................................................................................................................................................................................................................11

Filter drier............................................................................................................................................................................................................................................11

Sight glass...........................................................................................................................................................................................................................................11

Automatic water valve....................................................................................................................................................................................................................12

Finned evaporator............................................................................................................................................................................................................................13

Refrigeration plant with oil separator and heat exchanger......................................................................................................................................................14

Oil separator.......................................................................................................................................................................................................................................15

Heat exchanger.................................................................................................................................................................................................................................15

Refrigeration plant for a larger cold store.......................................................................................................................................................................................16

Shut-of valve.....................................................................................................................................................................................................................................17

Solenoid valve....................................................................................................................................................................................................................................17

Key diagram, control current for refrigeration plant, ig. 20.............................................................................................................................................18

Motor starters....................................................................................................................................................................................................................................19

Central refrigeration plant for cold store temperatures above freezing point..................................................................................................................20

Evaporating pressure regulator...................................................................................................................................................................................................21

Check valve.........................................................................................................................................................................................................................................21

Key diagram, control current for refrigeration plant ig. 25..............................................................................................................................................22

Refrigeration plant for freezer display counter.............................................................................................................................................................................23

Diferential pressure control.........................................................................................................................................................................................................24

Crankcase pressure regulator.......................................................................................................................................................................................................25

Condensing pressure regulator...................................................................................................................................................................................................25

Diferential pressure valve.............................................................................................................................................................................................................26

Evaporator thermostat...................................................................................................................................................................................................................26

Key diagram, refrigeration plant for freezer display counter, ig. 29 .............................................................................................................................27

Main wiring diagram for contactors..........................................................................................................................................................................................28

Refrigeration plant for ventilation air...............................................................................................................................................................................................29

Manual Automation of Commercial Refrigeration Plant

Hand-regulated

refrigeration plant

Fig. 1

A hand-regulated refrigeration plant is usually

built up of these components:

temperature t o . The E-curves represent eva-

porator capacity, which rises with the increasing

temperature diference t r - t o between room

temperature (t r ) and evaporating temperature

(t o ). Where the C-curve (winter operation) and

E-curve (summer operation) intersect each other,

compressor, condenser and evaporator capacities

are in equilibrium.

Compressor (1)

Condenser (2)

Evaporator (3)

In order to maintain the cold store temperature t r

at the desired level, it is necessary to equip the

plant with adjustable valves (4) and (5) in that

changes in the loads on the evaporator and

condenser under varying refrigeration demands

can be reckoned on.

As can be seen from ig. 1, the room temperature

will fall from t r to t r ' when refrigeration demand

falls from Q o in summer to Q o ' in winter. To meet

this condition the capacities of the compressor,

condenser and evaporator must be adjusted, for

example by regulating the compressor operation,

and by throttling the water low to the condenser

and refrigerant liquid low to the evaporator.

For example, the plant will be unable to maintain

the same temperature summer and winter with

permanently set regulating valves and a

continuously operating compressor. This can

easily be shown graphically, as illustrated in ig. 1.

The full lines represent summer operation and

the dotted lines winter operation, (e.g.

condensing temp. winter +25°C, summer +35°C).

The C-curves represent compressor capacity,

which rises with increasing evaporating

Manual Automation of Commercial Refrigeration Plant

Refrigeration plant with

thermostatic expansion

valve and air-cooled

condenser

Fig. 2

Thermostatic expansion valve

Automatic expansion valve

In this plant an air-cooled unit has replaced the

water-cooled condenser. Air-cooled condensers

are normally used where no cooling water is

available or where the use of cooling water is

forbidden.

Superheat in an evaporator is deined as

t 1 – p s = °C superheat, where t 1 is the temperature

measured at the point on the evaporator where

the expansion valve sensor is placed, and p s is the

pressure measured at the same point. (The

relevant pressure is converted to °C).

Replacing the manual valve ahead of the

evaporator with a thermostatic expansion valve

(pos. 1) ensures that the evaporator is

continuously supplied with the amount of

refrigerant necessary to keep a constant

superheat in proportion to the load.

For further details on superheat, see page 7.

This of course presupposes that the selected

expansion valve suits the evaporator concerned.

A factor here is that in conditions of maximum

load the expansion valve supplies precisely the

amount of refrigerant the evaporator is able to

evaporate. In addition the superheat setting of

the valve must match the evaporator.

Superheat is generally understood as being the

number of °C the evaporator has minus the

boiling point of the medium at the existing

pressure and with all liquid evaporated.

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