RACA Journal August 2024 RACA_August_2024_digital | Page 39

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7 .
Select the proper thermostatic charge based on the type of
refrigerant in the system and the evaporating temperature .
8 .
Check whether an externally equalised valve is required .
Always use an externally equalised valve when a
refrigerant distributor is used .
9 .
Determine the appropriate connections required .
10 .
Determine the length of capillary tube required .
11 .
Determine the order number according to the
manufacturer ’ s instructions and select the valve .
The following alternative to steps 3 , 4 and 5 above may be used :
3 .
Select the valve from the appropriate capacity table for the
evaporator temperature and the pressure drop available
( maximum evaporator load vs . pressure drop across valve at
the correct evaporating temperature ).
4 .
Determine the correction factor for the temperature of the
liquid entering the valve .
5 .
Calculate the corrected valve capacity by multiplying the
valve capacity according to the catalogue by the liquid
correction factors .
Refrigerant liquid temperature correction factors
Liquid temp .
R134a
R404a / R507
- 18 º C
1.69
2.00
- 12 º C
1.63
1.90
- 7 º C
1.56
1.80
- 1 º C
1.49
1.70
4 º C
1.42
1.60
10 º C
1.35
1.50
16 º C
1.28
1.40
21 º C
1.21
1.30
27 º C
1.14
1.20
32 º C
1.07
1.10
38 º C
1.00
1.00
These factors include corrections for liquid refrigerant density
and net refrigerating effect and are based on an average
evaporating temperature of – 18 º C . However , they may be used
for any evaporator temperature from – 40 º C to + 4.4 º C since the
variation in the actual factors across this range is insignificant .
Andre , let us move on to thermostatic expansion valve
( TEV ) bulb location and installation . The thermostatic
expansion valve sensing bulb that controls the thermostatic
expansion valve is clamped to the refrigerant suction line
where it monitors the temperature .
For TEVs that use an external pressure equalisation , the
equalising pressure line must be connected to the suction line
immediately after the temperature sensing bulb that operates
the TEV . In addition , it is good practice to connect the equalising
line to the top ( twelve o ’ clock position ) of the suction line .
This prevents oil and possible liquid refrigerant ingress into
( Courtesy of Danfoss ) the equalising line which may cause erratic operation of the expansion valve .
As far as feeler bulb location is concerned , we find some differences between manufacturers . Some state the twelve o ’ clock position while others the one o ’ clock position . Always follow the manufacturer ’ s specification . The twelve and one o ’ clock positions are typically for small suction lines up to 19mm in diameter .
This is in contrast to a system that uses a larger diameter line ( more than 19mm in diameter ). In this case the thermostatic expansion valve sensor bulb is clamped at the four or eight o ’ clock position on the lower portion of the suction line .
Some manufacturers specifications give more detail stating that : “ The bulb is best mounted on a horizontal suction line tube and in a position corresponding to between one o ’ clock and four o ’ clock . The location depends on the outside diameter of the tube .” This is to give a more accurate reading as should any liquid refrigerant be present in the suction line exiting the evaporator , this liquid refrigerant will most likely be found just above the oil flowing on the bottom of the tubing on its way back to the compressor .
The intent is to place the TEV ’ s sensing bulb close to but above the level of liquid refrigerant in the line . For larger diameter refrigerant tubing the two , three , or four o ’ clock position may be specified whilst on small diameter tubing the twelve , one o ’ clock position is used .
The reason we don ’ t place the sensor bulb on the bottom of the suction line is that oil traveling along the bottom of that tubing can act as an insulator to prevent the sensor bulb from accurately sensing the temperature of the refrigerant . It then stands to reason that one should never attach the feeler bulb , or any sensor for that matter to the bottom of the refrigerant tubing ( six o ’ clock position ). In addition , never place the bulb downstream of a p trap . Bulb placement before a p trap ( upstream ) is recommended .
In some instances , one can install the feeler bulb in the vertical position ; however , this is not ideal and only should be done when one has no other option . When putting the bulb on a vertical line
Diagram from a manufacturer with clear of placement recommendations in accordance to pipe sizes .

www . refrigerationandaircon . co . za RACA Journal I August 2024 37