Compressor VI

[Superfridge]

Hi all,
The MYCOM V series manual states
Vi = Volume of suction refrigerant when compression begins
Volume of same quantity of refrigerant at discharge port
In other words, Vi is the ratio of the groove volume after competition of suction to the volume when the
discharge port opens.
Conventional screw compressors have three fixed Vi values, that is 2.63, 3.65 and 5.80, termed “L port,” “M port” and “H port,” respectively.
The relationships are:
Vi = (Pd/Ps)1/k or Vik = Pd/Ps
For calculation purposes could I not use the Specific Volumes at the desired conditions eg:
Ammonia Freezer
-20degC = 0.6237
+30degC = 0.1106
VI = 5.6
Ammonia Water Chiller
0degC = 0.2895
+30degC = 0.1106
VI = 2.6

[HVACRsaurus]

Hi

You are missing “K” Ratio of Specific Heat

Find K in table part way down this page
https://www.engineeringtoolbox.com/specific-heat-ratio-d_608.html

I can put together a mock calculation a bit later if you like, uses “to the power of X” or indices (sorry can’t do it just now, wash the dog & feed the child etc.)

[HVACRsaurus]

Ps,

I just re read your message
Specific Volume probably already accounts for K
I’m sure you’ve got the same result

Cheers

[RANGER1]

Page 10 has a graph in this older manual if any help.

http://www.midstatesrefrigsupply.com/assets/v_series_manual.pdf

[Superfridge]

Thanks all,
my math is rubbish so just looking for an easy way to calculate it, will check against a FRICK pannel.

I have read if the VI is wrong for the application the oil in the oil separator can foam excessivly. Could someone please explain the reason for this? Over/under compression causes oil foaming? Cheers

[RANGER1]

Thanks all,
my math is rubbish so just looking for an easy way to calculate it, will check against a FRICK pannel.

I have read if the VI is wrong for the application the oil in the oil separator can foam excessivly. Could someone please explain the reason for this? Over/under compression causes oil foaming? Cheers

Superfridge,
Where did you read this, as never heard of it before, or experienced issues that I remember.
Only issues is motor amps out of control, or possibly noisier operation with oil compression.
Keep in mind though, a lot of things written out of USA nearly always have liquid injection oil cooling as well,.
The only thing from link below that could possibly cause foam would be when over compressed, it re-expands to the lower pressure causing "foaming".

https://www.irc.wisc.edu/export.php?ID=42

https://www.irc.wisc.edu/export.php?ID=423

[NH3LVR]

I have always calculated VI this way.
Discharge pressure + 14.7 lbs divided by Suction pressure + 14.7 lbs.
Did I do it wrong all these years?

[Superfridge]

Superfridge,
Where did you read this, as never heard of it before, or experienced issues that I remember.

You got me there, probably read it in a FRICK manual. I see oil foaming in oil separators alot on compressors with fixed VI and running out side of their VI envelope. Early season, low loads and low discharge pressures, compressors noiser than normal.

[RANGER1]

You got me there, probably read it in a FRICK manual. I see oil foaming in oil separators alot on compressors with fixed VI and running out side of their VI envelope. Early season, low loads and low discharge pressures, compressors noisier than normal.

Maybe machines unloaded. usually noisier in a lot of cases.

It as also been stated that head pressure can drop rapidly causing oil foaming, that might be a winter or low load thing as well when condenser fans cycle.
With most packages I work on has separate sight glass off side of separator, so not looking directly in lie with a bullseye.
Most bullseyes there is usually a bit of foam on top, when separating 100-300 litres of oil a minute has to be something going on.
Mycom "L" or "M" ports not a lot of difference.
Usually pretty simple booster "L" port, but changes to "M" port if -40 deg C & below
"M" port for highside compressor -10 deg C suction as a general rule.
"H" port is single stage economised below - 30 deg C SST
Even with Mycom "V" series, set & forget, as probably not going to change Vi seasonally.

Is it just an observation, or do you have a problem?

In the links I posted it shows in a table Vi for different seasons or pressures, also to check if correct Vi on auto Vi machine.

Also don't believe everything you read, especially what I write.

[HVACRsaurus]

Apologies, it’s only taken a week for me to get this together..
Photos & Screenshot below,

Vi = ?

Pd Pressure Discharge in Absolute, equivalent +30 Celsius = 1166 kPa
Ps Pressure Suction in Absolute, equivalent -30 Celsius = 119 kPa
k Ratio of Specific Heat, Ammonia = 1.32

Answer via screenshot of Google Calculator

Cheers

1539115392

[Superfridge]

Hey thanks for that, that makes the calculation look easier.

Using the Specific volume dosn't give the same answer