Estimation of Gas charge for Screw Chillers

[tadipatrir]

Dear Members,

Normally we select the refrigerant charge quantity for screw chillers by mass flow rate per ton and then fine tuning it during test.

Whether there is any other way of estimating it which is closer and accurate to the requirement?

Cheers!

[mohamed khamis]

By knowing the absolute suction pressure, enter P-h chart or calculator u can get the refrigerating effect, hfg ((kJ/(kg.K)) or latent heat (vapor enthalpy - liquid enthalpy). Thus:

minimum mass flow rate = (tons of refrig.*3.517)/Latent heat(kJ/(kg.K)) = ---- kg/s

the product is the minimum mass flow rate and if u want to know also the actual but approximetly by this:

normally the inlet refrigerant dryness fraction to the evaporator (Dx coil but others = 0) = 0.2 to 0.25


Mass flow rate = (tons of refrig.*3.517)/((1- 0.2 or 0.25)* Latent heat(kJ/(kg.K)*1.05) = ---- kg/s

the figure 1.05 accounts for the effect of superheating load.

this is method is valid for any refrigeration application screw or scroll or recp...

Cheers

[tadipatrir]

Dear Mohamed,

Thanks for your reply. Your reply gives the estimation of mass flow rate. Can we also estimate the charge quantity in Kg or any other unit.

I know that this will be tricky thing as it depends on condenser and cooler and other accessories.

Any specific guidelines?

Best regards,

[Dalmo]

tadipatrir,

It is not so accurate , but help imediatelly:

I use 0.8Kg per Ton of Refrig. I use this calc only for R22

cheers!

[tadipatrir]

Dear Dalmo,

Thanks for the reply.

Any such thumb rule for R134a refrigerant?

Regards,

[tadipatrir]

Dear Dalmo,

Thanks for the reply.

Any such thumb rule for R134a Refrigerant?

Regards,

[Dalmo]

tadipatrir,


I use for 134a 1.0 kg per ton, It's not accurate again, but it is better has a little more gas recipes to charge . Maybe you try to be able to find a tecnicall bulletin of you Screw Chiller knowing the right charge!

cheers!
post scriptum:

excuse for reply, because the GMT!

[absrbrtek]

There are so many variables anything posted would just be a guess. The above estimates are accurate for the situations that the individual posters are experiencing.

Ive seen 250 ton R22 packaged screws with 1000HP motors running -20 evap holding 7000#s of refrigerant.

On the other end of the spectrum I've seen 250 ton packaged R22 screws with 40F evaps using 600#s of gas.

If you have piping to add and the unit is other than a packaged skid, your charge could be anywheres from 2#s per ton to the skies the limit. JMHO

[mohamed khamis]

If u multipy the prevoius equations that i have posted earily to RPS u can get easily the mass kg i.e.

Mass (kg)= (tons of refrig.*3.517)/((1- 0.2 or 0.25)* Latent heat(kJ/(kg.K)*1.05)*RPS

RPS = compressor rotational speed in revoltion per second. Otherwise u use the roughly estimation of "Dalmo" gave. I wish it could help

Best regards