Method for Calculating Discharge Capacity of Positive Displacement Com

[ianmcq26]

Dear All

I am currently trying to determine the discharge capacity for the scenario of blocked outlet (closed valve) of a screw compressor to enable me to specify a replacement relief valve (compressor side). I have spoken to the compressor manufacturers and they have replied in simple terms to "install a 1/2" NPT device", with no justification and no mention of orifice size!

I am not safisfied with this and need to apply some chem eng.

Can someone advise regarding a method for determining the discharge capacity in this scenario?

Is this simply a function of the swept volume?

Info

- The compressor is in a chilled water refrigeration system, using R407c

- The system design pressure is known, ~30barg

- Relief temperature can be determined from a mollier (p-h) diagram as can specific volume at the relief valve set-point 27.5 barg

- compressor swept volume can be determined (volume x no. of rotations)

I have found a method in BS 13136

Q (kg/h) =60 x V x n x phi x vol eff

V=vol (m3)
n=rotational freq (1/min)
phi = density of heat flow rate (KW/m2)
Volumetric efficiency, est = 0.9

Can someone explain what the flux term refers to?

I can't for the life of me balance the units to give a mass flow either!

I have found a more simple method in ASHRAE 15 that consists simply of the swept volume, volumetric efficieny, density and an actuated minimum flow setting (not applicable in this case). This method makes sense and the units balance!

Can someone confirm from their experience which methods they would use to determine the discharge capacity?

Do I need to take into account anything else in the scenario development other than valve closed?

Also, could the temperatures be effected in another way? I am currently assuming the temperature to be based on saturated vapour temp at relief pressure (from p-h diagram). I have read in ASHRAE that compressor suction temp should be assumed to be 10deg C - how does this come into the calculation?!

Thanks in advance

Ian

 

[TD2K]

To size the PSV you need a flow rate, pressure, temperature and some physical properties.

Flow. A screw compressor, to my thinking, is just a PD device so whatever volume it is taking in on the suction end comes out at the high pressure end (not the same physical volume because of the change in pressure/temperature of course). The swept suction volume is theoretically the greatest that can enter the compressor and be delivered but the actual delivered flow would be affected by the volumentric efficiency and internal slippage which might be handled by a single 'efficiency' term as well as how that is affected by the increase in pressure to relieving pressure for a blocked discharge case.

Temperature. Is this a flooded or dry unit? A oil flooded unit's discharge temperature doesn't vary as much with discharge pressure as a dry unit as the oil absorbs a lot of the heat of compression. if the vendor can't advise you of the additional rise in discharge temperature at relief, and your 'normal' operating pressure isn't a long way below relief I'd ignore it as a first pass. If it's a dry unit I would take the vendor's normal temperature rise across the unit and determine the thermodynamic efficiency of the compressor and then use that to predict the discharge temperature at relieving pressure.

Pressure and physical properties (MW, k and Z) are straight forward to determine.

 

[Chance17]

A critical item for sizing the discharge safety valve is specification of the suction pressure upper limit.
This sets the mass flow through the machine and thus the required safety valve rate.
There is always a normal pressure but there may also be "upset" conditions.
For "blocked outlet" contingency, is it reasonable for the suction to remain at normal pressure ??