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Grizzly
26-11-2009, 07:23 PM
Hands up!
How many Techs out there would assume 4 bar (approx 60psi).
Was a satisfactory differential between a systems H.P Cut-out setting and The HP PRV setting?

Basically we had a situation recently where a system condenser fan speed inverter received a electrical spike.
Causing the fans to stop and the H.P pressure to rise to-wards it's H.P. cut-out setting of 20.0 bar.
(H.P. PRV set at 24.0 bar)
3 comps on a common circuit with individual mechanical HP cut-outs.
Comp 1 trips on H.P. comps 2 and 3 do not.
The PRV lifts and then resets!
When the above was re-simulated.
Comp 1 H.P. cut-out at approx 20 bar whereas the other 2 stayed shut until the Technician shut the system down at 21/21.5 bar for fear of lifting the PRV again.

This system is periodically tested for H.P. cut-out operation.
Yet because of the inaccuracy of Mechanical HP switches and a possibly slightly low 24bar set PRV.
A reasonable 4bar diff was proved to be un-suffice!

Obviously we have recommended their replacement.
And I suspect these will be wired in series not parallel as before.
Can anyone think of a reason why they are individually wired?
A long story I know but maybe someone will benefit
Moral.... Periodic reset using nitrogen of the pressure switches is recommended!
Cheers Grizzly

Andy
26-11-2009, 07:35 PM
Hands up!
How many Techs out there would assume 4 bar (approx 60psi).
Was a satisfactory differential between a systems H.P Cut-out setting and The HP PRV setting?

Basically we had a situation recently where a system condenser fan speed inverter received a electrical spike.
Causing the fans to stop and the H.P pressure to rise to-wards it's H.P. cut-out setting of 20.0 bar.
(H.P. PRV set at 24.0 bar)
3 comps on a common circuit with individual mechanical HP cut-outs.
Comp 1 trips on H.P. comps 2 and 3 do not.
The PRV lifts and then resets!
When the above was re-simulated.
Comp 1 H.P. cut-out at approx 20 bar whereas the other 2 stayed shut until the Technician shut the system down at 21/21.5 bar for fear of lifting the PRV again.

This system is periodically tested for H.P. cut-out operation.
Yet because of the inaccuracy of Mechanical HP switches and a possibly slightly low 24bar set PRV.
A reasonable 4bar diff was proved to be un-suffice!

Obviously we have recommended their replacement.
And I suspect these will be wired in series not parallel as before.
Can anyone think of a reason why they are individually wired?
A long story I know but maybe someone will benefit
Moral.... Periodic reset using nitrogen of the pressure switches is recommended!
Cheers Grizzly

Hello Grizzly:)

my reading on it is H.P. cutout staggered up to 0.9 of PRV value. I would tend to use a manual reset HP but keep a good range between each compressor cutout. HP should be wired directly on the safety chain to the compressor starter.
All safety settings should be checked each year as part of the maintenance.

Kind Regards Andy :)