Hi to all on my first post,

Brief background - UK time served engineer, been working offshore (oil rigs) for 16 years.
I have an idea for a soultion to an ongoing design issue, but can't believe I am the first person to think of this. So why don't we do it? What am I missing?

The issue...
A common system set up for offshore room temperature control is duct mounted DX coils with ATEX certified condensing units. For ATEX compliance we cannot use invertors or speed controllers, and most units are prety basic; open drive reciprocating compressors (although the last 5-6 years has seen more use of ATEX certified 'Bock' Semi-hermetic units).

Because we have limited capacity control for the compressor there is a requirement to ensure evaporator pressure is not allowed to fall too low to avoid the risk of icing up the coil. This is of particular concern offshore, where supply airflow keeps modules and rooms positively pressurised as part of the safe design.

If evaporator pressure regulation is controlled, then it will be with a hot gas bypass system, but the problem is, that almost every system I have seen or worked on has the bypass valve screwed in closed because they end up constantly bypassing and refrigeration duty falls off.


So, I want to know if anyone has seen a set up as follows, or if there is any reason that it wouldn't work?

Use cool gas from the top of the receiver tubed into the compressor suction line via a 'Close on Rise of Outlet' valve. The CRO valve would be adjusted to the lowest allowable evaporating pressure (e.g. 0-5degC for the refrigerant being used).
On a drop in system duty, with the evaporator pressure falling, the valve would open and maintain the minimum desired pressure.

This has the advantage of being completely removed from the evaporator and expansion valve operation and does not require a 3rd pipe run to the evaporator.
By using cool saturated vapour from the top of the reciever there would not be a significant increase in superheat at the compressor inlet.

The systems we install also utilise head pressure control valves to maintain condenser and receiver pressures so the removal of vapour from the top of the reciever should not have any detrimental effect.

I would include a solenoid valve in the 'bypass' line to allow pumpdown and avoid migration when offline (the same as hot gas bypass systems), and would likely include a delay timer on the solenoid to let the system settle following start-up, before opening the bypass valve line.

System sizes are anything from 5 - 30 kW cooling.

So, that's the plan. Will it work? What have I overlooked?
Please be critical...

Thanks,
Douglas