Heat Pump Boilers and BPHEs with variable water flow

[DTLarca]

So far I have been designing plant rooms having an Air Source Heat Pump Boiler outside feeding a BPHE in the plant room, via a glycol loop, then from the other side of the plant room BPHE hot glycol-free water is pumped to a Low Loss Header from which all the load circuits are fed.

One particular customer wants all future plant rooms designed even more simple than this - he wants to eliminate the need for the low loss header and its associated pump.

This means I have to try design variable flow through the plant room BPHE.

The idea then is that flow through the load side of the plant room BPHE varies from 2.1 litres per second to 0.5 litres per second. This simply means that the dT will vary from 5K to 20K, on that secondary side, with no discernible capacity loss at the plant room BPHE.

The lowest flow rates occur with the lower return-temperature heating circuits, such as towel radiators, which permits the higher dT's.

So long as the water side pressure drops through any BPHE are above 5kPa then there is ample turbulence to maintain capacity while only the dT increases.

The pressure drop through the BPHE will vary, at first glance, between 40kPa at 2.1 l/s and 2.5kPa at 0.5 l/s. That is 2.5kPa too low to maintain BPHE capacity.

So, the plant room BPHE will lose capacity at just 0.5 l/s - but I do not need its full capacity at that stage anyway and so this is not such a worry so long as I meet the minimum primary loop volume requirement.

But then my load circuit pumps are sized to provide the design volume at the max 40kPa which means that when the smallest pumps are working alone needing only 0.5 l/s they will in fact do around 0.8 l/s as they ride down their pump curves having less BPHE pressure drop to overcome. I have to be careful not to overload the pump motors at this point.

My question is - does anyone have any better ideas for designing a simpler plant room of such description?

[MikeHolm]

Is there a reason why you cannot use Grundfos Versaflow (or another brand) variable speed pumps to start with, just to keep on a good pump curve?

[DTLarca]

Mike, I haven't considered variable volume compared to the letting the last running pumps ride their curve.

Allowing the last running pumps to ride their curves allows for mass flows to increase in their circuits and so too through the BPHE which in turn helps to maintain heat flux density there when fewer pumps are running - the total pressure drop for each circuit drops as the total flow through the BPHE drops.

But, I suppose, what you mean, is that I could go for a double-pump set on the heat load side, which supplies all including DHW, Pool Heating, UF Heating, Towel Radiators, Radiators, Fan Coils and so on then simply have the pump vary speed maintaining a fixed discharge pressure - creating new curves. I should look into this on Monday - possibly the way I should go.