Variable Flow on Both Sides of a Plate Heat Exchanger

[EPullin]

To provide separation between existing Landlord and Tenant in a Commercial Office HVAC heating and cooling pipework circuits, I am considering the introduction of plate heat exchangers.

The fluid on both sides will be water, with standard water treatment, and operating with an approach of 2K. The secondary circuits are variable volume flow, and the existing fan coil units are two port control; therefore, the tertiary pumps will also need to be variable volume flow to avoid stripping out large numbers of valves and re-commissioning.

So far I have been unable to locate guidance on a suitable control strategy, and would be interested to hear horror stories of similar installations that could not maintain control, OR that a simple PI loop will provide adequate control.

 

[cdxx139]

One concern is using variable flow on a plate and frame, which may lead to fouling. You should consider a 3way mixing valve control strategy to keep the flow through the HX constant (to minimize fouling).

knowledge is power

 

[EPullin]

cdxx139

Thank you for your comment.

Both sides of the PHX are closed circuits and will have a strainers in front of the circulator and PHX; which should reduce the risk of fouling. In addition, a valve exercise routine will be run once a week, which should help reduce any build up on the plates.

 

[Drazen]

if you have some heat exchanger software, you should check tow conditions - nominal ones, and lowest flow on secondary condition, where you would need to determine lowest achievable flow on primary.

as regards to operating conditions, few times i had some very unusual temperature regimes, and manufacturer's specialists convinced me that exchanger itself will not have trouble with that.

plate exchangers originate from process industry where demands are usually more complex so they are expected to be very robust.

 

[rmw]

Fouling is not the only reason not to vary the flow through a PHE. They are sensitive to velocity across the plates to obtain the heat transfer needed. So unless you are oversized, be careful. Actually, oversizing is not a good thing to do with PHE's for both reasons mentioned - fouling and heat transfer.

rmw

 

[Ziggypump]

How about you check the different heat transfer rates at differing flows across the one type of heat exchanger. It may be as simple as maintaining a delta T across the one side of the exchanger as the other varies for the fan coil control.

Use a pressure transmitter on the fan coil side to maintain flow, and a temperature sensor on the outlet side of the heat exchanger on the fan coil side. Use the temperature sensor to vary the flow on the other side of the exchanger to maintain a set leaving water temperature of the heat exchanger. A temperature sensor on the heating side of the exchanger will be needed to control the heat. A high turndown ratio will be needed on the boiler.

This is just a quick thought, but will hopefully work for you.

PS: an auto-flow regulator will be needed on the boiler(?) side of the exchanger, because as the flow through the exchanger drops, so will the pressure loss.