Do I want a higher flow rate through my system or the lowest flow rate possible?

I have seen this question in various forms from many different sources. Generally speaking, installers will assume that the lower the flow-rate the better. Their logic is “I want to see a significant temperature drop across the heat exchanger otherwise the heat exchanger isn’t doing it’s job.” Another justification for the low flow rate is frequently to match the “design” or tested flow rate. Almost without exception this approach is wrong.

For uniformity of testing ASHRAE established a design fluid flow rate for solar collectors so they could be tested with a uniform set of criteria. Since the flow rate has an impact on the performance of the solar collector they needed a standard flow rate to provide consistency in their testing. This flow rate was not designed to represent the optimal flow for any particular collector arrangement.

The assumption that you keep the flow rate down to achieve better heat exchange is based on a limited understanding of heat transfer. All heat exchangers work by passing a hot liquid (or gas) along one side of a heat transfer surface and a colder liquid along the other side of the surface. How much heat passes from the hot side to the cold side depends on; the temperature difference between the sides, the flow rate on each side, and the heat transfer surface. Assuming no change to the heat transfer surface, if you increase either the temperature differential or the flow rates you increase the heat exchange rate. For example; if you take a plate heat exchanger with the following parameters; Hot inlet temp – 120, Cold inlet temp – 100, collector flow rate 5.5 gpm, storage flow rate 3.5 gpm you would transfer 24,113 btu/hr for a given size heat exchanger. Using the same heat exchanger, temperature settings, and collector flow rate but increasing the storage flow rate to 5.5 gpm you would increase your heat transfer rate to 29,906/hour. That’s a 24% increase in heat exchanger output. You argue, “but I will pay more to run my pumps to get that increased heat exchanger performance.” Here is the rub; the incremental cost to run the pump at the higher flow rate for that hour is about 30whr. The incremental heat available is 1.7KWH. In other words you will capture 57x the amount of energy expended.

Is it fair to claim that you are saving 57x the energy by running the higher flow pump if the solar collector can’t produce that much additional energy? Yes. If your collector array can’t produce all of the extra heat to keep up with the higher heat exchange your system will shut down. Using the numbers above if your solar panels are only producing 24,000 btus per hour then your higher flow rate system would only run 24/29.9 % of the hour. Either way this ratio holds.

Although the heat exchanger and collector will increase their performance by running at a higher flow rate there are a few reasons you don’t want to go overboard;

* Your flow rate should be constrained by the size of the pipe that you use.
* Too high a flow rate on the tank side can destroy stratification if used improperly.
* Higher initial cost to go with the larger pump.

Next time somebody tells you they run a lower flow to get better heat exchange just smile.