Vermont Energy Control Systems

Practical monitoring and control for the real world

Appendix B

Heat transfer in heat exchangers is a very complex topic. In this case, we did not have any specifications for the heat exchanger, so we can only model heat transfer one each side of the heat exchanger independently, and make reasonable assumptions about the performance of the heat exchanger itself.

Heat Transfer Equations

Water weighs 8.34 lbs/gal and 1 BTU will raise 1 lb of water 1℉. The equation for heat transfer is therefore:

BTU/hr = Gallons/Minute * Minutes/Hour * Pounds/Gallon * (T1-T2)
BTU/hr = Gallons/Minute * 60 * 8.34 * (T1-T2)

Solving for DeltaT (T1-T2):

(T1-T2) = BTU/hr / (Gallons/Minute * 60 * 8.34

Solving for flow rate:

Gallons/Minute = BTU/hr / ((T1-T2) * 60 * 8.34)

Greenhouse Zones

The greenhouse has four zones. BTU loads are from the design documents. Office and bench flow rates are actual measured values. Note that the bench zone can be supplied with water at two different temperatures and presents a different BTU load depending on supply temperature.

Zone Temp SupplyBTU Load Flow Rate
Office 120 5000 2gpm
Mech Room 120 5000 2gpm
Slab 120 110,000 9gpm
Benches 120 30,000 4gpm
Benches 150 70,000 4gpm

Using the equation for Delta T above, we can calculate the drop in temperature that we would expect from each zone, and the resulting return temperature:

Zone Supply Temp BTU Load Flow Rate Delta T Return Temp
Office 120 5000 2gpm 5 115
Mech Room 120 5000 2gpm 5 115
Slab 120 110,000 9gpm 24 96
Benches 120 30,000 4gpm 15 105
Benches 150 70,000 4gpm 35 115

Under peak load conditions, the greenhouse is supplied with hot Glycol at 120℉, and the return temperature will be about 100℉, with a total heat load of about 150,000 BTU/hr.

Supply (Hot Water) Side

The supply side of the heat exchanger is fed with hot water at about 170℉. In the original configuration, P15 was a fixed-speed circulator delivering about 10gpm. This was reasonable at peak load, but with smaller heat demands it resulted in 10gpm of very hot water being returned to the bottom of storage:

Condition Supply Temp BTU Load Flow Rate Delta T Return Temp
Office Only 170 5000 10gpm 1 169
Peak Load 170 150,000 10gpm 30 140

Predicted Values

The ideal flow rate under these conditions would be slow enough to minimize the return temperature to storage. If the heat exchanger has adequate surface area, we might be able to achieve a return temperature on the hot water side that's close (perhaps 10℉ above) to the return temperature on the Glycol side:

Condition Supply Temp BTU Load Glycol Return Temp Target Water Return Temp Flow Rate
Office Only 170 5000 115 125 0.2gpm
Slab & Bench 170 140,000 100 110 4gpm
Peak Load 170 150,000 100 110 6gpm

Observed Values

Logged data for office only and combined slab and bench zone operation shows good correlation with predicted values. Note that circulator speed is only available as a percentage of electrical drive signal, which does not have a linear relationship with actual flow rate. At this time, peak load conditions have not been observed.

Condition Supply Temp BTU Load Glycol Return Temp Water Return Temp Flow Rate
Office Only 180 5000 100 110 5%
Slab & Bench 180 140,000 100 110 30%
Peak Load 150,000