Case study: Ground source heat pump system upgrade
Recommended specifications of a heat exchanger
Most engineers determine flow and capacity of the heat exchanger based on water. Generally speaking, the heat exchanger’s flow and capacity with antifreeze fluid is not the same as water, and the selection shows different operating values because an engineer does not consider the effects of the glycol solution. In this case the flow should be adjusted by approximately 16% to compensate for the effects when you are using a 50:50 ratio of glycol: water at 100 F.
Keep in mind that when you select heat exchangers for winter and summer duties, select the units for both seasons with the same plate corrugation and avoid selecting mixed plates due to the effect on the pressure drop.
Often, engineers mistakenly select the largest area of the two seasons and the heat exchanger works only for one season and not the other because the plate corrugation type selected (NTU) and pressure drop were not satisfactorily chosen.
Engineers should be concerned about the debris in the groundwater that could travel to the plate heat exchanger. Most manufactures offers two options for avoiding this problem:
- Specify a port strainer on the plate heater. This reduces the possibility of plugging by preventing unwanted solid particles from entering the channel of the plate’s package.
- An automatic back flush system (ABS), commonly used in the Northeast, is the most expensive solution. The ABS automatically cleans the plates and frame heat exchanger without interrupting normal equipment operation.
The ABS consists of a four-way reversing valve that fits into the supply and return piping, allowing the reversal of the water flow direction in the heat exchanger. This flow reversal has been found to significantly reduce the fouling in the plate and frame heat exchanger. The flow reversal is controlled by a control panel mounted to the plate and frame heat exchanger or at a separate location as desired by the customer.
Plate materials of the heat exchanger are regulated by ASME BPV Code, Section VIII Division I: Design and Fabrication of Pressure Vessels. Usually they are built on stainless steel or titanium plates, determined by the groundwater. In most cases, stainless steel plates are specified in ground source water. If the water has a high concentration of chloride, titanium plates should be use to avoid corrosion problems in the future.
Ground source well water combined with the plate and frame heat exchanger give the owner and designer a more efficient heat source and heat sink because the plate and frame heat exchanger can maintain a small variation of the water temperatures between the ground loop and the building loop with a minimal heat transfer loss.
At Cold Spring Harbor Laboratories, the proper selection, optimization, and specification of the heat exchanger should be suitable to perform with groundwater temperature variation all year long to maximize the energy savings of the buildings.
More importantly, plate and frame heat exchangers are the most widely used for ground source water systems because they are easy to maintain, flexible because plates can be added in the future, and compact for space saving in a mechanical room.
Jose R. Rodriguez is a technical services engineer with Wallace Eannace Assocs. where he has more than 15 years of experience in HVAC and plumbing. Mike Sammut, partner at AKF Group, has more than 30 years of experience in HVAC. Arslan Asovic is project engineer at AKF Group and has more than 10 years of experience in HVAC.
- Rodriguez, J.R., “Demystifying the Specification of the Heat Exchanger for Special Plumbing Application” Plumbing Engineer, February 2013
- ASHRAE, 2011 ASHRAE Handbook – HVAC Applications
- New York Construction Magazine, “Drilling for Efficiency, Columbia University Project Goes Underground for Renewable Energy,” June 2009
- ASHRAE, 2012 ASHRAE Handbook – HVAC Systems and Equipment
- Breese J., “Predicting the Performance of Heat Exchangers,” HPAC Engineering, Jan. 2005
- ITT Bell & Gossett, Basic Heat Transfer Seminar, September 2003