An investigation was undertaken into the expected loss in performance when a compact heat exchanger is run at a reduced capacity. The transfer conditions of several actual exchangers were measured, and the resulting performance U value and corresponding total required transfer area (both gauges of overall performance and efficiency) was determined for the exchanger by analytical textbook theory. The same properties were also determined by the ASPENTech software, although with reduced geometry to match the reduced capacity and thus potentially increased efficiency.
The analysis was performed on three different exchanger types, each found in Heating, Ventilation and Air Conditioning (HVAC) circuits. Through a collaboration with FLOTH Sustainable Design Consultants three distinct sizes of each unit were able to have their flow properties logged and recorded, with measurements made using the respective Building Management Systems (BMS) and through on site flow metering.
The three compact exchanger units analysed were Air Cooled Coil Bundles, found in Air Handling Units, Shell-and-Tube Exchangers, functioning as evaporators in Chillers and Plate Frame Exchangers, facilitating Condenser Water and Generator Jacket Water Cooling. An investigation into the function of each type of exchanger – and its corresponding place in a HVAC circuit is presented in Section 3.0 Heat Exchanger Types Considered and Section 4.0 Experimental HVAC Units.
In almost every case, the measured flow conditions were at least fifty percent below the particular units’ design conditions – and thus all the measurements were made at a significantly reduced capacity. It was observed that the change in temperature of the fluids in each unit also dropped, although the flow rates through each unit was typically less affected.
It was found that the drop in performance was most significant in the larger units, and less considerable as the units decrease in size, although still present. However, for the most part, there was not a significant reduction in total required transfer area when the flow conditions were used to design a smaller and ideally more efficient unit – considered to largely be a side effect of the measured flow rates still being at or near full capacity.