General Overview
A liquid-to-liquid heat exchanger transfers heat between two liquids using a temperature difference without direct contact between them.
In a building, this type of heat exchanger can be installed in condensate cooling, vent condensing, boiler blowdown, and waterside economizer (free cooling), and in refrigeration applications such as evaporators and condensers. Typical liquid-to-liquid heat exchangers are plate-and-frame heat exchangers and tube heat exchangers.
Table 1 shows the plant and system configurations that may contain a liquid-to-liquid heat exchanger.
| Plant | System | Component | Controlling Variable |
|---|---|---|---|
| Water-cooled Chilled Water Plant | Waterside Economizer | Liquid-to-liquid heat exchanger | Outdoor air temperature (F) |
| Steam Plant |
|
Liquid-to-liquid heat exchanger | Blowdown water temperature (F) |
| Service Hot Water Plant | Service Hot Water Tank | Liquid-to-liquid heat exchanger | Schedule and occupancy |
Evaluation of Heat Transfer
In a liquid-to-liquid heat exchanger, heat is transferred from a liquid with excess heat (waste heat stream) to a liquid where the recovered heat benefits the process (supply stream). This is the principal energy phenomenon that reduces the overall energy usage of the entire plant or system where the heat exchanger is installed. A pump and motor may be required to circulate the liquids in the waste heat and supply streams.
Table 2 provides a summary of measurements needed to quantify the annual energy transfer and operating characteristics of a liquid-to-liquid heat exchanger.
| Component Quantification | Values to be Quantified | Measurement |
|---|---|---|
| Heat transferred by the heat exchanger to the supply stream |
Average hourly Btu/h transferred |
|
| Heat recovery system electricity consumption (if applicable), non-weather dependent system |
|
Hourly true RMS power (kWh) |
| Heat recovery system electricity consumption (if applicable), weather dependent system |
|
Measurement Strategy
The measurement strategy for a liquid-to-liquid heat exchanger involves measuring the supply stream flow through the heat exchanger and the temperatures at the supply stream inlet and outlet. If a pump is used in the system, the flow rate can be measured at the supply stream pump. Measurement locations are generically represented in Figure 1.
Measurement Equipment
Table 3 provides the equipment required to carry out the measurements of this component.
| Equipment | Description | Measurement (Units) |
|---|---|---|
|
Designed for systems engineers to quickly troubleshoot problems and verify performance during system commissioning and diagnostics. Allows measurement of flows throughout the plumbing infrastructure without intrusion. | Water Flow Rate (GPM) |
DENT 16” RoCoil Flexible Rope Current Transformers (CT-R16-A4-U) |
Provides a measurement of true RMS power from voltage and current inputs and records long-term power (kW) and energy (kWh) measurements. Requires ELOG19 software and a USB connection cable for programming and downloading data files. | True RMS Power (kW) |
|
An analog logger that supports up to four external sensors allowing you to measure temperature, current, voltage, air flow, pressure and more in one single logger. HOBOware Pro or HOBOware free software is required for logger operation. | Pipe Surface Water Temperature |
Calculation Methodology
The general methodology for quantifying the useful energy supplied by a liquid-to-liquid heat exchanger is determined by the temperature differential and flow rate of the supply stream. These values are multiplied by the heat capacity and density of the liquid (e.g., water, water-glycol mix) to find the energy flow rate. The energy flow rate can be regressed against a controlling variable (such as outdoor air, pump runtime or flow rate) to develop a regression model. Depending on operational variability, daily or weekly models may be developed to better characterize the component.
Click the button below to go to the calculators for this component.
Further Reading
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Apogee Interactive (2022). “Free Cooling.” Commercial Library. https://c03.apogee.net/mvc/home/hes/land/el?utilityname=union-power&spc=cel&id=1094; accessed February 4, 2021.
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ASHRAE (2019). ASHRAE Handbook: HVAC Applications. Chapter 48. DESIGN AND APPLICATION OF CONTROLS. I-P Edition.
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ASHRAE (2020). ASHRAE Handbook: HVAC Systems and Equipment. Chapter 40. COOLING TOWERS. I-P Edition.
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Carrier (2016). “How to Model a Waterside Economizer Application.” Carrier Engineering Newsletter, Vol. 4, Issue 1.
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Gordon, J.M.; Ng, K.C. (2001). “Cool Thermodynamics: The Engineering and Physics of Predictive, Diagnostic and Optimization Methods for Cooling Systems,” Cambridge: Cambridge International Science Pub; pp. 159-177.
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Trane (2008). “’Free’ Cooling Using Water Economizers.” Engineers Newsletter, Vol. 37-3. Also available at https://www.trane.com/Commercial/Uploads/PDF/11598/ News-%20Free%20Cooling%20using%20Water%20Economizers.pdf; accessed February 4, 2021.
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Trane (2010). CDS-PRM001-EN. TRACE 700 User’s Manual-Building Energy and Economic Analysis, Version 6.2; pp. 43-49. Also available at https://tranecds.custhelp.com/ci/fattach/get/55941/0/filename/FreeCooling%5B1%5D.pdf; accessed June 17, 2022.
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Taylor, S (2014). “How to Design & Control Waterside Economizers,” ASHRAE Journal, Vol. 56, No 6. American Society of Heating, Refrigerating and Air Conditioning Engineers; pp. 30-36.
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Trane (2016). “Waterside Economizers - Keeping the ‘Free’ In Free-Cooling.” Engineers Newsletter, Vol. 45-2. Also available at https://www.trane.com/content/dam/Trane/Commercial/global/products-systems/education-training/engineers-newsletters/waterside-design/ADM-APN058-EN_06012016.pdf; accessed February 4, 2021.
