University of Minnesota
University of Minnesota: Department of Mechanical Engineering
http://www.me.umn.edu/

ME 4331: Heat Exchanger

Analysis of Heat Exchanger: Performance and Flow Rate Measurement

Objective:

Students will examine the heat transfer characteristics of a high-performance coaxial heat exchanger operated in both coflowing and counterflowing configurations. An essential element needed to document the heat exchanger performance will be the accurate evaluation of the fluid flow rates through the heat exchanger. This will be accomplished, in part, by metering the fluid using a variety of flow measuring devices, including: a turbine meter,a Coriolis meter and an oval gear meter. The relative energy content of the fluid streams will be evaluated using thermocouple probes located at the inlet and outlet ports of the heat exchanger and sampled using a data acquisition system.

Introduction

Heat exchangers are devices that allow heat transfer between two fluids at different temperatures. The hotter fluid will lose energy through heat transfer to the cooler fluid. An advantage of most heat exchanger configurations is that the two fluid streams remain physically separated, but maintain thermal contact to allow efficient energy transport, via heat transfer, between them. Heat exchangers are ubiquitous in practice, being found in automotive radiators, air conditioners, refrigerators, home/industrial heating systems, and numerous other applications requiring the energy exchange between fluids.

The heat exchanger found in our lab is constructed of coaxial tubes manufactured from cupronickol, a corrosive-resistant alloy which maintains some of the favorable thermal properties of copper. (The make-up of cupronickol is 90% copper and 10% nickel, having a thermal conductivity of 26 W/m·K). The working fluid we will examine is water, which will be configured to pass through the exchanger in either the coflow or the counterflow arrangement. The heat exchanger employs a special kind of inner tube to enhance the heat transfer between the two fluids. The tubes have a star-shaped cross-section, with the star shape spiraling along the length of the tube; a tube sample is available in the laboratory. This has the effect of creating a secondary (swirling) flow pattern and turbulence which, together, work to effectively mix the fluid and improve the thermal transport between the streams.

Analytical Background

Two common methods are used to examine the performance of heat exchangers; such measures of performance might include the determination of unknown fluid inlet or outlet temperatures as well as the evaluation of the overall heating rates between the two fluid streams. These methods are the Log Mean Temperature Difference (LMTD) method and the Effectiveness-Number of Transfer Units (ε-NTU) method. The LMTD method is normally used to determine heat transfer rates when both the inlet and outlet temperatures are known. The ε-NTU method method is a more useful tool to employ when the inlet temperatures of the heat exchanger and the heat exchanger design are known. These methods will be used in conjunction with the first law of thermodynamics to arrive at the desired performance parameters.

Experimental Facility

The experimental hardware used to evaluate heat exchanger performance consists of a "flow board" and the exchanger itself. The flow board, donated to the university by Rosemount Inc.(now a division of Emerson Pioccas Management), consists of modern flow metering devices. These include rotameters, a turbine meter, an oval gear meter and a Coriolis meter. The Coriolis meter is considered to be our laboratory "standard," which simply means that the bias errors associated with this device are small and are assumed to be known. The meters are connected in both series and parallel on a flow board which also includes a number of valves (both 2- and 3-way) and sight gages. Pressure ports on the board can be readily monitored with differential and absolute pressure gages mounted on the board. Stainless steel plumbing connects the meters to cold and hot water supplies which will be used to feed the heat exchanger.

Procedure

The experimental procedure is described by the following broad goals. Details of each of these steps should be discussed among your laboratory partners and with the teaching assistant. Remember that each member of the group is responsible for all components of the lab operations, whether or not they actually assist in carrying out a particular procedure.

  1. With the assistance of the TA, run the flow loop to obtain basic operating principles and to identify safety issues. It is particularly important that all members of the team understand the role they will assume during the data acquisition. (Limitations in the hot water supply demand that the experiments be run rather quickly. This means that you need to "debug" your data acquisition procedures before you run the hot water side of the heat exchanger.) Remember to record in your laboratory notebook the procedures you use during data acquisition. Also note that the names of your lab partners should appear frequently in your lab notebook, identifying who participated in what aspects of the procedures. (During multiple runs of an experiment, it is assumed that individuals will switch roles to increase everyone's exposure to the equipment.) Each lab group will be required to provide a careful schematic of the flow board, labeling components and pipe connections.
  2. Calibrate the turbine meter and the oval gear meter using the Coriolis meter as the laboratory standard. Members of the lab team who are not directly involved in calibration procedures should begin to process the calibration data and plot the results. In this way, errors in procedure or data can be identified early and return trips to the laboratory during the week can be eliminated. The calibrations should be carried out over the largest possible operating domain of each meter (constraints may be imposed by the flow loop itself).
  3. Develop a procedure for evaluating heat exchanger performance; this must be done for the heat exchanger operated in coflowing and counterflowing configurations. Developing a procedure means your group will need to discuss how the performance measures can be evaluated through experimental testing. A good procedure will indicate "who does what" during an actual experiment as well as give details of the data acquisition and hardware configurations. As discussed in the lecture, the procedure must allow you to accomplish the objectives of the experiment and must be recorded in sufficient detail that someone of a technical background, but not necessarily knowledgeable about heat exchangers, could reproduce your results.

    To evaluate the heat exchanger performance, fix the power input to the hot side of the heat exchanger. This fixes m ⋅ Cp(T-Tamb); in practice this is best accomplished by holding constant both the mass flow rate and inlet temperature of the hot fluid. Your selection of m ⋅ Cp(T-Tamb) should be based on discussions within your group and should incorporate knowledge of the available equipment and limitations of the hot water supply system: final recommendations must be approved by the TA prior to testing.

  4. From these basic measurements, a great deal can be learned about the heat exchanger performance.
    • Plot the heat transfer rate from the hot fluid to the cold fluid versus cold side mass flow rate. (Both coflow and counterflow data should be represented on the same plot to facilitate comparison. Be sure you know the directions of the flows in all cases.)
    • Evaluate (UA) experimentally for all test conditions. Plot versus cold side mass flow rate. Note, as discussed in lab: (UA) = q /Δ Tlm, where Δ Tlm is known if inlet & outlet temperatures are known.
    • Evaluate ε experimentally for all test conditions. Plot ε versus cold side mass flow rate. Note, as discussed in lab: ε = q / qmax, where qmax = (m ⋅ Cp)min·(Th,i - Tc,i)
    • Using (UA) and ε determined above, evaluate NTU experimentally. Note that: NTU = (UA)/(m ⋅ Cp)min
    • For one representative case carefully specify results as follows:

      (UA) = (UA)ave ± δ(UA) @ 95%

      ε = εave ± δε @ 95%

      NTU = NTUave ± δ NTU @ 95%

      (If you do this on a spread sheet it is very easy to extend the results to all your test cases.)