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Air-Water Heat Exchanger Lab

Air-Water Heat Exchanger Lab. In this lab, YOU will design, conduct, and analyze your experiment. The lab handout will not tell you exactly what to measure or calculate!

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Air-Water Heat Exchanger Lab

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  1. Air-Water Heat Exchanger Lab • In this lab, YOU will design, conduct, and analyze your experiment. The lab handout will not tell you exactly what to measure or calculate! • You will use an air-water heat exchanger testing unit to verify the energy balance and determine important heat exchanger parameters for a variety of CFM.

  2. Lab Schedule • Week 1: discuss design of experiments and uncertainty, sketch and become familiar with apparatus, design experiment, and sign up for a time to take data • Week 2: take data and begin analysis • Week 3: analyze data and write report; take additional data if needed

  3. Design of Experiments • “Design of experiments (DEX or DOE) is a systematic, rigorous approach to engineering problem-solving that applies principles and techniques at the data collection stage so as to ensure the generation of valid, defensible, and supportable engineering conclusions. • In addition, all of this is carried out under the constraint of a minimal expenditure of engineering runs, time, and money. “ http://www.itl.nist.gov/div898/handbook/pmd/section3/pmd31.htm

  4. Types of DOE • “There are 4 general engineering problem areas in which DOE may be applied: • Comparative • Screening/Characterizing • Modeling • Optimizing • Comparative: The engineer is interested in assessing whether a change in a single factor has in fact resulted in a change/improvement to the process as a whole. “ This is what we’ll be doing. http://www.itl.nist.gov/div898/handbook/pmd/section3/pmd31.htm

  5. DOE for this lab • DOE is a complex engineering field. We’ll use only a few of the most basic ideas here. • If multiple input parameters are varying at the same time, it’s very difficult to know what’s causing a change in an output. • In this lab, we’ll vary the air CFM but keep all other parameters constant. • Two data points do not prove a linear/parabolic/exponential relationship. • We typically cannot extrapolate results beyond the range for which we took data with any level of certainty.

  6. Significance of Results • In our old HX setup (used several years ago), the temperature of the water changed only about 2 or 3ºC from inlet to exit. With a temperature uncertainty of >2ºC, was there a significant temperature change? • 95% confidence interval: 95% of data will fall within +/- 1.96s of the mean for a Gaussian distribution • We typically say differences are significant if they are outside of the 95% confidence interval

  7. Simplified Uncertainty Analysis • Random (precision) error • For temperature measurements, this typically includes fluctuations in the electronics of the data acquisition units as well as fluctuations in the quantities measured • Bias (fixed) error • For temperature measurements, this typically includes the finite resolution of the A/D card (if one is used), the use of a curve fit for the thermocouples, reading of calibration thermometers, and conduction and radiation errors. • Total uncertainty is found using the root mean square (RMS) of these two errors

  8. Random Error • 95% confidence interval – 95% of temperature readings will fall in this range • =+/- 2 (or 1.96) standard deviations • For your lab, you could estimate this error by taking more than 30 data points (N=30+) for one temperature, mass flow rate, and CFM for one of your CFM. Then the average and standard deviation could be calculated using the equations below. • Excel can be used instead. • Unfortunately, you will have limited time available to take data for this lab, so you may ignore random error.

  9. Bias Error • For our lab, we will do a simplified analysis using manufacturers’ provided uncertainties. • Mass flow meter uncertainty: +/- 1% of reading • CFM uncertainty: +/- 5% of reading > 185 CFM. Below 185 CFM, the uncertainty may be greater. • Thermocouples: +/- 1.8ºF each (varies with type of thermocouple) • Thermocouple reader: 1ºC + 0.1% of reading • You may assume that the uncertainty of properties such as specific heat and density are negligible.

  10. Temperature Uncertainty • To find the error of one temperature reading, use the RMS of the thermocouple and thermocouple reader errors • To find the bias error in your temperature difference, use the RMS: • Divide by the DT to find the percent uncertainty in DT • You can pick an average DT – don’t do this for every different DT that you measure • Since the air and water DT’s are different, they have different percent uncertainties in DT as well.

  11. Propagating Errors • To find the percent uncertainty in Q, you must propagate the errors. • In ME 120, you will study uncertainty analysis in detail. For this class, all you need to know is this: • If A=B*C*D, then Where U is a percentage uncertainty in each component • For this lab, find the percent uncertainty in Qair and Qwater. Do this at an average CFM reading – you don’t need to do it for each CFM.

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