Design Specifications for Wet-Bulb Aspirator Apparatus

259 Pages·2010·3.42 MB·English

Design Specifications for Wet-Bulb Aspirator Apparatus

by

Jacob P. Brenner

A thesis submitted in partial fulfillment of

the requirements for the degree of

Master of Science

(Mechanical Engineering)

at the

UNIVERSITY OF WISCONSIN-MADISON

2010

This thesis has been approved by

____________________________________ _____________
Dr. Gregory F. Nellis, Associate Professor date

____________________________________ _____________
Dr. Douglas T. Reindl, Professor date

____________________________________ _____________
Dr. John M. Pfotenhauer, Professor date

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Abstract

Aspirated psychrometers are commonly used for the measurement of the wet-bulb
temperature. By knowing the wet-bulb temperature, dry-bulb temperature, and pressure, the
thermodynamic state of a moist air stream can be determined. ASHRAE Standards 41.6-
1994 (RA 2006) and 41.1-1986 (RA 1991) currently specify the detailed design guidelines
and considerations required to construct an aspirated psychrometer that is capable of
measuring the wet-bulb temperature to within ±0.1°C.

The aim of this project is to be able to specify the design guidelines and considerations
required to construct an aspirated psychrometer capable of measuring the wet-bulb
temperature to within ±0.05°C. This is done by means of an analytical model used to predict
the error in the measurement of the wet-bulb temperature over a range of conditions. Also to
validate the model an aspirated psychrometer is built in accordance with the model and tested
over a range of experimental test conditions. The analytical model and the experimental test
apparatus are described in detail in this document.
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Acknowledgements

First and foremost I would like to thank my advisors, Professor Greg Nellis and Professor
Doug Reindl for the hours spent reading the various portions of this document and for their
technical support and guidance throughout this project. I would like to thank Professor John
Pfotenhauer for his participation on my defense committee. I would also like to thank the
American Society of Heating, Refrigeration, and Air-Conditioning Engineers (ASHRAE) for
their financial support, and the members of the ASHRAE Project Monitoring Subcommittee
overseeing this research for their continual feed-back throughout the project. Special thanks
to Charles Wright and TSI Inc. for the donation of the velocity transducer to the project.

I also need to thank my family for their continuous support of me throughout college. I
would especially like to thank my dad Joe, mom Joan, brothers Jason and Joe, sisters-in-law
Holly and Leah, and brother Jamie. I truly would not have been able to make it through the
past five and half years of college without your continual support and encouragement.

I would also like to thank Nick Edwards for his assistance relating to wicking height, as
described in Chapter 5. Finally I would like to thank all of the Solar Energy Laboratory. All
of you have made my time here enjoyable.
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Executive Summary

A wet-bulb aspirator apparatus is one instrument that can be used to measure the wet-bulb
temperature of a moist air stream. The wet-bulb temperature is an important psychrometric
property and accurate measurements are essential for testing and rating of various types of
HVAC&R equipment.

The goal of this research project is to develop and specify a technique that can be used to
accurately measure the wet-bulb temperature with an aspirated psychrometer. Specifically,
the research will provide the basis for improvements in ASHRAE Standard 41.6-1994 (RA
2006), Standard Method for Measurement of Moist Air Properties. The current ASHRAE
Standard 41.6 defines the design guidelines to construct a wet-bulb aspirator apparatus
capable of measuring the wet-bulb temperature to within ±0.10°C. One objective of this
research is to outline design guidelines and considerations needed to improve the accuracy of
wet-bulb temperature measurements to achieve an accuracy of ±0.05°C. This executive
summary provides an overview of the theoretical and experimental work carried out to
accomplish the project goal. A more detailed discussion of the work can be found in the final
report.

Adiabatic Saturation, True Wet-Bulb, and Measured Wet-Bulb Temperature
To be able to accurately measure the wet-bulb temperature, it is important to first understand
the definition of wet-bulb temperature and the closely-related quantity, adiabatic saturation
temperature. The adiabatic saturation temperature is defined as the temperature obtained by
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an air-water vapor mixture if it becomes saturated with water vapor in an adiabatic process
(ASHRAE 41.1, 1991). The true wet-bulb temperature is determined by a balance between
heat and mass transfer, as described in Nellis and Klein (2009).

To measure the wet-bulb temperature, a moist air stream is forced across a temperature
sensor kept wetted by a moist cotton sock. As water from the sock evaporates, the sensor
cools. The cooling of the sensor below the ambient dry-bulb temperature leads to convective
heat gain from the air stream to the temperature sensor. The “true” wet-bulb temperature is
the equilibrium temperature obtained when the energy loss by evaporation balances with the
convective heat gain. An expression for the true wet-bulb temperature is given by:

h
T =T − D (c −c )Δh (0.1.1)
true,wb db h v,wb v,db vap

where T is the dry-bulb temperature, h is the average mass transfer coefficient, h is the
db D
average heat transfer coefficient, c and c are the concentrations of water vapor at the
v,wb v,db
wet-bulb temperature sensor and in the free stream air, respectively, and Δh is the latent
vap
heat of vaporization for water.

Unfortunately, there are other forms of heat transfer to the temperature sensor, which cause
the observed or “measured” wet-bulb temperature to differ from the “true” wet-bulb
temperature. These “parasitic” forms of heat transfer to the temperature sensor principally
include radiation and lead wire conduction. When the additional heat transfer associated