The Modeling of a Natural Convection Heat Exchanger in a Solar Domestic Hot Water System

File(s)
Date
1994Author
Avina, John M.
Publisher
University of Wisconsin-Madison
Metadata
Show full item recordAbstract
The use of simulations can greatly aid in optimizing the design of natural convection heat exchangers
(NCHEs) in solar domestic hot water (SDHW) systems. Fraser et al. (1992) presented a NCHE
model that is used in the WATSUN solar simulation program, that requires experimental
measurements of the heat exchanger thermal performance and shear pressure losses. Two
TRNSYS models are here presented for a NCHE in a SDHW loop. The simple model, based on
Fraser et al.'s work, requires experimental testing on the particular heat exchanger. The simple
model can be used for optimizing SDHW system parameters (i.e. pipe lengths and diameters,
collector areas, tank volume etc.) excluding the NCHE itself which is represented by the
experimental curves. A detailed model, based upon cross flow correlations, requires geometric
specifications of the NCHE being simulated and is applicable to shell and coil and counterflow
configurations. By varying heat exchanger geometric parameters (such as the number of helices,
diameters of helices, diameter and length of the heat exchanger shell) the detailed model can be used
to design an optimum NCHE. Results comparing the detailed model with Fraser et al.'s
experiments show reasonable agreement. Using the detailed model and the least cost savings
economic analysis, simulations were performed to discover the optimal shell and coil NCHE
geometry. It was found that considerably reducing the heat exchanger size led to enhanced
economic performance over a 10 year period of economic analysis. Coil spacing and tube diameter
had a lesser impact upon system performance than heat exchanger shell length and number of
helices. Thermo Dynamics Inc. manufactures a shell and coil NCHE that contains 4 coils and is
0.635 m. The optimal heat exchanger design contains 2 helices and is 0.45 m long. For a given set
of system parameters, a SDHW system containing the optimally designed heat exchanger would
save the consumer an extra $110 in initial equipment cost, and $52 over a 10 year period. Heat
exchanger designs were subject to variations in system parameters, such as collector area, hot water
draw, location and glycol flow rate. Although each set of system parameters suggested a different
optimal design, overall, the optimal design found for the initial set of system parameters remained
adequate. As different economic assumptions will lead to differing optimal heat exchanger lengths,
this work can serve as a guide for those who desire to optimize a shell and coil NCHE based upon
a prevailing set of economic assumptions.
Subject
University of Wisconsin--Madison. College of Engineering.
Dissertations Academic Mechanical Engineering.
Thesis (M.S.)--University of Wisconsin--Madison, 1994.
Permanent Link
http://digital.library.wisc.edu/1793/7841Description
Under the supervision of Professors William Beckman and Sanford Klein; 255pp.
Citation
Avina, J. (1994). The Modeling of a Natural Convection Heat Exchanger in a Solar Domestic Hot Water System. Master's Thesis, University of Wisconsin-Madison.