MIL-HDBK-1003/13A

[tau] is transmissivity of cover glazes, [alpha] is absorptivity of collector

plate, UL is overall collector heat loss coefficient. The y intercept

(FR([tau][alpha]n) and the slope (FRUL) can be read directly from

Table 2-6 for a wide variety of collectors. The negative sign of the slope

is ignored and the absolute value used. The y intercept is called efficiency

intercept in Table 2-6. If a particular collector is not listed in Table

2-6, choose one with similar physical construction and use its value of slope

and intercept. If necessary, slope and intercept can be calculated for a

collector shown in Duffie and Beckman (1974), but this is not recommended.

If a manufacturer's brochure is used for obtaining test data, the slope will

be a constant if the [eta] versus [delta]T/I curve is a straight line;

however, if it is not a straight line, the slope to be used is the tangent to

the curve in the expected range of [delta]T/I. The units of FRUL must be

Btu/hr-ft2 deg. F for use on subsequent worksheets.

.

The term (mCp)c/Ac is the unit . heat capacity flowrate of working

fluid through the collector, where m is flow of working fluid through the

collector in lbm/hr, and Cp is specific heat of fluid in Btu/lbm deg. F.

The larger the flowrate, the lower will be the [delta]T through collector and

thus the higher . will be the collector efficiency. A practical limit

is reached at (mCp)c/Ac = 10 Btu/hr-ft2 deg. F, so the design

procedure is based on values of this order of magnitude. The latter figure

may be taken as constant in calculating subsequent parameters.

The value for [epsilon]c, effectiveness of the collector - tank heat

exchanger, is based on manufacturer's data for the conditions of flow through

the heat exchanger. If no heat exchanger is employed between the collector

and the tank, then this term equals 1.0. For most heat exchangers, the

effectiveness will vary from about 0.5 to 0.8

This is a relatively complex

term to calculate and a procedure is given in U.S. Dept HUD (1977), pages

A-34 to A-39. The simpler procedure is to ask a manufacturer for design

information. Most manufacturers will provide this factor or calculate it.

Be prepared to supply the following information:

a.

The physical characteristics of the two fluids in the heat

exchanger.

b.

The amount of heat to be transferred (Btu/hr).

c.

The flow rates (gal/min) on both sides of the heat exchanger.

d.

The approach temperature difference defined as the difference

between the temperatures of the hot fluid entering the heat

exchanger and the heated fluid leaving the heat exchanger.

Alternatively, one could assume an effectiveness that is reasonable (say

0.7), then complete the following worksheets. If the design appears

feasible, one could then go back and "refine" his estimate by consulting a

manufacturer to be sure that a heat exchanger is available that will provide

the assumed effectiveness. Such a heat exchanger might be employed if the

working fluid were expensive, to reduce the amount of fluid required, or if

it were desired to separate working fluid from potable water in a hot water

storage tank (see Section 2.1.7 and Figure 2-5).

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