UFC 3-440-01
14 June 2002
4-4
TRANSPORT SUB-SYSTEM
4-4.1
Transport Sub-System Design. Although the collector array layout may
differ for each building, the design of the transport sub-system should be similar for all
solar energy systems.
4-4.1.1
Heat Transfer Fluid. As discussed in Chapter 3, a solution of 30 percent or
50 percent food-grade, uninhibited propylene glycol and distilled water is required as
the heat transfer fluid for closed-loop solar energy systems. Ethylene glycol is highly
toxic and should never be used.
4-4.1.2
Heat Exchanger
4.4.1.2.1
Heat Exchanger Analysis. Two methods of heat exchanger analysis are
used in design: the log mean temperature difference (LMTD) method and the
effectiveness-number of transfer units (e-NTU) method. The LMTD method is used
most often for conventional HVAC systems and requires knowledge of three of the four
inlet and outlet temperatures. This method cannot be applied directly to solar systems
because the inlet temperatures to the heat exchangers from both the collectors and
storage are not constant. Since the goal of the solar system heat exchanger is to
transfer as much energy as possible, regardless of inlet and outlet temperatures, the e-
NTU method should be used. However, a complete e-NTU analysis can be avoided by
considering the impact of the heat exchanger on the overall system performance. The
annual system solar fraction is decreased by less than 10 percent as heat exchanger
effectiveness is decreased from 1.0 to 0.3. By setting a minimum acceptable
effectiveness of 0.5, the e-NTU method can be used to generate the temperatures
required by the LMTD method. These temperatures and the corresponding flow rates
can then be used to size the heat exchanger according to the LMTD method, with the
resulting heat exchanger satisfying the minimum effectiveness of 0.5.
4.4.1.2.2
Sizing. For proprietary reasons, manufacturer's representatives, through
the use of computer codes, typically size heat exchangers. These codes are usually
based on the LMTD method and require the designer to provide three temperatures
and the flow rates of both streams. To ensure that an effectiveness greater than 0.5 is
achieved, the following temperatures and flow rates should be used for sizing the heat
exchanger:
Temperatures:
Solar loop inlet
= 140 degrees F (60 degrees C)
Solar loop exit
= 120 degrees F (49 degrees C) or less
Storage side inlet
= 100 degrees F (38 degrees C)
Flow rates:
Solar loop
= AFR (see Figure 4-2 legend)
Storage loop
= 1.25 x AFR
The 120 degrees F (49 degrees C) solar loop exit temperature corresponds to an
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