Page 27 - HVAC Pump Handbook

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Physical Data for HVAC System Design

24 The Basic Tools

40
As loads and flows vary, differential pressure variation across control
valves is even more pronounced than the differential pressure across
the whole building.
35
Building differential pressure (psi) 30

25

20

15
Time over one day (Summer 2003)
Figure 2.3 Typical building differential pressure profile. (Courtesy of Flow
Control Industries. Woodinville, WA.)

2.5 Glycol-Based Heat-Transfer Fluid (HTF)
Solutions
Glycol-based heat-transfer solutions are prevalent in the HVAC in-
dustry. Glycol is used to (1) avoid freezing in equipment of an
HVAC water system such as heating and cooling coils and (2) to
transfer heat to and from energy storage tanks that use ice. There
is a substantial variation in both viscosity and density of the solu-
tion as the percentage of glycol is varied with temperature.
Information is provided on ethylene glycol–based heat-transfer flu-
ids, since they have been the most used in the HVAC industry.
Special applications of glycol-based heat-transfer fluids, such as
contact with potable water or food, may require the use of propy-
lene glycol–based solutions.
Figure 2.4 provides information on the viscosity and Fig. 2.5 on the
specific gravity of ethylene glycol–based heat-transfer solutions. Care
should be taken to avoid using glycol solutions near their freezing
curves because slush occurs there that will change radically the
pump’s performance. Figure 2.6 provides the freezing curve for an eth-
ylene glycol–based heat-transfer fluid. Verification of the minimum

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