Page 251 - HVAC Pump Handbook

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Configuring an HVAC Water System

248 The HVAC World

system Btu/h
Hot water gal/min (9.1)
500 temperature difference

system load (tons) 24
Chilled water gal/min (9.2)
temperature difference
These are general formulas based on a specific gravity of 1.0 for
water. If it is desired to secure more exact water flows for hot water,
then specific gravity and specific heat of water at the average temper-
ature of the system should be considered; Eq. 2.4 should be used.
Computer programs for load calculations should insert the specific
gravity automatically for the water temperature at all parts of the
water system.
The actual temperature difference that is selected for a specific
installation is determined by the cost of the coils for various tempera-
ture differences and the effect that higher differences may have on
the operating cost of the chiller or boiler. Reducing the leaving water
temperature of a chiller increases its energy consumption and may
offset the savings of higher temperature differences; likewise, elevat-
ing the leaving water temperature of a boiler decreases its efficiency
and can have the same effect on a hot water system. These are the
decisions that must be made by the designer for each application.

9.3 Modeling a Water System for System
Head and Area

The first task confronting the designer of a water system is to com-
pute the water flow and pump head required by that system after the
temperature differential is determined. It can be a daunting task to
compute these values by hand from minimum to maximum load on
the system.
As described in Chap. 1, the advent of high-speed computers and
special software has eliminated the drudgery of such water system
analysis. This enables the engineer to evaluate a system rapidly and
determine system flows and head losses under variable loads on the
water system. The designer can evaluate the building under various
load conditions, develop a better understanding of the energy con-
sumptions of the mechanical systems, and achieve an accurate esti-
mate of the diversity of the heating, ventilating, and cooling loads.
The significant fact that has been found about building loads is that
very few multiple-load chilled or hot water systems are ever uniformly
loaded. Analysis of many systems has demonstrated a decided varia-
tion in the water loading on the heating or cooling coils. Some coils on
a system will be fully loaded, while others will have very little load on
them. This fact must be taken into consideration during the system

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