ASHRAE Journal - March 2014 - 52

Column EnginEEr's notEbook

chillers whose kW/ton was 0.88 (because 0.88 kW/ton
× 3412 Btu/h = 3,000 Btu/h·ton, plus the chilled water
contribution of 12,000 Btu/h·ton, to arrive at 15,000
Btu/h·ton). If you are still using 1 ton = 15,000 Btu/h as
a rule-of-thumb for condenser water, you are, in effect,
still saying the chiller efficiency is 0.88 kW/ton.
The condenser water pump itself adds some heat to the
condenser water as kinetic energy imparted to the water
by the pump is converted to heat energy via friction in the
piping and losses at the pump itself. To account for the
heat imparted to the condenser water by the condenser
pump(s), you may conservatively add the peak design
brake horsepower of the pump(s), converted to Btu/h·ton.
Again, don't use rules-of-thumb because condenser water
pumping energy varies greatly, depending on pipe sizes,
number of fittings and other restrictions, and whether the
cooling towers are immediately adjacent to the chillers or
located a great distance away. For this example, imagine
a pump with a brake horsepower of 35.4 on a 600-ton
system. It adds 35.4 hp × 2545 Btu/h·hp or 90,093 Btu/h
divided by 600 tons for a result of 150 Btu/h·ton.
Continuing our example, to find the real energy rates

for condenser water, use 12,000 + 1,774 + 150 or 13,924
Btu/h per ton in place of 15,000 Btu/h·ton. (This is true
only for a 0.52 kW/ton chiller and a specific condenser
water pump. Use the figures for your system, and convert to Btu/h.) With a 95°F/85°F condenser water loop,
the true flow rate is 2.78 gpm/ton. Or, if you like to
think in terms of 3 gpm per ton, then your leaving and
entering condenser water temperatures are going to
be 94.3°F and 85.0°F, respectively. Similarly, using the
same chiller with 100°F/85°F condenser water, the true
flow rate is 1.85 gpm/ton. Or, if you prefer 2 gpm per ton,
then your leaving and entering condenser water temperatures are going to be 98.9°F and 85.0°F, respectively.

Review of the Math
1. By definition, a Btu (British thermal unit) is the
amount of energy required to raise the temperature of
one pound of water by 1 degree Fahrenheit. That is how
a Btu is defined. The specific heat of water is 1 Btu per
pound per °F (technically, this is only exact at 60°F, but
the inaccuracy at other temperatures for HVAC purposes
is trivial); i.e., 1 lb of water carries 1 Btu of heat for each
1°F of temperature change.
2. Water weighs 8.34 pounds per gallon, so one gallon
of water carries 8.34 Btu of heat for each 1°F of temperature change.
3. By definition, a "ton" is 12,000 Btu/h (its name
comes from the amount of energy required to freeze
2,000 lbs of water into ice). 12,000 Btu/h is the same as
200 Btu/min.
4. One gallon of water per minute (gpm) carries 0.0417
tons of heat (8.34 divided by 200) for each 1°F of temperature change. (Said inversely, 24 gpm carries one ton
of heat per each °F of ΔT).
5. So the flow rate (in gpm/ton) is 24 divided by the °F
ΔT. That is why a chilled water ΔT of 12°F means 2 gpm/
ton. A chilled water ΔT of 10°F means 2.4 gpm/ton. A
chilled water ΔT of 16°F means 1.5 gpm/ton.
6. But for condenser water, you must go back to Step
3 and use the correct factor for heat per ton, which will
be greater than 12,000 Btu/h, but hopefully not a simple
default of 15,000 Btu/h. Continuing with the same
example used throughout this column, I will use 13,924
Stephen W. Duda, P.E., is assistant director of mechanical engineering at Ross &
Baruzzini, Inc. in St. Louis. He is an ASHRAE certified Building Energy Assessment
Professional, High-Performance Building Design Professional, and Healthcare Facility
Design Professional.

www.info.hotims.com/49802-54
52

ASHRAE JouRnAl

ashrae.org

March 2014

ASHRAE Journal - March 2014

Table of Contents for the Digital Edition of ASHRAE Journal - March 2014