ASHRAE Journal - January 2014 - 60

COLUMN HVAC APPLICATIONS
With low loads being the norm, rather than the excep-
tion, minimum "capacity" is becoming a common problem.
When water flow becomes too slow in a coil tube, it
may become "laminar" and will make heat or cool difficult
to control. This does not mean it will not transfer
heat or cool, but rather we are unable to predict by how
much! Proportional water valves in conjunction with
modern thermostats work well to control space temperatures,
adjusting water flow to meet demand, even when
flow may become non-turbulent.
With non-condensing (sensible) cooling water coils
appearing on both fan terminals and chilled ceiling
induction units ("beams"), it is important that water coil
entering temperatures stay above the local dew point to
avoid condensation. Some building codes were recently
modified to allow installation without a drain pan or to
allow a catch pan on sensible coils, rather than a drain
pan. It is always a good idea to place a condensation sensor
on the inlet of a few sensible coils to alarm or close
water valves if the water temperature falls below ambient
dew points.
Common Water Coil Issues
Mixed Air Temperature. Mixed air temperature or
unit leaving air temperature (the combination of primary
and induced air temperatures) is dependent on the
location of the coil. It is calculated differently for series
or parallel fan powered VAV units:
Series Fan Units. For series fan units, heating coils are
typically located on the discharge, where coil entering air
temperatures are based on the minimum primary and
the induced airflow. The induced airflow is the fan cfm
less the minimum primary. With series flow units, the
heating coil leaving air temperature and the box leaving
air are the same. If a sensible cooling coil is used on the
induction inlet, it must be included in the calculation of
leaving air from the unit. If an ECM motor is provided,
both coils, VAV (outside air) and induced air, may be varied
by the on-board processor to allow for a wide range
of operating conditions.
Locating the heating coil on the induction inlet is a
possibility with an ECM fan motor, as it is able to deliver
the same amount of air when the unit is essentially pressurized
from the VAV inlet or when air is drawn through
the heating coil. Preliminary data suggests that an ECM
motor uses 40% more energy pushing (or pulling) air
through a multiple row coil, compared to delivering
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A SHR A E J O U RNA L
ashrae.org JAN UARY 2014
100% primary air with no downstream (or with an electric
heating coil downstream). With lowered water temperatures
requiring three- and four-row coils, this could
be a significant energy saving strategy.
Parallel Fan Units. Most parallel units have the water
coil on the discharge (despite the obvious pressure penalty
seen by the primary air handler); while some designs
allow locating them on the plenum inlet. This latter location
makes the coil entering air temperature the same
as the plenum air temperature. A parallel fan powered
unit's discharge air temperature is a mix of primary air,
typically at the minimum primary air flow rate, and plenum
air at the fan airflow rate. This is the entering air
temperature for a downstream coil. If the coil is on the
induction port inlet, it is recommended that the heating
coil leaving air temperature always stay below the
maximum recommended for the motor, which is in the
airstream.
Leaving Water Temperature. Air handler cooling coils
are often selected on the basis of water coilDT. This technique,
however, is not recommended for heating coils in
VAV boxes or fan coils, non-condensing boilers require a
high entering water (return water) temperature to avoid
"shocking" the system. Ensuring a high return water
temperature is best achieved through three-way valves,
secondary loops or other means, not through coil selections.
At part load, when the water valve is throttled, coil
leaving water temperatures will always be less than recommended
by these boilers.
Fluid Type. Three hot fluids are commonly used in
coil applications: water, ethylene and propylene glycol.
The use of glycol will significantly increase the minimum
gpm required for turbulent flow and also reduces
the specific heat of the fluid. Most programs compute
a Reynolds number to determine the amount of turbulence
in the fluid in the tubes, which is usually desired
to be greater than 5,000. As the percentage of glycol
increases, so too will the minimum gpm. Ethylene glycol
is falling out of favor for environmental reasons, and
propylene has a greater effect on minimum flow than
ethylene.
Coil Load vs. Room Load. A water coil is often selected
based on a given Btu/h. There are two loads that can be
used for this calculation. One is the coil load, which is
based on the airflow rate and the supply to discharge
temperature differential. The other is the room load,
which is the difference between the room and the unit
ASHRAE Journal - January 2014

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