ASHRAE Journal - November 2021 - 23

COLUMN ENGINEER'S NOTEBOOK
FIGURE 1 Uninsulated steam bayonet (left) and insulated bayonet (right).
For steam coils some controversy exists about where
to locate vacuum breakers to allow condensate to drain.
We believe they belong on the inlet of IFB (horizontal)
and VIFB (vertical) coils and on the outlet of distributing
coils.
Tied for second hardest to freeze are opposite-ended
steam distributing coils with top inlet and bottom outlet ,
although some of our staff has had trouble with control
of distributing coils.
Also tied for second are glycol coils which, depending
on the concentration of glycol used , are tough to freeze
or burst. In all cases pumping power and heat transfer
effi ciency will be worse than for water. Properties of glycol
solutions will be discussed in a future column.
Hot water coils with circulating pumps are effective
against stratifi cation freezes, which in our experience
are 90% of coil freezes. The circulating fl ow brings water
that was in a section of the coil that is in contact with
subfreezing air back into areas that are in contact with
warm (return) air. This also helps make the air temperatures
reaching downstream coils more uniform. But
recirculation will not protect a coil if the heating control
valve does not open or if the main heating system pump
quits when the entering air temperature is more than a
few degrees below freezing.
The only source of heat is the pump energy and a little
heat transfer from pipes outside of the AHU, while the
full heat transfer of outdoor air is cooling the coil fl uid.
Some engineers believe that recirculation will protect
coils in this situation, but fl owing water can freeze.
Another option is to add air blenders in addition to or
instead of recirculating coils. This works if the blender
you choose blends well and if you pick a good geometry.
When using multiple blenders, they should be placed
perpendicular to the outdoor air/return air (OA/RA) orientation.
If the OA comes in on the left and the RA on the
right and you have two side-by-side blenders, the result
is well-blended freezing air and well-blended warm air
crossing the downstream coil. Orient the blenders so the
OA and RA are blended. Mixing OA and RA far upstream
of the AHU is usually effective. AHU mixing box dampers
are usually not very effective.1
Energy recovery units also offer protection to coils in
addition to saving energy and reducing CO2 emissions.
If chilled water does not run year-round, an energy
recovery chiller can be an excellent solution for small
loads in cold weather. In one example, we used this
technology to heat a football practice fi eld while providing
chilled water to campus chilled water mains to serve
winter cooling loads.
Another source of unwanted heat that can cause economizer
dampers to open is heat from humidifi er tubes,
where the issue is not the heat of the steam, which
moves nearly vertically on a psychrometric chart (a hair
to the right), but the sensible heat transfer of the tubes
(Figure 1). ASHRAE Standard 90.1 now requires insulation
of humidifi er bayonets.
Piping Freeze Concerns
It is common to freeze pipes in boiler rooms with large
boilers that pull combustion air through a large combustion
air louver and then the boiler room. To address this,
we usually use a vertical unit heater near the combustion
air louver and set the fan to stay on whenever the
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ASHRAE Journal - November 2021

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