Page 209 - Boiler_Operators_Handbook,_Second_Edition

194 Boiler Operator’s Handbook

comfort. Many use the thermostat control to start and
stop the fans while others will regulate the flow of the
fluids through the coils. Ventilation is normally achieved
by a small damper connecting to the outside so the fans
will draw air in when they are operating. Refrigerant or
chilled water coils will remove moisture from the room
air by condensing it and the drain is typically run out
the building wall but can be piped indoors. Convectors
can be supplied with or without filters so filter replace-
ment or regular cleaning of the unit is required. Convec-
tors can also be two-pipe or four-pipe. Two pipe con-
vectors use the same coil for heating and cooling, using
heated or chilled water supplied by the central system.
These require the operator monitor the weather, decide
whether the building is to be heated or cooled and set up
the system to deliver water at the required temperature. Figure 5-49. Pitot tube application
Modern convectors sense the temperature of the water
supplied and automatically switch the thermostat re- mined by a formula equal to 1.3 multiplied by the width
sponse to control the flow of the water or the operation times depth of the square duct multiplied and raised to
of the fans. Four-pipe convectors have separate cooling the 0.425 power divided by the sum of width and depth
and heating coils and can heat or cool independent of raised to the 0.25 power. Width and depth are simply
the weather. the two horizontal dimensions for vertical ducts. You
should also be able to find a table for equivalent round
Achieving Comfort for Occupants duct sizes in one of the books available from the manu-
If someone is not comfortable you can count on re- facturers of your air conditioning equipment. Test ports
peated complaints and, in some cases, direct orders to should be on two quarter points of round ducts and at
do something about it. Unlike when they’re home, the least four on one side of a square duct. The more read-
occupants are seldom able to walkover and adjust the ings you take, the more accurate your calculated result.
thermostat because it’s seldom that the entire occupancy You connect the manometer to both ports on the pitot
is satisfied so the thermostat is locked up or concealed tube so you’re getting velocity pressure, the difference
because some people produce excessive adjustments between total pressure and static pressure. Velocity (in
that result in additional heating or cooling expense. The feet per minute) at each measurement is determined by
first step is determining what the space conditions are multiplying the square root of the velocity pressure (in
and that involves using a sling psychrometer (Figure inches of water column) by 4006. (I use 4006 because
5-50), a flow hood (Figure 5-47), a manometer set at a there are competing groups that call it 4005 and 4007)
ten to one slope (Figure 2-3), pitot tube (Figure 5-49) an With round ducts calculate the area of quarter points
infrared thermometer (Figure 5-51) and a scientific cal- sampled in square feet and multiply by the velocity for
culator. I mention these early on because you might just the flow in that quarter for small ducts. For larger round
be assigned to operate systems in a new installation and ducts determine the area in segments and multiply by
discover some of this equipment left behind by the in- the calculated velocity in that segment. For square ducts
staller or TAB (temperature and air balance) personnel. it’s much easier to simply select equal rectangles (that
If so, you’ll recognize it and put it away for your own sometimes come out to be squares) centered on each
future use. Much of the equipment and tools that still measured point, and multiply that area by the measured
clutter the trunk of my car came from that source. velocity.
A combination of the manometer and pitot tube Even when the air in a space is within the comfort
is used to determine air flow in a run of ductwork. It’s zone there may be other factors that affect the comfort
great for checking for leakage in a long duct run. For of the occupants. These can be attributed to solar insola-
best results the rest ports should be located twenty di- tion, warm ceilings, cold floors, warm or cold walls and
ameters downstream and four diameters upstream of drafts; any one of which can result in more than 50%
any change in the duct (elbows, transitions, etc.). Oh, for of the occupants being uncomfortable. Now you know
square ducts use an equivalent round diameter deter- why you need those instruments, you need to identify