Page 205 - Boiler_Operators_Handbook,_Second_Edition

190 Boiler Operator’s Handbook

Ductwork
Every system other than window units and the
thru-the-wall units at hotels and motels are fitted with
ductwork to convey the conditioned supply air from the
apparatus to the air conditioned spaces, return air from
the conditioned spaces, and supply outside air to the
inlet of the mixing box on the apparatus. As a general
rule air leaks in ductwork were typically in the range of
20% to 30%. Jointing methods for sheet metal duct were
developed for low cost installation and, except where
specified otherwise, subject to sloppy installation that
produced lots of leaks. Even insulated ducts had heavy
leakage; I can recall looking into a ceiling space to see
the insulation ballooning around the ducts because of
the air leaking out of the ducts and pieces of duct tape
flapping around in the breeze where the air came out
of the insulation. Newer standards have reduced the
Figure 5-46. Duct velocity and regain calculation
amount of duct leakage but don’t forget that it’s signifi-
cant in anything designed or built before 1973 when we
had the first so-called energy crisis. area can be empty with a large number of people gath-
Ductwork is designed one of two ways, constant ered in one part of the space to produce a concentrated
friction or static regain. The first is simply choosing duct load in one section. To handle situations where that can
sizes so the pressure drop is a constant (0.1 inches W.C. occur spaces served by a common piece of apparatus
per 100 feet of duct) so the required discharge pressure can be fitted with reheat coils that warm the air enter-
at the supply air fan could be calculated readily by mul- ing spaces where the cooling load is lower than normal
tiplying the length of duct (in feet) to the furthest reg- (no people, sometimes no lights). Conference rooms and
ister with corrections for elbows and other fittings by similar locations may also have time controls or occu-
0.001). Static regain design allowed for determination pancy sensors that actually reset the thermostatic con-
of the recovery of static pressure as the air velocity de- trol to allow the room to be cooler, or warmer, unless in
creases providing additional force to convey the air to use or scheduled to be in use.
that furthest register. Regardless of the design the oper- Reheat coils are typically thermostatically con-
ator only has to know that a boost in static pressure will trolled for the space in which they are located (which
occur when the air slows down, provided that it slows can be part of a number of spaces served by the same ap-
down in a manner that’s not produced by a damper or paratus and reheat coil). The control system commonly
other restriction where the regain is considerably less uses a method of detecting the warmest of those spaces
than 100%. The example in Figure 5-46 shows how stat- to control the temperature of the air leaving the appara-
ic pressure may be recovered from velocity regain in a tus and allows the remaining spaces to use their reheat
duct expansion. Different configurations provide other coils to achieve comfort in them.
values for the regain factor (R) but simply looking at a It seems strange to cool the air then heat it again
configuration and how abrupt changes are can give you but it’s one way of ensuring occupant comfort. One chief
an idea of how much of the potential regain can be re- operating engineer at a major college told me he could
covered. count on a considerable decrease in steam load when the
Don’t confuse a lack of air flow to a conditioned chiller plant was shut down for maintenance. To elimi-
space with blockage of ductwork. I’ve encountered sev- nate the energy waste associated with reheat some new
eral problems with duct failures, many including that system designs, including VAV boxes, have been devel-
flexible duct which is easy to crush. oped.

Reheat Coils VAV Systems
When apparatus serves spaces containing many Most modern systems have air handling units or
rooms, offices, or cubicles, it’s difficult to ensure occu- rooftop units with a variable speed drive controlling
pant comfort because the loads can change. Some of the the fan to supply conditioned air to VAV boxes. VAV