DOE-2 Engineers Manual Version 2.1A - DOE-2.com

where Tsurf is the coil surface temperature and Wsurf is the coil surface humidity
ratio at saturation. For a dry coil surface, Wexit and Wentering are
equal.
The coil bypass factor is a function of both physical and operational
parameters of the coil. Because the physical characteristics are constant, the
coil bypass factor is expressed as a product of the design, or rated, value and
two modifier functions. The most important variable is the coil surface air
velocity, which is directly proportional to the unit flow rate, CFM. Of secondary
importance are the entering coil wet-bulb and dry-bulb temperatures. The
coil bypass factor is
where
CBF = COIL-BF * COIL-BF-FCFM(PLRCFM) * COIL-BF-FT(TI,T2) (IV.l3)
TI =
T2 =
wet-bulb temperature entering the evaporator,
either the dry-bulb temperature of the air entering the
evaporator for chilled water systems, or the dry-bulb
temperature of the air entering the condenser for direct
expansion systems, and
PLRCHl = ratio of instantaneous flow rate to rated flow rate.
The values for the coil bypass factor can be calculated from manufacturer's
data by plotting the entering and exiting conditions on a psychrometric
chart, and then drawing a line through them to intersect the saturation
line. This intersection is the apparatus dew point temperature. Using this
point, along with Eq. (IV.II), a series of CSF values can be determined and
the rated value and modifier functions can be generated.
In the previous section, it was described how the various supply air and
space temperatures are calculated. During this process it was necessary to
know the range of dry-bulb temperatures of the supply air that could be made
available to the space. This required the estimation of equipment capacity
before the entire problem could be solved. To avoid iteration, the previous
hour's mixed air wet-bulb temperature is used to estimate the sensible capacity
using Eq. (IV.IO). Then, the minimum supply air temperature can be calculated
as
where
Texit = Tentering -
QCS
1. 08 * CFM
(IV.14)
Tentering = the estimated entering dry-bulb air temperature, using an
extrapolation of the return air dry-bulb temperature, along
with a simulation of outside air controls.
I V.12