Nailor 35SW Manual page 72

FAN POWERED TERMINAL UNITS
Electric Heating Coils
Application Guidelines
Discharge Air Temperature
When considering the capacity and airflow for the heater,
discharge air temperature can be an important factor.
Rooms use different types of diffusers and they are intended
to perform different functions. Slots that blend the air at the
glass and set up air curtains within the room, must be able to
blow the air very low in the room. Hot air will be too buoyant
to be effective in this case. Discharge air temperatures for
this application should be in the 85 – 90°F (29 – 32°C) range.
Diffusers in the center of the room blend their discharge
air as it crosses the ceiling. Discharge air temperatures in
this application can be as high as 105°F (41°C) and still
be effective. However, if the return air grilles are in the
discharge air pattern, the warm air will be returned to the
plenum before it heats the room. Again, the air temperature
C
needs to be blended down to an acceptable temperature
that can be forced down into the occupied space by the
time the air gets to the walls. Discharging warm air into the
room at temperatures above 105°F (41°C) usually will set up
stratification layers and will not keep the occupants warm if
there is a ceiling return because only the top 12" – 24" (300
– 600 mm) of the room will be heated.
The maximum approved discharge air temperature for any
Nailor Fan Powered Terminal Unit with supplemental heat
is 120°F (49°C). No heater should be applied to exceed this
temperature.
Electric Heater Selection
To properly select an electric heater, three things must be
determined: the heat requirement for the room, the entering
air temperature and the desired discharge air temperature.
The heat requirement for the room is the sum of the heat
loss calculation and the amount of heat required to raise the
entering air temperature to the desired room temperature.
Usually, the second item is small compared to the first for fan
powered terminal units in a return air plenum. MBH can be
converted to kW by using the chart or by calculation. There
are 3.413 MBH in 1 kW. If using the chart, find the MBH on
the left scale, then move horizontally to the right and read
kW.
Next, the desired discharge air temperature should be
ascertained. This will depend on the type of diffusers that
are in the room.
C162
(continued)
The desired heating airflow for the room can then be
calculated using the following equation:
cfm = kW x 3160
∆t (discharge air temp – inlet air temp.) °F
Assuming 70°F (21°C) supply air temperature to the heater,
the room airflow can be selected directly from the chart. Start
at the left at the design kW. Move horizontally to the desired
discharge air temperature. Then, move vertically down to the
cfm at the bottom of the chart.
The kW can be selected directly from the chart. Start at the
bottom with the design cfm into the room. Move vertically
up to the line that represents the desired discharge air
temperature. Then, move left to the kW.
The discharge air temperature can also be selected directly
from the chart. Start at the bottom with the design cfm into
the room. Move to the left side of the chart and find the
design kW. Move horizontally and vertically into the chart
until the lines intersect. The intersection will be the desired
discharge air temperature. Interpolation between the curves
is linear.
Heater Selection Chart
Assuming 70°F inlet air temperature at heater.
50
170
160
45
150
140
40
130
120
35
110
30
100
90
25
80
70
20
60
15
50
40
10
30
20
5
10
0
0
0 500 1000 1500
AIRFLOW, CFM
Diagonal lines are constant output temperature.
115°F
110°F
105°F
100°F
95°F
90°F
85°F
80°F
2000 2500 3000 3500 4000