Control Setup; General; Set Points; Maximum Airflow Set Points - Carrier 35E Installation And Start-Up Instructions Manual

CONTROL SETUP

The 35E single-duct VAV (variable air volume)
General —
terminal is designed to supply a varying quantity of cold prima-
ry air to a space in response to a thermostat demand. Some
units have reheat options to provide heating demand require-
ments as well. Most VAV terminals are equipped with pressure
compensating controls to regulate the response to the thermo-
stat independent of the pressure in the supply ductwork.
To balance the unit, it is necessary to set both the maximum
and minimum set points of the controller. The many types of
control options available each have specific procedures re-
quired for balancing the unit.
Maximum and minimum airflow set points
Set Points —
are normally specified for the job and specific for each unit on
the job. Where maximum and minimum airflow levels are not
specified on the order, default values are noted on unit ID label.
Field Adjustment of Minimum and Maximum
Airflow Set Points —
flow probe that measures a differential pressure proportional to
the airflow. The relationship between flow probe pressures and
cfm is shown in the Linear Averaging Probe Chart (Fig. 6).
This chart is attached to each unit. There are several conven-
tions (and no universally accepted method) in use for repre-
senting this flow factor:
MAGNIFICATION FACTOR — The magnification factor
may be expressed as the ratio of either velocity or pressure, of
the output of the sensor to that of a pilot tube.
For example, a velocity magnification may be used. All
probes develop an average signal of 1 in. wg at 2625 fpm. This
gives a velocity magnification of 4005 / 2625 or 1.52.
It may be a pressure magnification factor. In this case, the
ratio of pressures at a given air velocity is presented. For a ve-
locity constant of 2626, at 1000 fpm, this is 0.1451 / 0.0623 =
2.33.
UNIT SIZE 35E
INLET DIAMETER
VELOCITY MAGNIFICATION
VELOCITY CONSTANT
CFM K FACTOR
INLET AREA (SQ FT)
RECOMMENDED MIN CFM
Each 35E unit is equipped with a
Table 5 — Inlet Probe Area and K Factor
04 05
4.0 5.0
1.52 1.52
2625 2625
229 358
0.087 0.136
40 62
K-FACTOR — The K-factor may be represented in two
ways:
It may be a velocity K-factor, which is the velocity factor
times the inlet area, (which for all probes is 2625 fpm/in. wg).
Alternatively, it may be the airflow K-Factor, which is the
velocity factor times the inlet area. For an 8 inch unit, therefore,
this would be 2625 * 0.349, or 916. A separate airflow factor is
required for each size. All Carrier VAV terminals have round
inlets. Table 5 is a K-Factor table for all Carrier VAV terminal
inlets.

System Calibration of the Linear Averaging

Flow Probe —
operation, the velocity sensor and flow probe must be calibrat-
ed to the controller. This will ensure that airflow measurements
will be accurate for all terminals at system start-up.
System calibration is accomplished by calculating a flow
coefficient that adjusts the pressure fpm characteristics. The
flow coefficient is determined by dividing the flow for a given
unit (design air volume in cfm), at a pressure of 1.0 in. wg dif-
ferential pressure, by the standard pitot tube coefficient of
4005. This ratio is the same for all sizes regardless of which
probe is used.
Determine the design air velocity by dividing the design air
volume (the flow at 1.0 in. wg) by the nominal inlet area
(sq ft). This factor is the K factor.
Carrier inlet areas are shown in the table below. The design
air volume is also shown in this table. It can be determined
from this table that the average design air velocity for 35E units
is equal to 2656 fpm at 1.0 in. wg.
.
Record the information on a performance sheet (see
Fig. 7). This will provide a permanent record of the balanc-
ing information.
• installation location information
• box size
• cooling minimum airflow (cfm) limit
• cooling maximum airflow (cfm) limit
• reheat (cfm) limit (if applicable)
• heating minimum airflow (cfm) limit (if applicable)
• heating maximum airflow (cfm) limit (if applicable)
• calibration gain (after balancing)

• set points

06 07 08 09
6.0 7.0 8.0 9.0
1.52 1.52 1.52 1.52
2625 2625 2625 2625
515 702 916 1160
0.196 0.267 0.349 0.442
89 122 159 201
8
To achieve accurate pressure independent
10 12 14
10.0 12.0 14.0 16.0
1.52 1.52 1.52 1.52
2625 2625 2625 2625
1432 2062 2806 3665
0.545 0.785 1.069 1.396
248 357 486 635
16 22
22.0
1.52
2625
7000
2.667
1212