6
Single-Duct and Retrofit Terminal Units IOM
GENERAL
The LMHS / LMHS-LC single duct VAV terminal is designed
to supply a varying quantity of cold primary air to a space
in response to a thermostat demand. Some units have
reheat options to provide heating demand requirements
as well. Most VAV terminals are equipped with pressure
compensating controls to regulate the response to the
thermostat 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 required for balancing the unit.
SET POINTS
Maximum and minimum airflow 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
Each unit is equipped with an amplifying inlet airflow sensor
that measures a differential pressure proportional to the
airflow. The relationship between the inlet airflow pressures
and cfm is shown in the Figure 6 – Krueger Inlet Airflow
Sensor Chart. This chart is attached to each unit.
SYSTEM CALIBRATION OF AIRFLOW SENSOR
To achieve accurate pressure independent operation, the inlet airflow sensor must be calibrated 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. w.g differential pressure, by the standard Pitot
tube coefficient of 4005.This ratio is the same for all sizes, no matter which probe type is installed. Determine the design
air velocity by dividing the design air volume (the flow at 1.0 in. w.g) by the nominal inlet area (sq. ft). This factor is the CFM
K factor, list in the Table 1 above.
CONTROLS SETUP
TABLE 1: INLET AIRFLOW SENSOR AREA AND K FACTOR
LMHS, RVE
04
05
06
07
08
09
10
12
14
16
22
Inlet Diameter, in.
4
5
6
7
8
9
10
12
14
16
22
Velocity Magnification
1.52
1.52
1.52
1.52
1.52
1.52
1.52
1.52
1.52
1.52
1.52
Velocity Factor
2625
2625
2625
2625
2625
2625
2625
2625
2625
2625
2625
CFM K Factor
229
358
515
702
916
1160
1432
2062
2806
3665
7000
Inlet Area, Sq. ft.
0.087
0.136
0.196
0.267
0.349
0.442
0.545
0.785
1.069
1.396
2.667
Recommended Min CFM
(@ 0.03 WG)
40
62
89
122
159
201
248
357
486
635
1212
The controls on most new projects are Direct Digital (DDC)
Controls. These controls require that flow parameters be
loaded during start-up to translate the sensed pressure into
a measured flow rate. There are several conventions (and
no universally accepted method) in use for representing this
flow factor:
1. 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 pitot tube.
b. For example, a velocity magnification may be used.
All Krueger probes develop an average signal of 1
w.g. @2625 fpm. This gives a velocity magnification of
4005/2625, or 1.52.
c. The magnification factor may be a pressure factor. In
this case, the ratio of pressures at a given air velocity
is presented. For a velocity constant of 2626, at 1000
fpm, this is 0.1451 / 0.0623 = 2.33.
4. K-Factor: The ‘K-factor’ may be represented in two ways
a. It may be a velocity K-factor, which is the velocity
factor independent of the inlet area, (which for all
Krueger inlet airflow sensors, both Linear and the
new LineaCross, is 2625 fpm/in w.g.).
b. Alternatively, it may be the airflow K-Factor, which is
the velocity factor times the inlet area. For an 8 inch
Krueger unit, therefore, this would be 2625 * 0.349, or
916. A separate factor is required for each size. Below
is a K-Factor table for all Krueger VAV terminal inlets.
c.
CFM=K√ΔP
i. CFM=ft
3
/minute
ii.
ΔP= Pressure Differential (WG)
iii. K=Sensor Constant