84
McQuay IM 487-4
Unit Options
Unit Options
Enthalpy Control
Outside Air Enthalpy Control (OAE)
Units with MicroTech II control and an economizer come
standard with an electromechanical enthalpy control device
(OAE) that senses both the humidity and temperature of the
outside air entering the unit. This device has an enthalpy scale
marked A through D. Table 14 shows the control points at 50%
RH for settings A through D. Figure 92 shows this scale on a
psychrometric chart. When the outside air conditions exceed
the setting of the device, the outside air dampers are positioned
to the minimum outside air intake position by the MicroTech II
controller.
Differential Enthalpy Control (OAE/RAE)
An optional electric differential enthalpy control arrangement
(OAE/RAE) is available on units with MicroTech II control. In
this configuration a solid-state humidity and temperature
sensing device is located in both the return (RAE) and outside
intake (OAE) airstreams. This OAE device has the same A
through D scale as the device described above. However, with
the OAE/RAE arrangement the switch on OAE must be set all
the way past the “D” setting. With this done, the MicroTech II
controller adjusts the return and outside air dampers to use the
airstream with the lowest enthalpy.
Ground Fault Protection
The ground fault protection is designed to protect motors from
destructive arcing ground faults. The system consists of a
ground fault relay and a ground fault current sensor. The
ground fault relay employs solid state circuits that will
instantaneously trip and open a set of relay contacts in the 115
volt control circuit to shut the unit down whenever a ground
fault condition exists. The ground fault relay is self powered.
The ground fault sensor is a current transformer type of device
located on the load side of the power block through which the
power wires of all phases are run.
Figure 92. Enthalpy control settings
Phase Voltage Monitor (see page 111)
The phase voltage monitor protects against high voltage, phase
imbalance, and phase loss (single phasing) when any one of
three line voltages drops to 74% or less of setting. This device
also protects against phase reversal when improper phase
sequence is applied to equipment, and low voltage (brownout)
when all three line voltages drop to 90% or less of setting. An
indicator run light is ON when all phase voltages are within
specified limits. The phase voltage monitor is located on the
load side of the power block with a set of contacts wired to the
115 volt control circuit to shut the unit down whenever the
phase voltages are outside the specified limits.
External Time Clock
You can use an external time clock as an alternative to (or in
addition to) the MicroTech II controller’s internal scheduling
function. The external timing mechanism is set up to open and
close the circuit between field terminals 101 and 102. When
the circuit is open, power is not supplied to binary input
MCB-BI1. This is the normal condition where the controller
follows the programmable internal schedule. When the circuit
is closed, power is fed to BI1. The MicroTech II controller
responds by placing the unit in the occupied mode, overriding
any set internal schedule.
For more information, see the “Digital Inputs” section of
Bulletin No. IM 696, “MicroTech II Applied Rooftop Unit
Controller.”
Table 14: Enthalpy control settings
Control curve
Control point temperature at 50% RH
A
73°F (23°C)
B
70°F (21°C)
C
67°F (19*C)
D
63°F (17°C)
3 5
( 1 . 5 )
4 0
( 4 . 5 )
4 5
( 7 )
5 0
( 1 0 )
6 0
( 1 5 . 5 )
6 5
( 1 8 . 5 )
7 0
( 2 1 )
7 5
( 2 4 )
8 0
( 2 6 . 5 )
8 5
( 2 9 . 5 )
9 0
( 3 2 )
9 5
( 3 5 )
1 0 0
( 3 8 )
1 0 5
( 4 0 . 5 )
5 5
( 1 3 )
1 2
1 4
1 6
1 8
2 0
2 2
2 4
2 6
3 0
3 2
3 4
3 6
3 8
4 0
4 2
4 4
4 6
2 8
E N
T H
A L
P Y
B
T U
P
E R
P
O U
N D
D
R Y
A
I R
C
B
A
D
D
C B
A
0 .
5 0
0 .
4 0
0 .
3 0
0 . 2
0
0 . 1
0
0 .
6 0
0 .
7 0
0 .
8 0
0 .
9 0
R
E L
A T
IV
E
H
U
M
ID
IT
Y
3 5
( 1 . 5 )
4 0
( 4 . 5 )
4 5
( 7 )
5 0
( 1 0 )
5 5
( 1 3 )
6 0
( 1 5 . 5 )
6 5
( 1 8 . 5 )
7 0
( 2 1 )
7 5
( 2 4 )
8 0
( 2 6 . 5 )
8 5
( 2 9 . 5 )
9 0
( 3 2 )
9 5
( 3 5 )
1 0 0
( 3 8 )
1 0 5
( 4 0 . 5 )
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