Installation manual

ManualsBrandsMcQuay ManualsAir HandlersRAH 077C

Page 60 IM-487

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) which senses both the humidity and temperature of

the outside air entering the unit. This device has an enthalpy

scale marked A through D. Table 12 shows the control points

at 50% RH for settings A through D. Figure 55 shows this

scale on a psychrometric chart. When the outside air condi-

tions exceed the setting of the device, the outside air dampers

are positioned to the minimum outside air intake position by

the MicroTech II controller.

Table 12: Enthalpy control settings

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 out-

side 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 will adjust the return and outside air

dampers to use the airstream with the lowest enthalpy.

Figure 55. Enthalpy control settings

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.

Phase Voltage Monitor

The phase voltage monitor protects against phase loss (single

phasing) when any one of three line voltages drops to 74% or

less of setting. This device also protects against phase rever-

sal 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 volt-

age 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

An external time clock can be used as an alternative (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 digital

input DI (terminal DH1-1)on the ADI board. This is the nor-

mal condition in which the programmable internal schedule

is followed. When the circuit is closed, power is fed to DH1-

1. The MicroTech II controller responds by placing the unit

in the occupied mode, overriding any set internal schedule.

For more information, please see the "Digital Inputs" sec-

tion of Bulletin No. IM 696, "MicroTech II Applied Roof-

top Unit Controller."

Smoke Detectors

Optional smoke detectors can be located at the supply and

return openings. The wiring for these smoke detectors is

shown on any of the "Typical Main Control Circuit" wiring

schematics within the section “Wiring Diagrams” on page 48.

The sequence of operation for these detectors is as follows:

When the smoke is detected by either sensor, the normally

closed sensor contacts open. This removes power from digi-

tal input MCB BI 18 on the Main Control Board.

The MicroTech II controller responds by shutting the unit

down. The controller is placed in the Alarm Off state, and

cannot be restarted until the alarm is manually cleared. Refer

to the operation manual supplied with the unit for informa-

tion on clearing alarms (OM138 or Om137).

The smoke detectors themselves must be manually reset once

they have tripped. Power must be cycled to the smoke detec-

tor to reset.

CONTROL CURVE

CONTROL POINT

TEMP. 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

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 I V E

H U M I D I T Y

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