Modine MDB Série Manuel D'installation Et De Maintenance page 15

oPtional controls
timed Freeze Protection (tc-24)
The optional timed freeze protection system is factory installed
with the sensor (30°-75°F adjustable) factory mounted in the
discharge airstream for single blower units (unit sizes 110-130)
and field installed for twin blower units (unit sizes 220-230).
For field installed sensors, the sensor should be mounted near
the discharge air sensor as noted on page 13.
On initial start-up, the timed delay in the system allows the unit
to go through the normal ignition sequence. The timed delay is
a manual reset switch and adjustable for 1-10 minutes. In the
event that the unit fails to fire after this period, the discharge air
sensor will sense the cold air and will shut down the entire unit.
Figure 15.1 - timed Freeze Protection control
mild temperature inlet on/off stat (tc-03)
An optional mild temperature On/Off inlet stat may be supplied
with the unit, factory mounted in the piping compartment.
The purpose of the mild temperature On/Off stat is to shut
the main burner off 100% during mild weather so undesirable
temperature build-up in the space will not occur.
Figure 15.2 - mild temperature inlet on/off stat
variable Frequency drive (vFd)
(Shipped separately for field mounting by others)
Variable air volume 100% Make-Up units are equipped with a
VFD. The VFD controls the speed of the blower motor to vary
the air flow (cfm) from 40% to 100% of the full rated air flow.
The VFD is controlled by either a two-speed interlock, building
pressure control or a building management system.
Figure 15.3 - variable Frequency drive
Photohelic Building Pressure control (Ps-13)
(Optional for VFD equipped MDB Units or 75/25 MRB units)
The control includes a photohelic pressure switch with pressure
gauge, an outdoor air pressure pick up tube and a pick-up tube
wind and rain shield kit. Return air units also include an outside
air burner bypass damper linked to a return air damper and
a floating damper motor to control the mixture of outside and
return air.
The purpose of the control is to sense the difference between
the indoor building pressure and the outdoor atmospheric
pressure. This system is used when one make-up air unit
handles varying exhaust loads. As more exhaust fans are
turned on, pressure in the building will tend to become negative.
The photohelic pressure switch senses this change in pressure
and controls the unit operation to bring in more outside air to
bring the building pressure back to near atmospheric.
For MDB units, the VFD will vary the motor speed between
40% and 100% of the rated airflow for the selected unit. For
MRB units, the floating damper motor adjusts the position of
the outside air bypass damper and return air damper to vary
the outside air/return air ratio from a minimum 25% outside air
(75% maximum return air) to 100% outside air (0% return air).
The damper motor is a non-spring return type motor so when
power is interrupted to the motor via the photohelic pressure
switch, the motor will remain in its current position.
Note: The building pressure control's low pressure tap must be
vented to the outdoors and fitted with the wind and rain shield
provided with the photohelic control.
Adjusting the pressure adjusting knobs on the face of the
pressure gauge sets the desired building pressure set point
range. The smaller the difference between the high and low
pressure needles, the more sensitive the setting will be. The
larger the difference, the less sensitive the setting.
Figure 15.4 - Photohelic Building Pressure control
(Shipped separately for field mounting by others)
inlet or discharge air shut-off dampers
If an optional inlet or discharge outside air shut-off damper
is supplied with the unit, the damper will include an electric
damper operator motor. The motor is connected to the damper
blades via damper linkages.
The damper operator motor
includes a damper motor end
switch that is interlocked with
the unit's blower motor and
burner controls. The burner
and blower controls cannot
operate unless the damper
motor end switch has closed.
7-500.10
Damper
Actuator
Enclosure
(cover removed)
15