Carrier 48TC 04 Series, Service And Maintenance Instructions

Page: 45 / 152

45
Control Circuit, 24--V
The control circuit is protected against overcurrent
conditions by a circuit breaker mounted on control
transformer TRAN. Reset is manual.
GAS HEATING SYSTEM
FIRE, EXPLOSION HAZARD
Failure to follow this warning could result in death,
serious personal injury and/or property damage..
Disconnect gas piping from unit when pressure testing
at pressure greater than 0.5 psig. Pressures greater
than 0.5 psig will cause gas valve damage resulting in
hazardous condition. If gas valve is subjected to
pressure greater than 0.5 psig, it must be replaced
before use. When pressure testing field-supplied gas
piping at pressures of 0.5 psig or less, a unit connected
to such piping must be isolated by closing the manual
gas valve(s).
!
WARNING
General
The 48TC gas heating system consists of a tubular heat
exchanger assembly, multiple in--shot burners on a
manifold with manifold pressure regulating gas valve, an
induced draft combustion air fan system, electronic
ignition system and safety controls. The heat exchanger
tubes are located under the supply fan deck; the burner
section is located behind an access panel in the unit’s
condenser section, under the main control box.
The heat exchanger tubes are large diameter round
U--tubes. These tubes are smooth in the primary pass
regions and dimpled in the secondary pass regions. See
Figs. 60 and 61 for tube assemblies.
On 48TC sizes 04--14, the tubes are arranged in a
four--pass flow pattern. The in--shot burners fire into the
smooth primary tubes which pass through the heating
plenum and return to a combustion plenum above the
burner level. In the combustion plenum, the heated gas
flow converges into the dimpled secondary tubes. The
secondary tubes pass through the length of the heating
plenum and return to the induced draft fan inlet next to the
burner section.
On 48TC size 16, the tubes are arranged in a two--pass
flow pattern. Each tube has a smooth primary tube
segment on the bottom pass through the heating plenum
and a dimpled secondary tube segment on the top pass as
they return to the induced draft fan inlet. Each tube has its
own in--shot burner.
The induced draft fan wheel discharges into a flue passage
and the flue gases exit out a flue hood on the side of the
unit. The flue hood was shipped inside the unit and
required mounting by the unit installer.
Ignition and control of the heating system are provided
through the unit’s Integrated Gas Control (IGC) board.
The induced draft fan motor includes a Hall Effect sensor
circuit that confirms adequate draft fan wheel speed via
the IGC logic. Safety switches include a Roll--out Switch
(at the top of the burner compartment) and a limit switch
(mounted through the fan deck, over the tubes). See Fig.
55 and Fig. 57.
The 48TC gas heating section is designed for use with
Natural Gas (NG) fuel and installed at elevations less than
2000 ft (610 m). Accessory kits are available to adjust the
burner firing rates for use at higher elevations with NG
and to convert the burners to use liquefied petroleum (LP
or Propane) fuel.
See section Orifice Replacement for information in
modifying this unit for installation at elevations above
2000 ft (610 m).
The 48TC’s gas valve will provide one--stage or
two--stages of heating capacity, based on unit size, heating
capacity and electrical phase of the unit. See Table 9.
INDUCED-
DRAFT
MOTOR
MOUNTING
PLATE
BURNER
SECTION
INDUCED-
DRAFT
MOTOR
MANIFOLD
PRESSURE
TAP
ROLLOUT
SWITCH
FLUE
EXHAUST
VESTIBULE
PLATE
BLOWER
HOUSING
GAS VALVE
C06152
Fig. 55 -- Burner Section Details
48TC