Market Forge STP-6E Service Manual
1 GENERAL
The Steam Tech Plus pressureless steam cooker
consists of two identical cooking compartments, one
above the other, in a single cabinet assembly. Each
compartment Is fitted with independent electrically
operated steam generators and control circuitry.
Compartments can be used separately or
simultaneously in any cook mode with no effect on
cooking times or quality.
This section includes explanations of the steam
generation, control, drainage, suppression and cold air
evacuation systems.
NOTE: The various systems which have been
separated in the following sections to simplify their
descriptions are actually very closely interrelated to,
and dependent on one another to achieve precise
steam control and high efficiency. See Figures I and 2
on pages 7 - 9 for schematic illustrations.
2 FILL/COLD WATER INLET
When the POWER switch Is pressed into the ON
position, two normally closed solenoid operated fill
valves energize and open. One valve supplies cold
water to the tempering tank, while the other valve
supplies cold water to the corresponding steam
generator. At the same time, two normally open
solenoid operated drain valves energize and close
allowing the tempering tank and steam generator to fill
with water. Both the tempering tank and steam
generator will simultaneously fill until the factory
predetermined level is reached.
The water level in the tempering tank is sensed
by a normally closed magnetic reed float switch. As the
float rises to the factory predetermined level, a
magnetic sensor opens the circuit to the tempering tank
fill/cool solenoid valve. This shuts off the flow of water
Into the tempering tank. The fill/cool solenoid valve also
regulates the temperature of the drain water. See
Section 9 for more details on drainage.
The water level in the steam generator is sensed
by high and low water sensor rods which are mounted
on the front of the generator. The rods are connected to
the electronic water level control board which, controls
the opening and closing of the generator fill valv e.
When the water level has reached the high water level
sensor rod, the electronic board cuts power to the
generator fill valve. As steam is created and used up,
the water level in the steam generator will drop. When
the water level falls below the level of the low water
level sensor rod, the electronic board will close the
circuit to the generator fill valve, allowing fresh water to
enter the generator until the water has risen back up to
the level of the high water level sensor.
3 PREHEATING & STANDBY
When the water level in the generator has risen
up to the low water level sensor rod, the electronic
water level control board closes a circuit which
activates a relay. This relay supplies power to the
heater. The heater will stay on at full power until the
water temperature in the steam generator reaches
193°F. The steam generator water temperature is
sensed by a thermostatic switch mounted inside the
steam generator, below the water level. The steam
generator will standby or idle at 193°F, ready to make
steam when it is called for.
4 STEAM GENERATION
The Steam Tech Plus will not create steam until it
is called for. Steam is called for by:
a) setting the TIMER (in conjunction with the
MODE SELECT switch), or
b) pressing the CONSTANT STEAM button.
The cooking compartment doors each contain a
magnet which activates a magnetic reed switch.
These are located behind the front face of the unit,
next to each of the cooking cavity openings. When the
door is opened, the magnet and switch are separated.
This opens one of the circuits to the heater, which will
halt the creation of steam, but allow the heater to
maintain 193°F water temperature in the steam
generator.
When the unit is on, the heater will maintain
193°F water temperature in the steam generator, to
insure quick generation of steam when it is called for.
When steam is called for, a circuit is closed
which activates a relay. This relay doses the 208v,
240v, or 480v circuit which supplies power to the
heater. The generator now begins to create steam.
Steam passes from the steam generator directly
through the cooking compartment side wall opening,
into the cooking compartment. The perforated
sheetmetal baffling on the right side pan support rack
(which covers the steam inlet opening), insures
uniform distribution of steam throughout the interior of
the cooking compartment.
The fresh steam entering the cooking compartment forces the colder air to the bottom of the
compartment and down the cooking compartment
drain line. The cooking compartment drain line
branches into 3 lines. The first branch terminates at a
check valve which serves as a vacuum breaker. This
branch also contains an extremely sensitive pressure
switch, which will be further discussed in Section 5.
The second branch terminates inside the tempering
tank, below the water level which creates a water seal.
The third branch passe s through a steam trap valve.
As long as the air passing through the steam trap
valve Is below 200°F, it remains open, and the system
is open to the atmosphere. While the system is open to
the atmosphere, it will not build any pressure and
steam will be continuously generated by the heater. As
the temperature of the air being pushed out of the
cooking compartment, down the drain line, and through
the steam trap, rise s above
200°F, the steam trap valve will close. This will seal the
system off from the atmo sphere. As excess steam is
generated, a slight pressure will begin to build. When
the pressure switch senses a pressure of a 2" water
column, it opens the circuit to the relay which supplies
full 208v, 240v, or 480v power to the heater. Before
any more steam is created, the food inside the cooking
compartment must absorb all of the steam energy that
is already present. As the steam energy is absorbed by
the food in the cooking compartment, the pressure will
begin to drop. When the pressure drops below a I"
water column, the pressure switch will again close the
circuit to the heater, creating more steam.
5 STEAM CONTROL AND EFFICIENCY
The Steam Tech Plus pressureless steam cooker
uses an integrated steam control system which allows
it to achieve its high cooking efficiency. The system
monitors the slight pressure fluctuation in the cooking
compartment and steam generator. The pressure in
these areas is a direct reflection of how much steam is
being absorbed by the food in the cooking
compartment and the rate at which it is being
absorbed.
At the beginning of a cooking cycle, the food will
absorb nearly all of the steam that can be generated
and almost no pressure will build in the system. As the
food cooks and its surface temperature rise s, it will
absorb less and less steam, at progressively slower
rates. When pressure has built up to a 2" water
column, (about the same pressure it takes to blow
bubbles through a straw), the pressure switch will open
the circuit to the relay which supplies power to the
heater. This causes the heater to shut down until the
food has absorbed enough steam to lower the pressure
to a I" water column. At a I" water column, the pressure
switch will close the circuit to the relay which supplies
full power to the heater. The heater will then energize
again to create more steam to replace what was
absorbed by the food. This cycling on and off of the
heater will
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