All forms of energy can be put to safe and productive use
when properly controlled. Food service cooking equipment uses a wide and sometimes
complex variety of controls that can be generally categorized as either operating
controls or safety controls.
Operating controls regulate the flow of energy to the appliance
or run other mechanisms that ensure proper operation of the equipment such as
thermostats and blower fans. The more important safety controls act independently
of operating controls and serve to override the automatic operation of the appliance
to prevent a hazardous condition.
Safety controls act to avoid a dangerous situation by relieving
excess pressure or temperature or, if necessary, shutting off an appliance. Some
examples of safety controls on gas appliances are automatic pilot devices and
oxygen depletion sensors. Electric safety controls include GFI circuit breakers
and temperature controls.
Manual vs. Automatic Controls
Until modern times, most control devices were manual. Manual
controls are simple, relatively inexpensive and rely upon the user's judgment
for operation. When operating the burner on a range top, the cook simply turns
a knob to adjust the flame to a desired size based on an estimation of the amount
of heat needed.
Most appliances today use automatic controls, which are generally safer, more
precise, and in some instances the only practical option for operating an appliance.
The oven thermostat is an example of an automatic control used for its precision.
In the case of gas appliances, every automatic control system contains three elements.
The sensor is designed to keep track of the conditions of operation.
If the oven temperature falls below a set point, the sensor notes this condition
and relays a signal to the actuating means, which may be a fluid
in the sensing device that expands when heated creating pressure that activates
the third element, the final control element. The final control
element is a valve that opens or closes to let in more, less or no gas.
Once the main gas is flowing, either a match or some type
of ignition system must ignite it. Many appliances are equipped with standing
pilots which 1) automatically light the gas at the main burner and 2) cut off
the supply of gas if the pilot is unable to ignite the gas (either because it
is extinguished or too low to operate properly). The pilot burner operates the
pilot safety device and provides the flame that lights the main burner.
Intermittent Ignition Devices
In systems where the pilot burner is lit each time the appliance
is operated, the pilot burner igniter is either a spark or a heater coil and replaces
the standing pilot. These are known as pilot relight systems or intermittent pilot
systems. They save energy and respond faster to the presence or absence of a pilot
flame than a thermocouple. Today, all gas appliances are required by law to have
an intermittent ignition system. The two main types of intermittent ignition devices
for direct burner ignition are direct spark ignition and hot surface ignition.
Direct Spark Ignition
Direct spark ignition replaces the standing pilot with a set
of electrodes. Some use separate igniter and flame sensors, others use a common
igniter/sensor. When signaled by the thermostat, an internal transformer generates
a high voltage spark and the gas is allowed to flow to the main burner. These
events occur simultaneously, and the gas burner is lit. If the main burner does
not ignite, gas flow shuts off, a condition known as lockout. Although some systems
allow multiple ignition attempts before lockout, once lockout occurs the system
must be manually reset.
Hot Surface Ignition
The hot surface ignition system consists of a
silicon carbide glow bar positioned within the burner flame. In response to a
call for heat, the control system provides line voltage to the glow bar, which
reaches a temperature of about 2,500°F in about 18 to 45 seconds. The control
then starts the flow of gas to the burner and ignition occurs. Either the hot
surface element or a separate probe are used to sense the flame.
Cooking Appliance Controls
Typical controls in modern gas ranges and ovens
include thermostats, automatic pilots, pilot relight and intermittent ignition
systems, gas pressure regulators and automatic gas valves. Oven thermostats can
be the modulating type, cycling type, or a combination of both. The differences
between these types of thermostat controls is discussed in the Basics section
Temperature Control Methods.
Some ranges also incorporate programmed cooking
controls for start-and-stop or cook-and-hold warm features. With start-and-stop,
the oven automatically starts itself, cooks the food and shuts itself off. Cook-and-hold
warm systems provide a convenient way to cook foods and hold them at serving temperatures
for extended periods of time. At the end of a prescribed length of time, the oven
temperature is automatically lowered to a temperature that keeps the food warm
without drying or further cooking.