Common Terms

Heat Tranfer

Watt Density

Common Nema Enclosure Types

Common Terms

AMBIENT- The immediate surrounding air or other medium in which a system is operated.

AMPERES (AMPS)- The common unit of measure for the flow of electric current. Typically designated with the letter "I".

ASME- American Society of Mechanical Engineers. The organization that governs several weld code standards and test programs that apply to pressure vessels and similar constructions.

AUTOMATIC RESET- A device that returns to the "ON" or "OFF" state (depending on it's normal operation) without manual input. Term is often used in reference to over temperature or limit type thermostats.

BOILING POINT- The temperature at which a liquid changes from a liquid state to a gas state. Example: Boiling point of water is 212° F (100° C).

BRITISH THERMAL UNIT (BTU)- Common unit for heat energy. 1 BTU is equivalent to the amount of energy required to heat 1 pound of water 1° F from 32° F to 33° F at standard atmospheric pressure. One kilowatt-hour equals 3412 BTU.

CALIBRATION- The process of adjusting a device such as a test instrument or control thermostat to insure that the indicated or controlled values are accurate.

CFM- Flow volume of a material expressed in cubic feet per minute.

CONDUCTORS- Materials that facilitate the flow of electric current. The better the conductor the more easy the flow of electric current. Example: Copper is a good conductor and a common material used for electrical wiring.

DENSITY- Weight of material per unit volume typically expressed in pounds per cubic foot. Ex: Water has a density of 62.4 pounds per cubic foot at or near room temperature.

DIELECTRIC- Defines the electrical insulating ability of a material. A higher dielectric value indicates better insulating qualities. All Vulcan units are subjected to a high voltage test applied between live and grounded metal parts. The test voltage is typically 1000 volts plus a multiplier of the rated voltage.

DIFFERENTIAL- The temperature difference between the "ON" and "OFF" temperature switch points of a thermostat or temperature controller.

FPM- Flow velocity of material expressed in feet per minute.

FPS- Flow velocity of material expressed in feet per second.

GPH- Flow volume of material expressed in gallons per hour.

GPM- Flow volume of material expressed in gallons per minute.

HEAT OF FUSION- The amount of energy required to change one pound of material from a solid to a liquid state. Expressed in BTU per pound.

HEAT OF VAPORIZATION- The amount of energy required to change one pound of material from a liquid to a gaseous state. Expressed in BTU per pound.

HEAT TRANSFER- The process of transferring thermal energy from one object to another. See page titled "HEAT TRANSFER" for more detailed explanation.

INSULATION RESISTANCE- A measure of a material's insulating properties usually expressed in megohms (millions of ohms). A high megohm reading is desirable. A low heater megohm reading is often an indication of moisture. Proper manufacture and sealing of the heater ends will reduce moisture absorption into the heater. The test for insulation resistance is often referred to as a "megger" or "megohm" test.

KILOWATT- Unit of measure for electrical power equivalent to 1000 watts.

KILOWATT-HOUR- Unit of measure equivalent to 1 kilowatt (1000 watts) applied continuously for 1 hour.

MANUAL RESET- A device that requires manual action in order to reactivate to normal operational mode. An over temperature, limit, or safety switch often employs manual reset feature to ensure fault condition is removed prior to reactivation of device.

MgO- Designation for magnesium oxide a common insulating material used in the construction of electric heating elements. MgO has excellent high temperature qualities including electrical insulation properties and thermal conductivity.

NEC- National Electric Code. Standards and specifications that apply to safe electrical wiring and installation practices.

NEMA- National Electrical Manufacturers Association. An organization that issues electrical related standards and guidelines.

NPT- Abbreviation for "National Pipe Thread" . A common mounting fitting thread type used on electric heaters. Standard sizes range from 1/8 through 2-1/2 inch NPT.

OVERSHOOT- A rise in temperature beyond the intended temperature control set point. Often the result of issues related to the rate of temperature rise, sensor location, and/or heater sizing. Excessive overshoot on an electro-mechanical type thermostat can cause permanent damage to the device.

OHM- The common unit of measure for electrical resistance. Common designations are the letter "R" or symbol "½".

PID- Proportional, integral, and derivative control modes.

RADIATION- Transfer of heat energy from a hot object to a cold object through a vacuum or medium such as air without heating the medium.

REFLECTIVITY- Ratio between energy reflected by a surface and the total energy hitting the surface.

RELAY- Mechanical or solid state switching device. Employed as a means of switching power loads such as that of a heater. Triggered by a temperature control or other similar device.

RESISTANCE- The opposition to electric current. The resistance or opposition to electric current results in the generation of heat. This electrical principle forms the basis for electric heat. An example of an excellent resistance material is the nickel-chromium resistance wire used in Vulcan heaters.

RESISTANCE TEMPERATURE DETECTOR (RTD)- A temperature sensor construction incorporating a sensing element of a winding of fine platinum wire that produces a predictable resistance value change corresponding to temperature change.

SILICON CONTROL RECTIFIER (SCR)- A power switching device that incorporates no moving parts.

THERMISTOR- A temperature sensor construction that incorporates a material composite encapsulated in epoxy or glass. The resistance value changes with temperature change.

THERMOCOUPLE- A temperature sensor construction incorporating 2 dissimilar materials connected at one end that produces a small voltage that changes value with temperature.

VOLT- Common units of measure for the electrical force between two points referred to as the "Electromotive Force" (EMF). Designated as "E" or "V".

WATT- The common unit of measure electrical power. Used in the heater industry to express heat energy. Designated as "W".

SENSOR- A device that detects temperature, pressure, or other property. The sensor with supporting electrical circuitry and components can provide indication and switching of other devices such as electrical heaters. Due to typical heater environments, most sensors are enclosed within a tube. Common temperature sensors include thermocouples, RTD's, and thermistors.

SET POINT- Thermostat or temperature controller set temperature value.

SPECIFIC HEAT- The energy required to change the temperature of a unit volume of material. Common unit of measure in the heating industry is BTU/lb °F.

TEMPERATURE- Indicates degree of hot or cold on a specific scale. The most common units are Fahrenheit and Centigrade. The two temperature scales are related as follows:

Degrees F = (1.8 x °C) + 32
Degrees C = (°F - 32) / 1.8

THERMOSTAT- A device that switches "open" or "close" an electric circuit as the result of a temperature event occurring. A thermostat is often used in conjunction with a heater to shut-off the heater when a set temperature value is achieved.

THERMOWELL- A tube utilized to protect a sensor or sensing bulb. Tube is typically welded liquid tight in order to prevent exposure of the sensor to a wet or corrosive environment.

WATT DENSITY- The power loading expressed in watts over a unit surface area. Common units are watts/in2 and watts/cm2. See paragraph titled "WATT DENSITY" and example calculations.

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Heat Transfer

"The process of thermal energy (heat) flowing from one object to another"

I. For heat transfer to occur, there must be a temperature difference between the objects
II. Heat will only transfer in the direction of the lower temperature object
III. There are three types of heat transfer:

1. Conduction - The transfer of heat energy from one material to another while in direct contact. The materials remain in a fixed position relative to each other. The rate of con duction varies depending on the material and is called the "thermal conductivity". Most metals are good conductors of heat (high "thermal conductivity").

A Vulcan strip heater clamped to a block of metal (in direct contact) heats the metal as a result of conduction.

2. Convection - The transfer of heat energy as a result of material particles in motion relative to each other. Mixing causes heat transfer from the warmer particles to the colder particles. Convection is the primary method of heat transfer in liquids and gases.

A Vulcan finned tubular heater assembled into a pipe or duct with air forced over the heater. The colder moving air is heated by the warmer air particles near the heater as the air particles mix together

3. Radiation - The transfer of heat energy in the form of rays, similar to light, from one body to another through a space. All bodies at a temperature above that of their surroundings radiate heat. The sun heating the earth is one example.

A Vulcan tubular assembled into a toaster oven. The heater radiates heat to the item being toasted.

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Watt Density

"The power loading in watts over a unit surface area of a heater"

I. Normally expressed in watts per square inch

II. The higher the watt density - the hotter the heater sheath operating temperature under the same application

III. Watt densities greater than recommended for the heater type will result in premature heater failure

IV. Watt densities greater than recommended for the material being heated will result in degradation of the heated material or could generate hazardous conditions such as a fire or toxic fumes

V. Calculations combined with utilization of tables and graphs are key factors in the successful application of Vulcan heaters

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Common Nema Enclosure Types

NEMA 1 - General purpose indoors. Basic enclosure intended to prevent accidental contact with internal electrical components. Suitable for use indoors where not subjected to unusual conditions.

NEMA 4 - Watertight and dusttight indoors or outdoors. Designed to exclude water applied in the form a hose stream. Also suitable where apparatus may be subjected to a stream of water during cleaning operations.

NEMA 4x - Watertight, dusttight, and corrosion resistant - indoors or outdoors. Protects internal components against splashing water, falling or hose directed water, and severe external condensation. Sleet resistant. Corrosion resistant material or finish.

NEMA 7 - Explosion resistant in hazardous class 1 locations (areas where flammable gases or vapors are or may be present in quantities to produce explosive conditions or fire).

NEMA 12 - Dusttight and driptight indoors. Enclosures are designed to protect internal components from fibers, lint, dust, dirt and light splashing, and dripping of non-corrosive liquids.

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