Technical library

OHM'S LAW

Safety is always a consideration when using electrical equipment. Some common concerns are contamination due to unsanitary equipment, protection from and limits on electromagnetic interference, and meeting strict safety requirements in potentially explosive environments. Customers rely on certifying agencies such as 3A, Factory Mutual, Canadian Standards Association, and others to ensure safety in these applications. These agencies examine, test, and certify that each product has been designed to meet specific standards for sanitary applications, hazardous locations, or specific electrical situations. Unlike independent testing laboratories who are unauthorized to issue any label but their own, certifying agencies enable the manufacturer to mark approved products with the corresponding standard committee's label, ensuring the end user that these products have been tested and meet those specific standards.

Sanitary Equipment

3A: Sanitary Standards Administrative Council

The objective of the 3A Sanitary Standards Committee is to formulate standards and accepted practices for equipment and systems used to process milk and milk products. Such standards are developed through the cooperative efforts of local, state, and federal sanitarians, equipment manufacturers, and equipment users so that the standards are acceptable to those involved in the sanitary aspects of dairy and related industries. The 3A Symbol Administrative Council authorizes manufacturers to display the 3A symbol on processing equipment that is in compliance with 3A Sanitary Standards.
USDA: United States Department of Agriculture
The Federal Meat and Poultry Products Inspection Acts authorize the USDA to require that the slaughter of animals and the subsequent processing of meat and poultry products be done in a sanitary manner. The Food Safety Inspection Service (FSIS) inspects to USDA sanitary guidelines for equipment and facilities engaged in these operations. As of November 1997, the USDA no longer approves product to these guidelines but rather requires the facility engaged in these processes to combine equipment in their plant that will pass a USDA Inspection. The manufacturer of the equipment will be solely responsible for ensuring their product will meet the USDA guidelines for inspection.

Hazardous Locations Equipment

FM: Factory Mutual Research Corporation

The Factory Mutual Approvals Division determines the safety and reliability of equipment, materials, or services utilized in hazardous locations in the United States and elsewhere. Factory Mutual certifies to NEC (National Electrical Code) standards for hazardous locations, NEC Standard 500 (Division classification) and also to the new NEC Standard 505 (Zone classification), which attempts to harmonize American and European classifications. For a product to receive approval, it must meet two criteria. First, it must perform satisfactorily, reliably, and repeatedly as applicable for a reasonable life expectancy. Second, it must be produced under high quality control conditions. Factory Mutual also has interlaboratory agreements and can certify to Canadian and European standards.

CSA: Canadian Standards Association

The association includes Canadian consumers, manufacturers, labor, government, and other regulatory agencies among its actively participating influences. These groups work together to generate standard requirements (CSA standards) that demonstrate product quality, enhance market acceptability, and improve quality and safety control procedures in manufacturing and construction for the Canadian marketplace. The standards generated by CSA are the cornerstone for determining a product's eligibility for certification in hazardous locations in Canada. CSA also performs product evaluation, testing, and ongoing inspection to these standards, and also to American and European standards through new interlaboratory agreements.

INERIS/ NEMKO/ LCIE/ BASEEFA

These are some of the recognized European approval agencies that have certified Viatran transmitters to Cenelec (European Committee for Electrotechnical Standardization) and/or IEC (International Electrotechnical Commission) standards for hazardous locations. Cenelec attempts to harmonize the electrical standards of its member countries. Generally, IEC standards are used. However, in certain instances where IEC standards are considered too vague, Cenelec defines more precise requirements. The member nations of Cenelec, which include and exceed those of the EEC (European Economic Community), are bound to adhere to these international regulations.

For an area to be classified as hazardous, the following three requirements for a Fire Triangle must be present simultaneously: Flammable gas, dust, or fiber Ignition source Air/oxygen.
Hazardous locations are broken into Divisions, Zones, Classes and Groups. These enable the manufacturer to specify exactly the type of hazardous location for which the product has been certified. The first classification describes the presence of flammable material in a hazardous location, either continuously, intermittently, or abnormally. The apparatus grouping states what type of flammable material is present: either gas, dust, or fiber. The temperature codes indicate the maximum temperature the device's external enclosure can reach. This is summarized in Table 1.

Table 1: Hazardous Locations Classifications

TABLE 1 (HAZARDOUS LOCATION CLASSIFICATION) TO GO HERE

For a product to be approved for a hazardous location, it must be designed so that an explosion of the flammable or combustible material surrounding the device does not occur. There are different methods of protection to achieve this. Viatran uses the three most accepted methods in the pressure transmitter market: Intrinsic Safety, Explosion Proof (Flame Proof), and Suitable for Use in Hazardous Locations.

Intrinsic Safety

An Intrinsically Safe piece of equipment is an electrical device that is incapable of causing an ignition of the prescribed flammable gas, vapor, or dust, regardless of any spark or thermal effect that may occur in normal use, or under any conditions of fault likely to occur in practice. This means that the device design is limited in such areas as PC Board layout, surface temperature, protection of electrical components, and power supply to the device. The devices are certified with either specific Intrinsic Safety Barriers (Loop certification) or general Intrinsic Safety Barrier parameters (Entity certification). These barriers are used outside the hazardous location and limit the amount of current, voltage, capacitance, and inductance entering the certified device. Often considered the safest and most technically elegant approach, there are many benefits of an Intrinsically Safe device to the customer. Expensive and cumbersome explosion-proof enclosures and conduit connections are not needed, electric shock is minimized, and controls can be maintained without shutting down the process.

Explosion Proof / Flame Proof

An Explosion Proof (or Flame Proof, as classified in IEC and Cenelec standards) device is an electrical device designed with an enclosure capable of withstanding, without damage, an explosion within it of a specific gas, fiber, or dust. In turn, it prevents ignition of these same materials surrounding the enclosure by a spark or flame from the explosion within. Factory Mutual formerly limited its Explosion Proof standard by requiring that the explosive external material be able to enter the device to cause an explosion. This excluded hermetically sealed devices from approval consideration. FM has recently modified their definition to include these devices. This certification usually requires that devices be designed with sturdy and durable enclosures with conduit connections. The primary benefits of this type of protection are that the device is not limited by low available power nor does it restrict PC Board layout. Viatran has also designed some of the smallest Explosion Proof devices in the industry.

Suitable for Use in Hazardous Locations

Factory Mutual developed this unique approval as a way for products to receive hazardous location approvals that cannot conform to existing protection concepts. There is no documented standard and the definition of this certification is unique to each product. In Viatran's case, this protection concept was utilized for our hermetically sealed products that did not meet FM's former Explosion Proof definition. Products that receive this approval are certified to the same Divisions as a comparable Explosion Proof or Intrinsically Safe device.

TABLE 2 (CODE EXAMPLE) TO GO HERE

"CE" marking is a declaration from the manufacturer that their product conforms to a specific Directive(s) adopted by the EEA (European Economic Area) and is a requirement for the product to be sold into any of the countries in this 18 member group. CE is an abbreviation for the French phrase Conformité Européene, meaning European Conformance. Unlike hazardous location approvals, the manufacturers are solely responsible for ensuring their product's conformance to these Directives which were developed using IEC and Cenelec standards. The Directives that affect transmitters are the EMC (Electromagnetic Compatibility) and LVD (Low Voltage) Directives. These state that the products must meet specific electromagnetic emission and immunity, as well as electrostatic discharge standards. Transmitters that meet EMC standards, as declared by the manufacturer, must be able to withstand interference from the radio frequency spectrum, electrostatic discharge, surges, etc., without the unit's performance being affected. The transmitter must also emit a minimum of the above charges so that it does not affect other nearby electrical devices or systems such as emergency communications or radio and television broadcasts. The LowVoltage Directive addresses basic electrical shock and fire hazard issues. These directives are currently only a requirement for the EEA member nations and are not required for products sold outside this community.

General Conversion Chart:

FLOW

CONVERSION FACTOR TABLE TO GO HERE

THERMOCOUPLE TYPES TABLE TO GO HERE

Type E

The Type E thermocouple is suitable for use at temperatures up to 900°C (1650°F) in a vacuum, inert, mildly oxidizing or reducing atmosphere. At cryogenic temperatures, the thermocouple is not subject to corrosion. This thermocouple has the highest EMF output per degree of all the commonly used thermocouples.

Type J

The Type J may be used, exposed or unexposed, where there is a deficiency of free oxygen. For cleanliness and longer life, a protecting tube is recommended. Since JP (iron) wire will oxidize rapidly at temperatures over 540°C (1000°F), it is recommended that larger gauge wires be used to compensate. Maximum recommended operating temperature is 760°C (1400°F).

Type K

Due to its reliability and accuracy, Type K is used extensively at temperatures up to 1260°C (2300°F). It's good practice to protect this type of thermocouple with a suitable metal or ceramic protecting tube, especially in reducing atmospheres. In oxidizing atmospheres, such as electric furnaces, tube protection is not always necessary when other conditions are suitable; however, it is recommended for cleanliness and general mechanical protection. Type K will generally outlast Type J because the JP (iron) wire rapidly oxidizes, especially at higher temperatures.

Type N

This nickel-based thermocouple alloy is used primarily at high temperatures up to 1260°C (2300°F). While not a direct replacement for Type K, Type N provides better resistance to oxidation at high temperatures and longer life in applications where sulfur is present.

Type T

This thermocouple can be used in either oxidizing or reducing atmospheres, though for longer life a protecting tube is recommended. Because of its stability at lower temperatures, this is a superior thermocouple for a wide variety of applications in low and cryogenic temperatures. It's recommended operating range is— -200° to 350°C (-330° to 660°F), but it can be used to -269°C (-452°F) (boiling helium).

Type S,R and B

Maximum recommended operating temperature for Type S or R is 1450°C (2640°F); Type B is recommended for use at as high as 1700°C (3100°F). These thermocouples are easily contaminated. Reducing atmospheres are particularly damaging to the calibration. Noble metal thermocouples should always be protected with a gas-tight ceramic tube, a secondary tube of alumina and a silicon carbide or metal outer tube as conditions require.

W-5 Percent Re/W-26 Percent Re (Type C*)

This refractory metal thermocouple may be used at temperatures up to 2315°C (4200°F). Because it has no resistance to oxidation, its use is restricted to vacuum, hydrogen or inert atmospheres.

CALIBRATION AND STANDARD CALIBRATION POINTS TABLE TO GO HERE

TABLE OF EQUATIONS TO GO HERE

Typical 3-Phase Wiring Diagrams and Equations for Resistive Heaters

TABLE OF DEFINITIONS TO GO HERE

Calculations for Required Heat Energy

When performing your own calculations, refer to the Reference Data screens for values of materials covered by these equations. The total heat energy (kWh or Btu) required to satisfy the system needs will be either of the two values shown below depending on which calculated result is larger.
A. Heat required for start-up
B. Heat required to maintain the desired temperature
The power required (kW) will be the heat energy value (kWh) divided by the required start-up or working cycle time. The kW rating of the heater will be the greater of these values plus a safety factor. The calculation of start-up and operating requirements consist of several distinct parts that are best handled separately. However, a short method can also be used for a quick estimate of heat energy required.

Short Method

Start-up watts = A + C + 2/3L + Safety Factor
Operating watts = B + D + L + Safety Factor
Safety Factor is normally 10% to 35% based on application.
A = Watts required to raise the temperature of material and equipment to the operating point, within the time desired.
B = Watts required to raise temperature of the material during the working cycle.

Equation for A and B (Absorbed watts-raising temperature)

TABLE HERE

C = Watts required to melt or vaporize material during start-up period.
D = Watts required to melt or vaporize material during working cycle.

Equation for C and D (Absorbed watts-raising temperature)

Weight of Material (lbs) · Heat of Fusion or Vaporization (Btu/lb)
Start-up or Cycle Time (hrs) · 3.412

L = Watts lost from surfaces by:

• Conduction — use equation below
• Radiation — use heat loss curves
• Convection — use heat loss curves

Equation for C and D (Absorbed watts-raising temperature)

TABLE HERE

For more detailed equations used in solving power estimating problems, see Power Equations under Equations. See Heat Loss Factors and Graphs in Reference Section.

You should always include a safety factor of varying size to allow for unknown or unexpected conditions. The size of the safety factor is dependent on the accuracy of the wattage calculation. Heaters should always be sized for a higher value than the calculated figure. A factor of 10% is adequate for small systems that are closely calculated; 20% additional wattage is more common. Safety factors of 20% and 35% are not uncommon, and should be considered for large systems, such as those containing doorsthat open or are large radiant heat applications. You'll also want to predict how long your system will operate without failure, so examine the amount of heater life you'll be needing. And because electricity costs money, take efficiency factors into account so your system will cost as little as possible to operate. With these considerations in mind, carefully review them all to be sure you do, in fact, have definitive information to decide on a particular solution to your heating problem. Some of this supporting information may not be readily available or apparent to you. You may find it necessary to consult the reference tables and charts in this reference data section, or reference a book that deals with the particular parameter you need to define. At the minimum, the thermal properties of both the material(s) being processed/heated and their containing vessel(s) will be required.

Figuring a safety factor requires some intuition on your part. The list of possible influences can be great. From changing ambient operating temperatures, caused by seasonal changes, to a change in material or material temperature being processed, you must carefully examine all the influences.

Generally speaking, the smaller the system with fewer variables and outside influences---the smaller the safety factor. Conversely, the larger the system and the greater the variables and outside influences — the greater the safety factor.

Here are some general guidelines:

The safety factor should be higher for systems that have production operations that contain equipment cycles subjecting them to excessive heat dissipations, e.g.: opening doors on furnaces, introducing new batches of material that can be of varying temperatures, large radiant applications and the like.

AMPERAGE CONVERSIONS TABLE HERE

GRAPHS HERE

(Click on graphs to enlarge.)

Heat Losses from Uninsulated Surfaces

Heat Losses at 70°F Ambient.

How to use the graph for more accurate calculations

Convection curve correction factors:

For losses from top surfaces or from horizontal pipes

For side surfaces and vertical pipes

For bottom surfaces

Radiation Curve Correction Factors

The radiation curve shows losses from a perfect blackbody and are not dependent upon position. Commonly used block materials lose less heat by radiation than a blackbody, so correction factors are applied. These corrections are the emissivity (e) values.

Total Heat Losses =

Helpful Hint

The graphs for losses from uninsulated and insulated surfaces are hard to read at low temperatures close to ambient. Here are two easy-to-use calculations that are only rule-of-thumb approximations, but they are reasonably accurate when used within the limits noted.

TABLE HERE

Rule #1

Losses from an uninsulated surface (with an emissivity close to 1.0): (This applies only to temperatures between ambient and about 250ºF).

TABLE HERE

Rule #2

Losses from an insulated surface: (This insulation is assumed to be 1 inch thick and have a K-value of about 0.5 Btu-in/hr-ft2-°F. Use only for surfaces less than 800°F.)

Heat Losses from Insulated, Water & Metal Surfaces

GRAPHS HERE

Some Material Emissivities/Metals

TABLES HERE

Some Material Emissivities/Non-Metals

TABLES HERE

Platinum RTD Tolerance Values

TABLES HERE

RTD Tolerance Class Definitions

DIN class A: ± [ (0.15 + 0.002 | t | ] °C
DIN class B: ± [ (0.30 + 0.005 | t | ] °C

RTD Resistance Wire Comparisons

TABLES HERE

LOAD GUIDE HERE FROM WORD DOC

Solution Type Of Heater Solution Type Of Heater Solution Type Of Heater

Reference Links: Material Properties www.matweb.com