Technical Library: Engineering Reference Tables

FLOW

• 1 cc/min = .001 Liters/min
• 1 cc/min = .000264 Gallons/min
• 1 Liter/min = 1000 cc/min (ml/min)
• 1 Liter/min = .264 Gallons/min
• 1 Gallon/min = 3.785 Liters/min
• 1 Gallon/min = 3785.41178 cc/min (ml/min)

Conversion factors

THERMOCOUPLE TYPES

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.

Initial Calibration Tolerances for Wire and Cable Reference Junction 0°C (32°F)

Type Wire Measuring Junction Temperature

• BX Greater than 1000°C (1832°F)
• SX Greater than 870°C (1598°F)

• Type E -200 to 0°C ±1.0°C or ±0.5% (whichever is greater)
• Type T -200 to 0°C ±0.5°C or ±0.8% (whichever is greater)

Equation 1

Heat Required To Raise The Temperature of A Material

Where:

• Q1 = Heat required to raise temperature
• W = Pounds of material
• CP = Specific heat of material (Btu/lb-°F)
• T = Temperature rise of material
• (TFinal - TInitial) °F

Equation 2

Heat Required To Vaporize A Material (To Melt a Substance, Substitute Latent Heat of Fusion ---Hf---for Latent Heat of Vaporization)

Where:

• Q2 = Heat required to vaporize
• W = Pounds of material
• Hv = Latent heat of vaporization (Btu/lb)

Equation 3

Heat Losses From Surfaces

Where:

• Q3 = Surface heat losses
• A = Surface area (ft2)
• FSL = Surface loss factors (W/ft2) evaluated at surface temperature
• t = Exposure time (hours)

Equation 4

Heat Losses By Conduction Through Materials

Where:

• Q4 = Conduction heat losses
• K = Thermal Conductivity (Btu · in/ft2 · °F · hour)
• A = Heat transfer surface area (ft2)
• X = Thickness of material (inches)
• T = Temperature difference across material (T2 - T1)°F
• t = Conduction time (hours)

Typical 3-Phase Wiring Diagrams and Equations for Resistive Heaters

Definitions

Equations

3-Phase Wye (Balanced Load)	3-Phase Open Wye (No Neutral)
IP = I L, VP = VL/1.73 , WWYE = VL2/R = 3 (VP2)/R , WWYE = 1.73VLIL	IPO = I LO, VPO = VL/2 , WOWYE = 1/2 ( VL /R) , WOWYE = 2 (VPO2/R)
3-Phase Delta (Balanced Load)	3-Phase Open Delta
IP = I L/1.73 , VP = VL, WDELTA = 3(VL2)/R , WDELTA = 1.73VLIL	VP = VL, IPO1 = I PO3 = I LO2, ILO3 = 1.73 I PO1, WODELTA = 2(VL2/R)

Calculations for Required Heat Energy

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)

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)

SAFETY FACTOR CALCULATION

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:

• 10% safety factor for small systems with closely calculated power requirements
• 20% safety factor is average
• 20% to 35% for large systems

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.

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

• Multiply convection curve value by 1.29

For side surfaces and vertical pipes

• Use convection curve directly

For bottom surfaces

• Mulitply convection curve value by 0.63

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 =

• Radiation losses (curve value times e)
• + Convection losses (top)
• + Convection losses (sides)
• + Convection losses (bottom)
• = Conduction losses (where applicable)

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.

Rule #1

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

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

k = 0.67 @ 200°F

k = 0.83 @ 1000°F.

For 2 or 3 inches Insulation: multiply by 0.84 For 4 or 5 inches Insulation:

multiply by 0.81

For 6 inches Insulation:

multiply by 0.79

Heat Losses from Oil or Paraffin Surfaces

Some Material Emissivities / Metals

Some Material Emissivities/Non-Metals

Platinum RTD Tolerance Values

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