Determining Heat Energy Requirements
 Constant Temperature Applications
 Variable Temperature Applications
 Process Applications
 Determining Heat Energy Absorbed
 Determining Heat Energy Lost
 Estimating Heat Loss Factors
 Design Safety Factors
 Total Heat Energy Requirements
 Basic Heat Energy Equations
 Summarizing Energy Requirements
 Equipment Sizing & Selection
The objective of any heating application is to raise or maintain the temperature of a solid, liquid or gas to or at a level suitable for a particular process or application. Most heating applications can be divided into two basic situations; applications which require the maintenance of a constant temperature and applications or processes which require work product to be heated to various temperatures. The principles and calculation procedures are similar for either situation.
Constant Temperature Applications
Variable Temperature Applications
Total Heat Energy Absorbed — The sum of all the heat energy absorbed during startup or operation including the work product, the latent heat of fusion (or vaporization), make up materials, containers and equipment.
Total Heat Energy Lost — The sum of the heat energy lost by conduction, convection, radiation, ventilation and evaporation during startup or operation.
Process Applications
 Calculated heat energy required for process startup over a specific time period.
 Calculated heat energy required to maintain process temperatures and operating conditions over a specific cycle time.
Determining Heat Energy Absorbed
StartUp Requirements (Initial HeatUp)

Operating Requirements (Process)

Determining Heat Energy Lost
Estimating Heat Loss Factors
Design Safety Factors
Total Heat Energy Requirements
 Q_{T} = The total energy required in kilowatts
 Q_{M} = The total energy in kilowatts absorbed by the work product including latent heat, make up materials, containers and equipment
 Q_{L} = The total energy in kilowatts lost from the surfaces by conduction, convection, radiation, ventilation and evaporation
 Safety Factor = 10% to 25%
Basic Heat Energy Equations
The following equations outline the calculations necessary to determine the variables in the above total energy equation. Equations 1 and 2 are used to determine the heat energy absorbed by the work product and the equipment. The specific heat and the latent heat of various materials are listed in this section in tables of properties of nonmetallic solids, metals, liquids, air and gases. Equations 3 and 4 are used to determine heat energy losses. Heat energy losses from surfaces can be estimated using values from the curves in charts G114S, G125S, G126S or G128S. Conduction losses are calculated using the thermal conductivity or "k" factor listed in the tables for properties of materials.
Equation 1 — Heat Energy Required to Raise the Temperature of the Materials (No Change of State)The heat energy absorbed is determined from the weight of the materials, the specific heat and the change in temperature. Some materials, such as lead, have different specific heats in the different states. When a change of state occurs, two calculations are required for these materials, one for the solid material and one for the liquid after the solid has melted.
Q_{A} =  Lbs x C_{P} x ΔT 3412 Btu/kW 
 Q_{A} = kWh required to raise the temperature
 Lbs = Weight of the material in pounds
 C_{p} = Specific heat of the material (Btu/lb/°F)
 ΔT = Change in temperature in °F [T_{2 (Final)}  T_{1 (Start)}]
The heat energy absorbed is determined from the weight of the materials and the latent heat of fusion or vaporization.
Q_{F} or Q_{v} =  Lbs x H_{fus} or H_{vap} 3412 Btu/kW 
Where:
 Q_{F} = kWh required to change the material from a solid to a liquid
 Q_{v} = kWh required to change the material from a liquid to a vapor or gas
 Lbs = Weight of the material in pounds
 H_{fus} = Heat of fusion (Btu/lb/°F)
 H_{vap} = Heat of vaporization (Btu/lb/°F)
The heat energy lost from surfaces by radiation, convection and evaporation is determined from the surface area and the loss rate in watts per square foot per hour.
Q_{LS} =  A x L_{S} 1000 W/kW 
Where:
 Q_{LS} = kWh lost from surfaces by radiation, convection and evaporation
 A = Area of the surfaces in square feet
 L_{S} = Loss rate in watts per square foot at final temperature (W/ft^{2}/hr from charts)
The heat energy lost by conduction is determined by the surface area, the thermal conductivity of the material, the thickness and the temperature difference across the material.
Q_{LC} =  A x k x ΔT d x 3412 Btu/kW 
Where:
 Q_{LC} = kWh lost by conduction
 A = Area of the surfaces in square feet
 k = Thermal conductivity of the material in Btu/inch/square foot/hour (Btu/in/ft^{2}/hr)
 ΔT = Temperature difference in °F across the material [T2  T1]
 d = Thickness of the material in inches
Summarizing Energy Requirements
Equations 5a and 5b are used to summarize the results of all the other equations described on this page. These two equations determine the total energy requirements for the two process conditions, startup and operating.
Equation 5a — Heat Energy Required for StartUpQ_{T} =  (  Q_{A} + Q_{F} [or Q_{V} ] t  +  Q_{LS} + QLC 2  )  (1 + SF) 
Where:
 Q_{T} = The total energy required in kilowatts
 Q_{A} = kWh required to raise the temperature
 Q_{F} = kWh required to change the material from a solid to a liquid
 Q_{V} = kWh required to change the material from a liquid to a vapor or gas
 Q_{LS} = kWh lost from surfaces by radiation, convection and evaporation
 Q_{LC} = kWh lost by conduction
 SF = Safety Factor (as a percentage)
 t = Startup time in hours^{2}
Q_{T} =  (Q_{A} + Q_{F} [or Q_{V}] + Q_{LS} + Q_{LC})(1 + SF) 
Where:
 Q_{T} = The total energy required in kilowatts
 Q_{A} = kWh required to raise the temperature of added material
 Q_{F} = kWh required to change added material from a solid to a liquid
 Q_{V} = kWh required to change added material from a liquid to a vapor or gas
 Q_{LS} = kWh lost from surfaces by radiation, convection and evaporation
 Q_{LC} = kWh lost by conduction
 SF = Safety Factor (as a percentage)
Equipment Sizing & Selection
The size and rating of the installed heating equipment is based on the larger of calculated results of Equation 5a or 5b.
Notes —
Loss Factors from charts in this section include losses from radiation, convection and evaporation unless otherwise indicated.
Time (t) is factored into the startup equation since the start up of a process may vary from a period of minutes or hours to days.
Operating Requirements are normally based on a standard time period of one hour (t = 1). If cycle times and heat energy requirements do not coincide with hourly intervals, they should be recalculated to a hourly time base.