SIZING HEATING EQUIPMENT
Specific heat is significant because the amount of heat required to change the temperatures of different substances is used to size equipment. Recall the example of the house and furnace earlier in this unit. The following example shows how this would be applied in practice. A manufacturing company may need to buy a piece of heating equipment to heat steel before it can be machined. The steel may be stored outside in the cold at 0°F and need preheating before machining. The desired metal temperature for the machining is 70°F. How much heat must be added to the steel if the plant wants to machine 1000 lb/h? The steel is coming into the plant at a fixed rate of 1000 lb/h, and heat has to be added at a steady rate to stay ahead of production. Figure 1.15 gives a specific heat of 0.116 Btu/lb/°F for steel. This means that 0.116 Btu of heat energy must be added to 1 lb of steel to raise its temperature
1°F.
Q = Â Weight * Specific Heat * Temperature Difference
where Q = quantity of heat needed. Substituting in the
formula,
we get:
Q = 1000 lb/h * 0.116 Btu/lb/°F * 70°F
Q = 8120 Btu/h required to heat the steel for machining.
The previous example has some known values and an unknown value to be found. The known information is used to find the unknown value with the help of the formula. The formula can be used when adding heat or removing heat and is often used in heat-load calculations for sizing both heating and cooling equipment.
Copied from REFRIGERATION & AIR CONDITIONING TECHNOLOGY by WILLIAM C. WHITMAN, WILLIAM M. JOHNSON, JOHN A. TOMCZYK and EUGENE SILBERSTEIN
