How is heat transfer duty calculated?

Heat transfer can be classified in two ways:

Sensible heat transfer, also known as temperature change.   For example, it takes one BTU to raise one pound of water one °F.  The key relationship here is:

Q = Cp • M • TD

Where:
Q is BTU/hr
Cp is heat capacity in BTU/lb-F (the Cp of water is 1)
M is mass flow in lb/hr
TD is Temperature difference in °F

Latent heat transfer, also known as phase change.  For example, it takes 1000 BTUs to boil one pound of liquid water into steam – at the same temperature.  The key relationship here is:

Q = Hfg • M

Where:
Q is BTU/hr
Hfg is the latent heat of vaporization in BTU/lb (the Hfg of water is 1000)
M is mass flow in lb/hr