# Fluid Fundamentals

How heat gets transferred from one fluid to another depends largely on the physical characteristics of the fluids involved, especially their density, specific heat, thermal conductivity, and dynamic viscosity. *Density* (ρ) is a fluid’s mass per unit volume, measured as lb m /ft 3 (where lb m represents pounds of mass) or kg/m 3 . Density can be used to convert a measurement from a mass-flow rate, such as lb m /hr, to the more common volumetric units, such as gallons per minute for liquids, or cubic feet per minute for gases.

Throughout a heat exchanger, the mass flow rate remains constant, but changes in temperature and pressure can change the volumetric flow rate, particularly for a gas. So a gas flow should be stated as a mass flow, a volumetric flow at standard conditions, or as a volumetric flow including temperature and pressure. In any case, the operating pressure should always be specified. *Specific heat* (*c* or *c* *p* for a gas, where *p* represents a constant pressure) is the amount of heat required to raise the temperature of one unit of fluid mass by one degree. Its unit s are B T U / (lb m °C) or J/ (kg °C). Specific heat relates the quantity of transferred heat to the temperature change of the fluid while passing through the heat exchanger. *Thermal conductivity* (*k) *represents the ability of a fluid to conduct heat. It is measured in BTU/ [ft 2 hr (°F/ft)], BTU/ (ft hr °F), or W/(m °C).