Basic Concepts and Process Variables

Process variables are chemical and physical properties involved in the streams of different processes of a complete system. The most common process variables at this stage of your development are mass, volume, density, moles, pressure, and temperature and will be discussed in some detail in terms of the main units and conversions between SI and English Engineering systems. 

As shown in Table 2.2, density is a derived unit, and its basic unit is mass per length cubic, i.e., mass per unit volume. In general, the density of solids is not temperature dependent and can be measured by immersing a known mass of a solid in a liquid and measuring the displaced volume. As we will see in Chap. 6, an estimate of the density of air, at atmospheric pressure, can be obtained through the ideal gas law. From the ideal gas law we can obtain the following expression to estimate density  

In the SI system, mass is measured in kilograms (kg) and in the English Engineering system in pounds (lb). As presented in Table 2.3, the equivalence between these systems is 

2 Fundamentals of Magnitudes, Unit Systems, and Their Applications in Process Engineering 

An easy and practical way to convert imits is by multiplying by 1, as developed in the following solution  

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