Pressure and temperature are two of the most important of chemical engineering process variables.

In addition to reminding you a bit about them, it is importnat to discus how one might find out what they are in a given process!

DEFINITION:

**Pressure** is the ratio of force to the
area over which that force acts (or the area over which that force
is transmitted).

What ar the units of pressure then?

Pressure on a solid surface is easy to follow.

The pressure in a fluid is tougher. One way to think of it:

DEFINITION:

**Hydrostatic Pressure** is the pressure
in a fluid due solely to the fluid's weight. [$\rho g h$].

EXERCISE:

Check the dimensions on $\rho g h$.

NOTE:

For this reason, pressure is sometimes given in terms of fluid height instead of regular pressure units. (e.g., "mmHg" or "ft of water")

Pressure measurements:

Manometers

OUTCOME:

Calculate pressures using a manometer (and explain the concept of gauge pressure)

TEST YOURSELF!

If we know P_{2} and the difference in
fluid heights, how do we calculate P_{1}?

Hydrostatic Pressure

Consider a cylindrical tank filler with water:

We want to measure the hydrostatic pressure on the bottom of the tank.

Volume of cylinder (fluid): $\pi r^2h$

Weight of fluid: $\rho g V$

Area of base: $\pi r^2$

Pressure on Base: F/A -> $\frac{\rho g(\pi r^2h)}{\pir^2}$
-> $\rho g h$

So, hydrostatic pressure is independent of radius of cylinder!