Engineering Fundamentals: An Introduction to Engineering, 4th ed.c

236 Chapter 9 Mass and Mass-Related Parameters

9.4 Mass Flow Rate

In Chapter 8, we discussed the significance of volume flow rate. The mass flow rate is another

closely related parameter that plays an important role in many engineering applications. There are

many industrial processes that depend on the measurement of fluid flow. Mass flow rate tells engi-

neers how much material is being used or moved over a period of time so that they can

replenish the supply of the material. Engineers use flowmeters to measure volume or mass flow

rates of water, oil, gas, chemical fluids, and food products. The design of any flow measuring de-

vice is based on some known engineering principles. Mass and volume flow measurements are also

necessary and common in our everyday life. For example, when you go to a gasoline service station,

you need to know how many gallons of gas are pumped into the tank of your car. Another example

is measuring the amount of domestic water used or consumed. There are over 100 types of com-

mercially available flowmeters to measure mass or volume flow rates. The selection of a proper

flowmeter will depend on the application type and other variables, such as accuracy, cost, range, ease

of use, service life, and type of fluid: gas or liquid, dirty or slurry or corrosive, for example.

Themass flow rateis simply defined by the amount of mass that flows through something

per unit of time.

(9.5)

Some of the more common units for mass flow rate include kg /s, kg /min, kg /h or slugs /s or lbm/s.

How would you measure the mass flow rate of water coming out of a faucet or a drinking fountain?

Place a cup under a drinking fountain and measure the time that it takes to fill the cup. Also, mea-

sure the total mass of the cup and the water and then subtract the mass of the cup from the total to

obtain the mass of the water. Divide the mass of the water by the time interval it took to fill the cup.

We can relate the volume flow rate of something to its mass flow rate provided that we

know the density of the flowing fluid or flowing material. The relationship between the mass

flow rate and the volume flow rate is given by

(9.6)

The mass flow rate calculation is also important in excavation or tunnel-digging projects in

determining how much soil can be removed in one day or one week, taking into consideration

the parameter of the digging and transport machines.

9.5 Mass Moment of Inertia

As mentioned earlier in this chapter, when it comes to the rotation of objects, the distri-

bution of mass about the center of rotation plays a significant role. The farther away the

mass is located from the center of rotation, the harder it is to rotate the mass about the

 1 density 21 volume flow rate 2

mass flow rate

mass

time



1 density 21 volume 2

time

 1 density2a

volume

time

b

mass flow rate

mass

time

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