## Introduction to Food Engineering

### Time-temperature calculations in Unsteady-State heat transfer

#### Learning Outcomes

At the end of this module, you will be able to:

1. describe how Heisler charts are constructed and used
2. use a Heisler chart for a spherical-shaped object
3. predict temperature change in a cylindrical can using Heisler charts

In this module, we will consider unsteady-state heat transfer when there is a finite resistance to heat transfer both at the surface of a solid object as well as inside an object. For this purpose, we will use Temperature-time charts, known as Heisler charts. These charts provide a relationship between the dimensionless temperature ratio and a dimensionless number called Fourier number. Fourier number contains the elapsed time during heating or cooling. In the next video, you will learn how these charts are constructed for simplified geometrical shapes such as infinite cylinder, infinite slab, and sphere.

In the next video, you will examine how to use a Heisler chart to determine temperature change with time or time required for a desired temperature change for a spherical-shaped object. The same procedure will be used for any of the other shapes such as infinite slab or infinite cylinder. You must make sure that the correct chart is selected when solving a problem for a given shape.

Now that you know how to use Heisler charts for the three standard shapes, in the next video, we will consider finite-shaped objects such as a cylindrical can that is most widely used in canning foods. We will also consider a brick-shaped object since we commonly encounter rectangular-shaped packages containing foods.

Recap

In the preceding videos, you learned how Heisler charts are constructed and then used a Heisler chart to determine temperature after some time of heating or cooling of a spherical-shaped object. You also considered how Heisler charts may be used for other finite shapes such as a finite cylinder or a finite slab.