31 Steam
Learning Objectives
Explain the production of steam in the plant
Explain the transportation of steam in a plant
Explain the use of steam in a plant
Introduction
There are three states or phases of water: ice, water, and steam. When water is heated past the boiling point, it forms steam. When water boils on the stove what we see is a mixture of steam and tiny water droplets that have condensed. Pure steam is invisible.
Steam is particularly useful because water can store much more heat energy than most other substances. A large amount of heat is required to boil water and make steam (think of how long it takes for a kettle to boil water.) This energy is stored in the steam and can be used to heat something up or do work. Water and steam are also relatively safe and can be transported easily through pipes.
There are 2 types of steam:
- Culinary steam is used to heat food product because it is sanitary, has no chemical contaminants, and can come in contact with food, or a surface that touches food.
- Plant steam may have chemical additives and must not come in contact with food or food surfaces. Plant steam is used for heating the plant and doing mechanical work.
Definitions
- Condensation – conversion of water from vapour to a liquid. Water vapour from the air condenses on your cold glass on a hot summer’s day forming liquid water droplets.
- Culinary steam – steam that is suitable for direct contact with food or surfaces that contact food.
- Heat transfer – heat energy moving from one object to another as the result of a temperature difference.
- Indirect heating – heat moving through an object into another. Heat can be easily transferred from a flame or steam, through a steel pot, into food.
- Plant steam – steam that is used for plant heating or other mechanical work. Plant steam can not come in contact with food.
Generation
Chemical energy contained in coal, gas, oil, or other boiler fuel is converted into heat energy when burned. This heat energy is transmitted through the wall of the boiler furnace to the water. The temperature of the water increases due to this added energy until it boils. The steam produced is contained in the closed boiler system- it has no place to escape so it produces pressure. Steam pressure can be very dangerous but is also useful in operating steam powered equipment.
Effective Heat Transfer
Steam is used to heat food products. The most efficient transfer of heat occurs when there is a large temperature difference between the steam, and the food product. However, certain things can get in the way and reduce the efficiency of the heat transfer.
Figure 1 shows examples of layers of films that can build up on the product and steam side of a heat transfer surface. Some of the most common examples of these films in food processing operations are: stagnant layers of product, burned-on product, rust, scale, water (condensate), layers of single-celled organisms, and air. These films must be eliminated or reduced to o