Solar water heaters.

Text 3

Words and word combinations to be remembered:

1.dead load	Вес конструкции	5. frequency	Частотность
2.live load	Временная (рабочая) нагрузка	6.factor of safety	Коэффициент прочности
3.resist	Сопротивляться	7.ratio	Отношение
4.magnitude	Размеры, величина

Read the text carefully so as to be ready to discuss it:

Gravitational forces

The gravitational force on a structure can be divided into dead loads and live loads. (1)Dead loads can be calculated accurately because they rarely change with time and are usually fixed in one place. (2)Live loads are always variable and movable, so no exact figures can be calculated for these forces.

Structures must also resist other types of forces, such as wind or earthquakes, which are extremely variable. It is impossible to predict accurately the magnitude of all the forces that act on a structure during its life;(3) we can only predict from past experience the probable magnitude and frequency of the loads.

Engineers never design a structure so that the applied loads exactly equal the strength of the structure. This condition is too dangerous(4) because we can never know the exact value of either the applied loads or the strength of the structure. Therefore, a number called “factor of safety” is used. The safety factor is defined as the ratio of the probable strength of the structure and the probable loads on the structure. (5)This factor may range from 1-1 (where there is little uncertainty) to perhaps 5 or 10 (where there is great uncertainty).

Text-based assignment:

I. General understanding. Answer the questions:

1. Can the loads from internal partitions of a building be estimated accurately? Why?

2. Can the loads from the storage in a building be estimated accurately? Why not?

3. How can an engineer predict the possible loads that will occur on a structure?

4. Why do engineers never design a structure so that the applied loads exactly equal the strength of the structure?

5. When there is great uncertainty about the loads on a structure and the strength of a structure, does an engineer choose a high or low safety factor?

6. When does failure occur? We mistake when we not take into account all forces and loads in a structure.

Read the text carefully and then do assignments after it:

Solar water heaters.

Solar water heaters consist of a solar collector and a well-insulated tank. The function of the collector is to absorb solar radiation during the day and transfer heat to the water. The tank serves to store the water with little temperature loss until it is required. Various types of absorber units are found, but the relatively simple flat-plate collector is most commonly used. The collector normally consists of the following components: an absorber or heat-exchanger element, insulation, cover and absorber box.

I. Read the following descriptions of the components of a collector and name them:

1) A thick layer of material with a low U-value (коэффициент теплоусвоения) placed behind the unexposed side of the collector plate. Insulation

2) A weatherproof container made of wood, metal, or plastic. Cover and absorber box.

3) Two flat sheets of transparent (прозрачный) glass or plastic. Absorber

4) A sheet of flat material constructed so that channels are formed through which water can circulate. It is painted black. Heat-exchanger element

II. Match these functions with the components of the collector:

1. To prevent rain from affecting the insulation and to provide a support for the transparent cover. Absorber

2. To absorb the maximum amount of radiation and to prevent the minimum of radiation being re-radiated. Insulation

3. To reduce heat losses to the rear to a minimum. Insulation

4. To reduce convection (теплообмен) and low-temperature radiation heat losses from the absorber surface. Insulation

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