5. Fill in the gaps.

Roll mills may contain one, two, or three rolls and can be arranged for either (1)____ or continuous production. The (2)____of material is cradled between the rolls or between the roll and a feed bar, where it is (3)____ and folded. Part of the material is (4)____ and continuously drawn down between the rolls or between the roll and the feed bar where it is subjected to crushing, shearing, and abrasion.

1). A) interrupting, b) batch, c) prolonged.

2). A) mounting, b) lifting, c) supply.

3). a) kneaded, b) baked, c) made.

4). A) used, b) separated, c) nipped.

6. Complete the sentences.

This action calls for the proper formulation as sufficient liquid must be provided to wet the constantly increasing surface of the 1)____ mate­rial. This is necessary both to 2)____ the particles as they pass through the mill and to avoid the formation of new agglomerates. Since all the broken or dispersed particles in the thin film must be immediately surrounded with a film of 3)____, the feed to the mill must be uni­form and not contain lumps of dry material. The single-roll mill con­tains a wide-faced 4)___ working against a smooth hardened roll. The larger contact area of the 5)____bar and the roll gives more hydraulic shear than in two- or three-roll mills. Accordingly, single-roll mills are best 6)____ for dispersion rather than grinding. Also, they cannot efficiently handle materials of a wide range of viscosity.

a)vehicle, b)considered, c)dispersed, d) grinder bar, e) lubricate, f) rubbing.

7. Group the antonyms.

a) low den­sity, b) internal, c) inner, d) intensively, e) timely, f) high density, g) external, h) extensively, i) continuous.

Unit 11

Text 1

1. Read, translate and retell the text.

Hydraulics. Momentum Transfer.

Pumps

A). Pumps of various kinds are required everywhere. A fundamental knowledge of basic types of pumps and their exploitation are of great importance for an engineer. The conditions that will influence the selection of the type of the pump are: the type of liquid to be handled; that is its viscosity; cleanliness, temperature and so on; the amount of liquid to be handled; the total pressure against which the liquid is to be moved; the type of power to be used to drive the pump.

Pumps may be divided into four major classifications: piston pumps or reciprocating pumps. These pumps are driven by engines or electrical motors; centrifugal pumps driven by steam turbines or electric motors; rotary pumps driven by steam turbines or electric motors; fluid-impellent pumps which are not mechanically operated but are fluid-pressure operated.

B). The centrifugal pumps is one of the simplest types. It consists of an impeller spinning in a circular casing. It is cheap, occupies very little space, runs for long periods takes little attention and possesses other good qualities. The centrifugal pumps have the disadvantages that their output is effected by changes in pressure on the delivery side and they are not suitable for viscous liquids.

Unlike the centrifugal pumps, piston and plunger pumps are constant output machines. Plunger pumps are generally used for high pressures and

Fig.30. Cutaway view of a simplex piston pump.

Fig.31. Peristaltic pump.

piston pumps for lower pressures.

C). Pumps may be classed into two main groups, positive-displacement

pumps and centrifugal pumps. Positive-displacement pumps may be either a reciprocating type or a rotary type. The prime feature of a positive-dis­placement pump is that a definite quantity of liquid will be delivered for each stroke or revolution of the prime mover. Only pump size, design, and suction conditions will influence the quantity of liquid that can be delivered. On the other hand, a centrifugal pump can deliver a variable volume of fluid with varying head for a constant speed.

Positive-Displacement Pumps. Principles of Unit Operations. Reciprocating Pumps. Figure 30 illustrates a simplex power-driven piston pump. In this type of pump, the piston is connected to a suitable crank shaft which is driven by an electric motor. Steam-driven pumps are also used in the process industry.

In these pumps, liquid enters the cylinder through a check valve which is opened by an external pressure acting on the fluid. The flow of the fluid through the inlet valves follows the piston movement backward through the cylinder on its inlet stroke. When the piston moves forward, the inlet valve closes, and a second valve is forced open to discharge the liquid. The piston must have a close fit with the cylinder walls in order to minimize liquid slip past the piston. If the piston carries with it its own packing, it is called a piston pump. In contrast to this, a plunger pump uses a close-fitting rod moving through the cylinder past fixed packing. The plunger is simply an extension of the shaft. A volumetric efficiency may be defined as the ratio of actual discharge to the discharge based upon piston displacement. The efficiency for well-maintained pumps should be at least 95 per cent.

D). Reciprocating pumps are particularly well suited for pumping viscous fluids because the high rate of shear acting on the cylinder walls serves as an additional "packing." This pump is also good for attaining high pressures, and, because of its positive-displacement characteristic, it is sometimes used for metering fluids. Liquids containing abrasive solids should not.be pumped with a reciprocating pump because of damage to the machined surfaces.

Notes. Fluid-impellent pump – рідинний насос; to be fluid-pressure operated – працювати під тиском рідини; to run – працювати.