Numerical Question



Numerical Question

Solution 1

Data:

P = 600 KN/m2

n = 2000/29 = 68.96

V = 2 kg

T =?

R = 287 J/kgK

A = 2?r2

A = 2x3.142x(3) 2

A = 56.566 cm2

PV = nRT

T = PV/nR

T = 600x2/68.96x287

T = 0.06oC

Every compressed-air system begins with a compressor — the source of air flow for all the downstream equipment and processes. The main parameters of any air compressor are capacity, pressure, horsepower, and duty cycle. It is important to remember that capacity does the work; pressure affects the rate at which work is done. Adjusting an air compressor's discharge pressure does not change the compressor's capacity — even though many people seem to believe it will.

There are a number of basic air compressor designs — and variations of them — on the market today. They all fall into two general categories: positive displacement and dynamic. Although the operating specifications for two different types of air compressors may be very similar on the surface, other installation and performance factors can make one design superior to the other in a real-world application. Let's review some of the basic designs and terminology.

Reciprocating Compressors

Reciprocating compressors are positive-displacement units that trap a charge of air and then physically reduce the space that confines it, causing its pressure to increase. Reciprocating units, often called piston compressors, use a piston, cylinder, and valve arrangement. Their operation is very similar to the familiar internal-combustion engine, but they simply trap and compress the air without adding fuel to explode it. Note that whenever air is compressed, heat is generated. Proper cooling of the internal parts of any air compressor is a critical part of its design.

There are three basic selection decisions that must be made about reciprocating compressors:

• single- or double-acting operation,

• single- or multi-stage configuration, and

• air or water cooling.

In a single-acting piston compressor, the piston only compresses air in one direction of its stroke. In a double-acting model, the piston compresses air with both directions of its stroke. Obviously, because both strokes perform work, a double-acting compressor is more efficient (in moving a volume of air per input hp) than a comparable-size single-acting unit.

A single-stage unit compresses air from inlet to discharge pressure in one operation. A multi-stage unit compresses from inlet to discharge pressure in two or more operations — generally passing the air through an intercooler to remove some of the heat of compression between each stage. This saves power and keeps the compressor's internal operating temperatures lower.

In air-cooled compressors, ambient air circulates around the compressor cylinders and finned heads to provide cooling. Heat transfers through the metal to the air. Air-cooled units are generally designed for 50 to 75% duty cycles, depending on the particular units and their application. In water-cooled compressors, integral water jackets surround the cylinders and heads. Heat transfers through the metal to the water — more effectively than through metal to air. Thus, water-cooled reciprocating units reduce internal temperatures more efficiently than comparable air-cooled units.

Most air-compressor manufacturers promote the two-stage compressor as the optimum machine for producing 100-psi ...