1 reply to this topic

[gatty]

Hi All,
can some one tell me that why does water get accumulated in the air reservoir of a compressor and how is water formed .moreover i would be greatful if someone could really tell me all about air compressors used in industries ......i.e. its parts ,their uses and what difficulties are faced in real life while working a compressor
gautam9954@yahoo.co.in

[Art Montemayor]

Gautam:

I don’t know your student level, but by the nature of your question I can surmise that you would not feel very comfortable with - nor understand - any Thermodynamic explanation of what occurs in the compression of gases – especially air. Therefore, I’m going to employ basic, empirical engineering explanations as to what is occurring.

Water accumulates in your air reservoir (downstream of your aftercooler) because it is cooled below the temperature corresponding to its vapor pressure. This water existed originally in the suction air coming into the compressor and constitutes a condensable component when the pressure is raised and the resultant temperature is cooled

Your air compressor sucks in atmospheric air and there is no such thing as truly dry, atmospheric air. All atmospheric air contains some degree of moisture in it. This is obvious when it is considered that one of the important and perpetual functions of the air is the conveyance and distribution of water over the earth. Whenever air is commonly spoken of as “dry”, it is only meant that it is relatively drier than some other air.

Moisture which is thus intermixed with the air is in the form of perfectly transparent and invisible vapor until the saturation temperature or dew point is reached. The saturation or dew point of air is determined by its pressure and temperature, especially the latter, and is the point beyond which any excess water vapor is precipitated as actual water. Fog is caused by atmospheric air being cooled to the dew point.

When air is at the saturation point, it is said to have a humidity of 100%. The relative humidity of the air at any time is the percentage of moisture contained in it as compared with the maximum amount it is capable of holding at the same temperature. The degree of saturation or relative humidity of the air is easily determined by the used of the dry and wet bulb thermometer.

Whenever air is compressed and transmitted at the ordinary working pressure of 90 psig, the saturation point is reached, the accompanying condensation of vapor to water occurs, and the importance of the moisture-carrying powers of air may be seen.

At a fixed temperature any given volume of free air is at the saturation point (or is "saturated") when it contains a certain maximum amount of water, which depends (of course) on the temperature. If the temperature is kept constant and the absolute pressure is doubled, thus reducing the volume one-half, the moisture-carrying capacity is reduced in the same proportion. The excess vapor in the air is condensed as water. In other words, half of the original moisture is dropped out of the air as a liquid.

If, however, the humidity of the free air at sea level is 50% at the beginning of compression, it becomes 100% when the absolute pressure is doubled. If the pressure is raised to 90 psig, the humidity becomes 350% and 250% of this the air will refuse to carry as vapor and it will be deposited as liquid water.

Thus far it has been assumed that the temperature would remain constant, but this is far from the fact. As air is compressed, its temperature rises rapidly and with each rise of about 20 ^oF its capacity for moisture is almost doubled. If free air at 60 ^oF is compressed to 90 psig in a single-stage reciprocating compressor, its temperature at delivery will be well above 300 ^oF. As a result of this temperature change, its capacity for moisture will have been doubled so many times that when it leaves the compressor discharge port its relative humidity will be quite low, although it still carries all the moisture with which it entered the compressor cylinder originally.

When the air gets into the transmission lines, however, its temperature drops, reducing the moisture–carrying capacity of the air. The excess moisture condenses into liquid water, but is still mechanically mixed with the air; the super-saturated air then appears as a fog or mist. If the air line is long enough, the freed water will settle out of the air onto the side of the transmission line and will trickle down to the bottom, where in time it will form a flowing stream and will be carried along with the air current.

It should be obvious that the presence of liquid water or moisture in an air transmission line is not desirable. It can cause corrosion, blockage, tool and instrument contamination, water hammer, freezing hazards, and other harmful effects.

The most satisfactory way of preventing any of the above difficulties is by removing the moisture immediately after compression and before the air enters the distribution system. This can be done very efficiently by the use of aftercoolers which reduce the temperature of the hot air from the compressor discharge to a point where almost all of the moisture is condensed and is removed. It has been found that air for general usage can be dried sufficiently if cooled to within 15 ^oF of the temperature of the cooling water used. Obviously, the colder the water used, the colder the air will be upon leaving the aftercooler – and the more water that will be removed.

Efficient, self-draining moisture separators should be used directly downstream of aftercoolers to ensure that liquid water is removed from the compressed air system. If drier air is required for water-sensitive processes downstream, then engineered water-removal processes should be employed to remove the water content down to parts per million (by volume). These processes can be of the refrigeration type or the Adsorption type.

Air is normally compressed in a variety of compressor types:

• reciprocating;
• centrifugal;
• rotary;
• diaphragm;
• liquid piston;

You are not going to learn “all about air compressors used in industries ......i.e. its parts, their uses and what difficulties are faced in real life while working a compressor” in an Internet Forum. You have to read and study books as well as obtain hands-on experience in installing and operating these machines. You would be well-advised to jump at any opportunity to take apart and re-assemble any compressor you can get your hands on – such as a common gasoline station air compressor used to fill automobile tires. The more you do this, the more you will understand the Thermodynamics and Mechanical design that goes into each one of these types of machines.

Good Luck.