In our previous post,you came to know how single stage reciprocating air compressor works.The working principle of air compressor is same in all types of reciprocating air compressor.The difference is in stages of compression, construction, cooling media etc. This post will give you clear idea of working principle of Double Stage Reciprocating Air Compressaor.

Double stage reciprocating air compressor consist of two cylinders. One is called LOW PRESSURE CYLINDER and another is called HIGH PRESSURE CYLINDER.When piston in low pressure cylinder is at it’s outer dead centre (ODC) the weight of air is zero (neglecting clearance volume), as piston moves towards inner dead centre (IDC) pressure falls below atmospheric pressure and suction valves opens due to pressure difference.The fresh air is drawn inside the low pressure cylinder through air cleaner.This air is further compressed by piston and pressure inside & outside the cylinder is equal, at this point suction valves closed.As piston moves towards ODC compression of air took place and when the pressure of air is in range of 1.5 kg/centimeter square TO 2.5 kg/centimeter square delivery valves opens & this compressed air is then entered into High pressure Cylinder through INTER COOLER.This called as Low Pressure Compression.If suction and discharge stroke took place on both side of piston then it is called Double Acting Low Pressure Compression.

When air pressure in high pressure cylinder is below to the receiver pressure,suction valves of high pressure cylinder opens & low compressed air from Low Pressure Cylinder drawn into High Pressure Cylinder.As piston moves towards the ODC, this air is further compressed.When air pressure from low pressure cylinder and inside the high pressure cylinder is equal,suction valves closed.Now air is further compressed by piston untill the
pressure in the High Pressure Cylinder exceeds that in the receiver & discharge valves opens.This desired high pressure air is then delivered to receiver.Same procedure is repeated in every cycle of operation.If suction & discharge stroke took place on both side of piston then it is called Double Acting High Pressure Compression.In Double Stage Reciprocating Air Compressor AIR PRESSURE can be developed in range of 5.5 kg/centimeter square TO 35 kg/centimeter square.

Normally where we require AIR PRESSURE above 7.0 kg/centimeter square & delivery of air above 100 cubic feet/min. this DOUBLE STAGE RECIPROCATING AIR COMPRESSOR is used. This is most common model
used in various plants.

Posted by: The Aeroflon Team

In single stage reciprocating air compressor the entire compression is carried out in a single cylinder.If the compression is affected in one end of the piston & cylinder then it is known as Single Acting & if the compression is affected in both ends of piston & cylinder then it is known as Double Acting reciprocating air compressor.

The opening & closing of simple check valve (plate or spring valve) is depend upon difference in pressure,if mechanically operated valves are used for suction & discharge then their functioning is controlled by cams.Now come to main point “How Single Stage Reciprocating Air Compressor Works?” The weight of air in the cylinder will be zero when the piston is at top most position,if we neglect clearance volume.When piston starts moving downwards,the pressure inside the cylinder falls below atmospheric pressure and suction valve/inlet valve opens.The air is drawn into the cylinder through air cleaner (SUCTION STROKE).

When piston moves upwards,compresses the air in cylinder & inlet valve closes when pressure reaches to atmospheric pressure. Further compression follows as the piston moves towards the top of its stroke until,when the pressure in the cylinder exceeds that in the receiver. (COMPRESSION STROKE.)

At the end of this stroke discharge/delivery valve opens & air is delivered to receiver for reminder of the stroke. (DISCHARGE STROKE)

When it is double acting reciprocating air compressor,suction stroke is in process at one end of piston while at same time discharge stroke is in process at other end of piston. In simple word we can say that suction & compression took place on both end of piston & cylinder in double acting reciprocating air compressor.

The working of Double Stage (Two Stage) reciprocating air compressor will be describe in next post.

Posted by: The Aeroflon Team

Factors that affect Cylinder Lubrication

Inlet Gas Debris – Even when the proper rate and lubricating medium are in use, dirt and foreign matter in the gas will prevent the lubricant from performing properly. Inlet gas debris screens with a maximum 50-micron mesh opening are recommended. Proper maintenance of inlet screens is required to minimize pressure losses in the inlet piping and between stages.

Excessive differential pressure is the best indicator of a plugged screen. Removal of the screen, cleaning and re-installing is absolutely critical to ensure as much as possible that all debris is collected and removed. Removal of a plugged screen without cleaning and re-installing it will allow debris directly into the cylinder and should never be practiced.

Oil Dilution – Cylinder lubrication requirements will vary with the operating conditions and the composition of the gas to be compressed. Careful consideration must be given to proper cylinder lubrication selection. The degree of cylinder oil dilution by the process gas stream is influenced by the following factors: process gas composition and specific gravity (SG) – usually the higher the SG, the higher the pressure, the greater the oil dilution; discharge gas temperature – the higher the cylinder discharge temperature, the less the oil dilution; lubricant selection – some types of oil are more prone to dilution than others; CO2 or H2S content – these two components will compound the dilution effects of the gas and create acidic conditions in the cylinders.

Liquids in Gas – The use of higher viscosity lubricants or specially compounded lubricants can compensate for the presence of liquids in the gas stream. When there are liquids present in the gas, the most effective lubrication of the cylinders and packing required removal of the liquids before the gas enters the compressor.

An interesting point to note is that if there is a small amount of liquid in the gas stream, it will go to one specific point in the compression system. In a plant with several compressors, a small liquid problem may cause a lubrication problem on one compressor but not the one next to it.

Source: An Overview of Compressor Lubricants and Compressor Lubrication, Lingel, Clinton D.

Posted by: The Aeroflon Team

Lubrication Optimization

The base rate is a starting point for every force need lubrication system. After the initial break-in period, the lube rate should be adjusted to the normal cycle time recommended by the manufacturer. With periodic inspections, however, it is possible to optimize the lubrication rates to meet the requirements of your particular operation.

Inadequate lubrication results in extremely rapid break-down of Teflon and PEEK piston and packing ring materials. Black, greasy deposits, which can be found in the distance piece, packing case, cylinder and valves are indicators of under-lubrication.

Excessive lubrication can result in excessive oil carryover into the gas stream, and increase quantities of deposits in the valves and gas passages. Valve plate breakage and packing failure due to “stickation” are also symptoms of over-lubrication.

Neither under-lubrication nor over-lubrication is an optimum operating condition. Manufacturer’s recommendations are designed to cover a wide range of conditions. For instance, one operation that has 5% CO2 with very clean, dry gas is significantly different than 5% CO2 with water saturated gas. However, the design must cover both of these possible scenarios.

To optimize the system, start by inspecting the cylinders and packing to ensure that they are in good condition. If so, reduce the lube rate by 10% (divide the cycle time by 1.10). In a month, inspect the cylinder discharge passage and the discharge valves for evidence of the black greasy deposit created by under-lubricated Teflon. Check the distance piece and the piston rod by removing a side cover door. Make sure there is not a build-up of Teflon at the back of the packing.

Also look at the oil in the gas passages and in the distance piece. If possible, take a sample of oil out of the packing vent or from the downstream scrubber. Make sure the oil looks new and is not darkened by Teflon or from thermal stress. An oil analysis may provide very useful information.

If none of the checks indicate any lack of lubrication, reduce the lube rate another 10% and repeat the inspection process again. If time is available to remove the cylinder heads, the cigarette paper test is also a very good inspection.

Lubrication Paper Test Method

The cigarette paper test method can provide a practical indication to check cylinders for the proper lubrication rates. Relieve and vent all pressure from all cylinders. Remove a head end suction valve and position the piston at inner center, for the cylinder to be checked. Use two layers of regular un-waxed commercial cigarette papers, together. Wipe the cylinder bore at the top with both papers using light pressure in circumferential motion through about 20 degrees. The paper next to the bore should be stained with oil, but the second paper should not be soaked through.

Repeat the test at both sides of the bore at about 90 degrees from the top, using two new clean papers for each side. When the paper next to the bore is not stained though, it may be an indication of under-lubrication. When both papers are stained through, it may be an indication of over-lubrication. In either case, it is normally recommended that the lubrication rate be changed accordingly and that all cigarette paper tests be repeated until passed. Repeat for all cylinders.

The cigarette paper test only gives an indication of oil film quantity. It does not give an indication of viscosity quality. Oils diluted with water, hydrocarbons or other constituents may produce what appears to be an adequate film. But the oil film may not have the required load-carrying capability because of dilution.

Cylinder and Packing Oil Recommendations

The following matrix shows recommended oil for a given discharge pressure and type of gas. These lubrication recommendations are general guidelines. If the recommended lubricants or flow rates do not appear to work adequately, flow rates and/or lubricant types may need to be changed. Please contact the lubricant supplier for specific lubricant supplier for specific lubricant recommendations.

Base Rate Calculations

There are many different ways to calculate the amount of oil that is injected into the cylinders. Most manufacturers historically have used a specific base amount of oil and factored that by the diameter of the cylinder or piston rod. Other components of the calculated lubrication rate may include piston stroke and speed. The total amount of oil has been specified by a total volume of oil or by the number of drops per minute. B far, the more accurate measure for lubrication is the volume of oil.

Drops per minute only provide the correct amount of oil at a specified temperature. If the temperature is colder or hotter, the size of the drop can change dramatically. This may either inject into the system. The smaller drop of hotter lubricant will not provide the same lubrication as the larger drop of colder lubricant.

A total volume of oil such as pints per day is more accurate because it will inject a consistent amount of oil into the system regardless of the temperature. A base rate set by a number of pints per day per inch of bore diameter is one common calculation method to determine the total amount of oil required by that point. The base rate is determined by:

Base Rate (pints per day) x Bore (inches) = Total pints per day at full rated speed

A second calculation is then needed to determine the cycle time for a given divider block and to modify that rate for reduced speed.
Another calculation method injects one pint per day per 2,000,000 ft2 of cylinder surface area.

(Bore x Stroke x rpm) ÷ 31800 = Pints per day

Again, a second calculation is needed to determine the cycle time for a given divider block.

Once the total pints per day are known for each injection point, the proportion of the total is used to determine the size of distribution block needed. Determining the proportions of the blocks needed is done by using ratios of the required rates and then selecting a block that provides the closest match. Usually a range of 90% to 115% will allow for selection of a block.

Finally, once all the blocks are known, the cycle time can be calculated by the following equation.

Cycle Time = (6 x Sum of all blocks) ÷ Total Quantity of Oil.

Compressed air is a versatile tool used widely throughout industry for a variety of purposes. Unfortunately, running air compressors often uses more energy than any other equipment.

• This is the usual unit of measure for discharge air from a compressor
• CFM is the acronym for Cubic Feet per Minute. A compressor is said to have so many cubic feet of compressed air per minute (CFM) of flow from it’s discharge port.
• When it comes to using compressed air in your plant or home workshop you will want to know how many cubic feet per minute you can expect from the discharge port of your compressor to help determine if that compressor has sufficient compressed air flow to power your air tools or other air-consuming applications.
• To do that you need to know what CFM a particular device or tool will require to function within it’s design parameters. The device or tool will require a certain number of CFM at a specific air pressure, to work properly.

A rule of thumg is that 1 HP generates about 4 CFM at the rate of 90 PSI

• This is an industry standard, though it doesn’t apply to most compressors under 10 HP. For compressors smaller than 10 HP, you will need to read the specifications for that particular unit to determine their flow and pressure rates or use the “guesstimate” of 2 CFM at 90 PSI per HP of electric motor
• When you’ve sized all of your applications and totalled up all of the air you’re going to need now and for the future expansion you may be undertaking in the future, and you are out searching for the right air compressor, you would divide the number of CFM you need by 4, and that will give you a rough idea of the horsepower rating of the compressor required.

Be Careful. Not all compressor manufacturers rate their compressor output the same way. You might see a compressor showing a discharge rate at what appears to be an acceptable CFM, but on closer inspection find that the figure is predicated on a much lower pressure than you might need.

Discharge rates in CFM at higher pressures are always quite a bit lower than discharge rates at lower pressures, for that same compressor.

Ensure that the unit you select will give you both the CFM you need, and the pressure your equipment demands to work properly for you.

 Posted by Mitul Choksi