BLOWERS EXHAUSTERS. Positive Displacement blowers (PD), as the name implies, compress CONTINENTAL INDUSTRIE

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1 COMPARISON Blowers Exhausters Page 1 of 6 Technical comparison... P 4 Positive Displacement blowers (PD), as the name implies, compress an incremental volume of gas between the inlet and outlet of the machine. In comparison to a centrifugal displacement blower s relatively horizontal performance curve the positive displacement blower has a nearly constant output volume at any pressure the process demands. Due to the compression of fixed volumes in each revolution, the PD blower s output volume is governed by the speed of rotation of the drive irrespective of discharge pressure. Volume can be varied only by change of speed, a mechanical change within the machine of venting excess gas. Weight flow can be varied by inlet throttling, but, that makes the PD very inefficient and may even damage it. PD blowers are used primarily where a comparably priced centrifugal machine cannot achieve the desired discharge pressure or where a constant volume is needed at a widely varying pressure. That implies that the PD blower s niche is producing a relatively high pressure at a small volume and not producing very high pressure at any volume.

2 COMPARISON Blowers Exhausters Page 2 of 6 Technical comparison... P 4 PD compressors should not be used where a centrifugal can conveniently do the job. Why? THEY ARE NOISY : As each incremental volume is compressed and suddenly released at the discharge of the compressor, a high-pressure pulse is generated. The ear translates these pressure pulses into noise. The noise is very annoying since it has fixed frequency determined by the rotational speed and the number of pulses per revolution. THEY VIBRATE : The act of compression and release generates repetitive stress in equipment and associated structures. The PD blower requires a very well designed base both to hold it in place and to prevent destruction of the building where it s located. Root s catalog recommends a reinforced concrete pad some four feet thick in order to adequately isolate a 4000 ACFM PD blower. They can produce a very high pressure if a blockage should occur. This characteristic necessitates a relief valve. Those are the main disadvantages of PD blowers. They are the ones that result in the requirement of expensive high quality silencers, pressure relief valves and extensive foundation work. THEY ARE FRAGILE : Every PD has somewhere within its inner recesses some highly machined close fitting parts. These parts don t like abrasives. They don t like corrosives. They especially don t like sticky crud. PD blowers don t pump liquids at all well. Centrifugals don t relate too well to most of that stuff either but in a number of applications they work just fine because proper design can circumvent the problem. A classic case would be boosting carbon dioxid from within a refinery down the pipe to a processing plant. Water is a fact of life here, both as an upset and from condensation. A little water passes right through a centrifugal but would blow up a PD. CONCLUSION : We can t stress strongly enough that if a centrifugal blower can do the job, there is no excuse for using a PD blower. The only exception to that rule would be if the process really needs a constant volume, but has widely varying pressure requirements. From the 77 size machine upwards centrifugal blowers have a better efficiency than a PD.

3 COMPARISON Blowers Exhausters Page 3 of 6 CENTRIFUGAL BLOWER EFFICIENCY : Constant efficiency -no wearing Of internal parts -ample clearance Throughout the blower. HORSEPOWER : The horsepower input to the centrifugal blower will vary inversely with the ratio of absolute temperatures at a constant air volume requirement. The increased horsepower due to lower air temperatures, (And increased pressure due to lower temperature) can be effectively throttled out with the use of a simple inlet butterfly valve. It should be noted that since weight flow increases with decreased temperature that the actual volume required is less at reduced temperature. Technical comparison... P 4 POSITIVE DISPLACEMENT BLOWER EFFICIENCY : Higher efficiency at design operating speed when new close tolerance - wearing parts cause loss in efficiency. HORSEPOWER : In the PD blower is a constant value and is independent of air temperature, except for increased slippage due to increased air density. Slippage depends upon a particular manufacturer s basic design.

4 COMPARISON Blowers Exhausters Page 4 of 6 CENTRIFUGAL BLOWER VOLUME: The centrifugal is a variable volume machine. The complete volume range can be achieved by simple throttling at the inlet by a butterfly valve. Usually the characteristic curve rises considerably from design volume to minimum volume ; however, flatter curves can be designed for special applications. When throttling the centrifugal blower, the horsepower decrease, is proportional to the air volume handled. The centrifugal blower can be throttled to any specific operating condition dictated by the plant. Technical comparison... P 4 POSITIVE DISPLACEMENT VOLUME: The PD blower is a constant machine. Unless the speed is changed, these machines deliver a constant volume of air. Control is accomplished by bleeding off discharged air to atmosphere. There is no horsepower saving.these machines cannot be throttled because their characteristic in this condition is increased horsepower and excessive pressures.

5 COMPARISON Blowers Exhausters Page 5 of 6 CENTRIFUGAL BLOWER PRESSURE : No emergency pressure relief is ever needed on a centrifugal blower since its curve does not permit it to reach excessive or destructive pressures. No check valve is required. TEMPERATURE : No high temperature safeguards are required since the design pressure ratio of centrifugal blowers cannot produce excessive or unsafe temperatures even if the inlet temperature reaches 104 F (40 C). NOISE : The noise level generated by the centrifugal blower is in the range of 70 to 95 db. The centrifugal blower is a naturally quiet machine without silencing equipment. Technical comparison... P 5 POSITIVE DISPLACEMENT BLOWER PRESSURE : The PD blower must be protected from overload by a pressure relief valve of suitable the size. A check valve should be fitted on outlet side to prevent the blower from running backwards when switched off whilst running on load. TEMPERATURE : The PD blowers can produce excessive discharge temperatures if the inlet temperature is higher than 20 C. In this case it is necessary to provide the blower with a water cooling system. NOISE : The noise level generated by the PD machine is in the range of 110 to 120 db. This can possibly be reduced to about 100 to 105 db with appropriate silencing equipment in the inlet and outlet. Both result in loss of efficiency.

6 COMPARISON Blowers Exhausters Page 6 of 6 CENTRIFUGAL BLOWER PULSATION : Centrifugal blowers produce a smooth non-pulsating air flow when operating at any point beyond the surge limit. Since horsepower is in direct proportion to the volumetric demand, an ammeter is calibrated in CFM for indicating airflow when required. Since our centrifugal blowers all have outboard mounted bearings, no chance exists for lubricant to contaminate the air stream. BEARINGS : Bearings and carbon ring seals are installed in outboard mounted housings and they are easy to reach for maintenance and replacement. SIMPLE REASON : The only rotating part is the shaft and impeller assembly. Absence of wearing parts like gears. Due to the relatively large clearance, it is not a necessity to install high efficiency filter on inlet. Most centrifugal blowers are grease lubricated. Oil lubrication is preferred only on large units. Relatively lightweight -no special foundation required. Technical comparison... P 6 POSITIVE DISPLACEMENT BLOWER PULSATION : PD Units all deliver a pulsating air flow. Airflow measuring device required. PD Units are internally lubricated and there is a strong possibility that oil will contaminate the air flow delivered at the discharge of the unit. BEARINGS : Bearings, gears and seals are not easily accessible. SIMPLE REASON : Complex design. The rotary lobes and the timing gears have to be machined to the highest degree of precision. PD blowers need oil lubricated high efficiency filters, and have ball and roller bearings. Normally a special foundation is required due to the heavy weight of the entire blower assembly.

7 Page 7 of 8 CONSTANT OUTPUT VOLUME VOLUME CAN BE VARIED ONLY BY CHANGE OF SPEED COMPLEXITY OF PD BLOWER Annex Fig.1 EXTREMELY NOISY 110 / 120 dba at 1 meter Acoustic enclosure is mandatory Multitude of BEARINGS (not easily accessible). Increased maintenance costs. SILENCER HIGH VIBRATION LEVEL SILENCER GEAR IS AFFECTING OVERALL EFFICIENCY GEAR COMPLEXITY OF POSITIVE DISPLACEMENT DESIGN = HIGH INVESTMENT MAINTENANCE COST

8 Page 8 of 8 PARTS LISTS (1) Interstage labyrinth (7) Eye seal (2) Double eye seal (8) Oil slinger (3) Baffle (9) Inner reservoir (4) Balance piston (10) Outer reservoir (5) Carbon ring seal (11) Impeller (6) Cooling fan (12) Bearing SIMPLICITY OF LOW SPEED BLOWER = RELIABILITY Annex Fig.2 LOW SPEED MACHINE (DIRECT DRIVE), SIMPLE SPLASH TYPE LUBRICATION DIRECT DRIVE FOR A BETTER EFFICIENCY Vaneless diffuser SIMPLICITY OF MULTI STAGE = LOW INVESTMENT MAINTENANCE COST