Feature Pumps Centralised pumping makes economic sense


is increased by running at a lower than necessary operating pressure, the size of the vacuum equipment is also increased along with operating costs. Referring to the previous example, assume there is a total system-wide leak rate of 50m3


/hr at atmospheric pressure: am3 am3


Centralised vacuum pump systems are suitable for use in the food industry


Engineering managers are always looking to reduce operating costs and become more energy efficient, and one way of doing this is through centralised compressed air systems. For those using vacuum as part of their processes, a similar approach can be applied, as Mark Sumnall, UK sales director at Busch (UK), discusses


ngineering staff may not be as familiar with vacuum as they are with compressed air, but correct specification and sizing is impor- tant when designing a new system or upgrading an existing installation. Over specification can be as problematic as a system that does not deliver enough vacuum and it is often perceived that vacuum efficiency operates near the end pressure capability of the pump rather than what is needed for the process or application.


E


A common vacuum application is the picking up or moving of products using suction cups. Where multiple machines and lines are used, they are often in a manifold arrangement which feeds into a piping system, supported by a centralised pump set.


The function of the pump set is to remove the volume of atmospheric air leaking into the vacuum system (expressed as standard cubic metres per hour (m3


/hr) as it is at atmospheric


pressure) to maintain the desired operating pressure level.


At an operating pres- sure of 500mbar the point is reached where the ratio of the pressure across the leak point is 2:1. Here sonic velocity is reached, where the rate of atmospheric air entering the vacuum system becomes fixed or constant.


This flow is expressed mathemati- cally by: Q (Flow, m3 Orifice x Velocity


/hr) = Area of the


Flow rates may continuously vary or fluctuate initially and then become fixed. To properly size the capacity of the vacuum source requires an under- standing of the operating conditions and dynamics of the process. As atmospheric air (or m3


/hr) enters


Many such systems operate at pres- sure levels around 200 mbar with mini- mal or no regulation of the vacuum source. While pressures at this level will not cause any issues, many will only require a vacuum level of around 400 mbar to operate efficiently. The lower pressure level does not provide any improvement in throughput, output or production levels over that which is required.


Process & Control JULY/AUGUST 2013


the vacuum system, the air within is expanded and the pressure is reduced. The air under the vacuum condition is now expressed as actual cubic metres per hour (am3


/hr), which is the volume


of the air at the operating pressure, or vacuum level.


Whether the leak is fixed or constant the lower the operating pressure level in the vacuum system, the higher the rate of expansion, which is am3 Therefore, when the volume of am3


/hr. /hr


Sizing the vacuum source requires a full understanding of the process (above)


A common vacuum application is the picking up or mov- ing of products using suction cups (left)


• To operate at 200 mbar requires 425 /hr


• To operate at 400 mbar requires 212 /hr


The size of vacuum equipment is determined by the level of am3


/hr


required at the operating pressure. With a constant leak rate, to operate continu- ously at 200 mbar would require a pump twice the size as the one operat- ing at 400 mbar (this is a factor of 2). For example, in comparing oil lubri- cated rotary vane vacuum pumps: • To deliver 425 am3


/hr @ 200 mbar


requires 15 HP (11 kW) of pump capacity • To deliver 212 am3


/hr @ 400 mbar


requires a 7.5 HP (5.5 kW) of pump capacity


The same ratio in pump capacity (and therefore the pump size) would hold true in a comparison of any simi- lar positive displacement vacuum pumps. Note that required levels of HP (kW) will vary depending on the tech- nology so further savings may be avail- able by changing to a more energy efficient pump.


Like compressed air equipment, vacuum system efficiency can be enhanced by changing the operating pressure level. Changes made by raising pressure (or decreasing the vacuum level) in a constant leak type applica- tion can have a dramatic impact on vacuum pump size, capital outlay, energy and maintenance costs. Advances in controls and regulation enable ‘on-demand’ vacuum system performance. For example, combining dry running vacuum pump technology with variable speed controls means the engineer can ‘dial in’ the exact level of vacuum required, minimising waste by running only those pumps needed, at the required speed.


Consideration should be given to pressure drop in the equipment, piping, or components and no more than 10% from the base operating pressure should be realised. Overcoming exces- sive pressure drop cannot be resolved with a larger pump and operating at lower pressure levels — it needs an effi- cient system design and specification at the outset.


Busch (UK) T: 01952 677432 www.busch.co.uk


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