HOW OVER-SIZED EQUIPMENT CAN BE A DRAIN ON YOUR RESOURCES

Where resource efficiency is concerned,
bigger is certainly not always better!

Capacity is such a vital aspect of manufacturing success that it is not unusual to find industrial sites where equipment is far bigger than it needs to be. Part of the reason for this is the need for production plants to be flexible, in which case excess capacity is probably justifiable. It is also often the case that over-sized equipment is installed with an eye on future expansion. The thinking in this case is that the slack will eventually be taken up. Then of course there is the case of the "prudent" engineer, who installs over-sized equipment simply to be absolutely sure that required production levels are reliably met. This “just in case” approach is in my experience probably the most prevalent reason for the large amount of over-sized equipment I come across.

Specifying equipment that is too large for the duty obviously incurs unnecessarily large capital costs. But a far more insidious impact is the inefficient manner in which oversized equipment consumes resources. When you consider that the life cycle costs associated with most process equipment are typically dominated by the costs of resource consumption rather than the initial capital outlay, it is easy to see how what seems like a prudent capacity decision at the time of purchase can have seriously detrimental impacts on profitability.

In general, equipment tends to operate best at the capacity level for which it was designed. To illustrate this point, let’s look at a few examples of oversized equipment and explore the impact of operating them at capacity levels significantly below design ratings.

CASE 1: UNDER-LOADED INDUCTION MOTORS

Induction motors have a nameplate capacity that determines the amount of shaft power they develop when under full load. The input power to the motor under full-load conditions may be estimated by dividing the rated capacity by the full-load efficiency of the motor. Hence, as an example, a 7.5kW motor with a full-load efficiency of 88.6% would have an input power at full load of 7.5 kW / 88.6% = 8.47kW.

Motors rarely operate under full load for extended periods of time. The loading of a motor may be assessed by measuring input power and then dividing it into this full-load input power figure.    

Motor loading = measured input power / (Rated full-load power / ƞ_full-load)

Hence, say for the 3-phase induction motor in our example we measured the input power of the individual phases under typical operating conditions and determined the total input power to be 4.3 kW. The loading of this motor would then be 4.3kW / 8.47kW = 51%. This motor would clearly be significantly under-loaded. A loading of less than 100% is not necessarily a bad thing, the key consideration is where the loading level places the motor along its efficiency continuum.

The efficiency of an induction motor varies significantly with load. Below is a typical induction motor efficiency curve.

As shown, at loadings slightly below 100%, motor efficiency is actually greater than at full load. However, at low loadings, we see that motor efficiency drops off sharply. Based on this efficiency curve, running a motor at the loading we calculated above would result in the motor operating at an efficiency level of some 90% of full-load efficiency. Hence the efficiency level of the motor would be 90% x 88.6% = 79.74%. Should this particular motor operate continuously, this efficiency deficit can cause operating costs to mount up very quickly.

For this situation, it should be clear that the use of a motor with a higher full-load efficiency than the one installed would not solve our problem. The solution would be to specify a smaller motor, which would mean operating at a higher loading and hence a movement up the efficiency curve to a level closer to full-load efficiency.

 CASE 2: UNDER-LOADED BOILER

Boilers are rated to deliver a specific quantity of steam at a given temperature and pressure, using a given fuel. At this “maximum continuous rating”, the boiler would operate at its optimal efficiency. However, when operated at throughput levels lower than the MCR, efficiency tends to decline. The reasons are to do with the fact that burners and other auxiliary equipment are designed to work best over a specific throughput range, and that shell losses, which are fixed, become a larger proportion of the overall boiler losses at lower throughputs. In addition, remember that boilers are heat transfer devices, and as such, reductions in the rate of gas flow through a boiler, as happens at lower throughputs, result in reduced heat transfer coefficients on the combustion side.

A typical boiler efficiency curve is as indicated below.

It is clear that operating a boiler at 30% of its design capacity can result in a significant efficiency drop, as compared to operating it at say, 60% of its design capacity. The difference in efficiency results in a significant increase in the amount of fuel required to produce a given mass of steam.

 CASE 3: UNDER-LOADED CIP SET

Cleaning in place (CIP) refers to the process of cleaning the product-side surfaces of processing equipment in the food industry. This is typically accomplished by pumping water and cleaning solutions through pipelines, or using spraying devices to distribute water and cleaning solutions inside process vessels. The CIP set itself comprises a series of tanks, pipelines, valves and pumps. Each tank holds an individual solution or process fluid, such as water, acid solutions, sterilising agents, caustic solutions or recovered rinse water. There are several ways in which an oversized CIP set can lead to wastage of resources. Some of these could include:

• If the tanks are too large, more fluids than are necessary will be heated and pumped for cleaning. This translates into more water, chemical and energy use than is necessary for constituting the cleaning solutions. Large tanks also translate into more heat transfer surface and hence more heat loss to the surroundings;
• If the flows employed are too high, a significant amount of energy could be wasted. Since centrifugal pumps are typically used for pumping CIP fluids, and energy consumption for these pumps varies with the cube of flowrate, small deviations in excess of minimum required flows could result in significantly more pumping energy being used;
• While CIP sets are typically designed to recover water and cleaning solutions, thereby reducing their resource consumption, some water discharge and loss of cleaning solution is unavoidable – essentially the water used for cleaning becomes too contaminated for reuse.  It is also necessary to periodically drain the system and make up fresh solutions. In the case of an oversized CIP plant, the resource losses associated with ongoing operation and intermittent refreshing of cleaning fluids tends to be higher.

There are many other common examples of oversized equipment I could cite, and in all cases, the amount of resources they consume per unit of production is higher than it would have been had they been correctly specified. Of course, it is not always possible to justify the removal of equipment and its replacement with a correctly-sized alternative, so what would the solutions to this challenge be in practical terms?

The first approach I would recommend would be to try to identify the drivers of the inefficiency behind each individual process. For example, if an under-loaded boiler performs poorly due to poor control of the air-to-fuel ratio at low throughput levels, investigate the control system. It may be the damper valve characteristic that is the problem, in which case use of a variable speed drive on the forced draft fan could help to increase efficiency levels. If the driver of inefficiency for an oversized heated vessel is the excessive surface area, look into insulation as an option. In short, try to establish cost-effective plant modification strategies as far as possible.

In many cases, modifications to the equipment itself may not be possible. For example, an under-loaded induction motor cannot be modified and requires replacement. However, this need not be expensive. It may be that a motor with the correct characteristics may be available elsewhere on your site. This is one of the reasons I recommend retaining redundant motors as spare units, even when replacements are made on the basis of installing higher-efficiency units. Old, standard efficiency motors that are properly loaded are more efficient than high-efficiency motors that are under-loaded, so there are often opportunities to deploy such motors more efficiently elsewhere should they have been replaced with more efficient alternatives. Of course, these motors need to be in good working order.

A final approach I would recommend would be to investigate production planning and scheduling to assess how this may be changed to take advantage of excess capacity in ways which minimise resource consumption. Can larger batches be produced less frequently, for example? Is there scope to operate at higher instantaneous capacities and extend maintenance and cleaning breaks as opposed to spreading production over longer periods while operating the facility at low throughput levels? Much of this approach comes down to business decisions, and considerations that impact on entire supply chains. Product characteristics also play a significant role – for example, perishable products have to be produced as close as possible to time of sale. It is therefore a case of intimately understanding your processes and taking advantage of the unique opportunities on offer, on the understanding that each facility will be different.

So, is over-sized equipment hurting your environmental performance and bottom line? Your first step should be to gather all capacity data for your facility, take the necessary measurements and find out. I can almost guarantee that you will find resource efficiency opportunities in this area.

Copyright © 2013, Craig van Wyk, all rights reserved