Fans, pumps, boilers and furnaces etc. are used in most industrial processes to convert material from one form into another — such as iron into steel, or pulp-fiber into paper. The conversion of one material, or mixture of materials, to a more complex form is often measured in tons per unit of time (such as minutes, hours or days) and is called the throughput.
When sizing industrial equipment one starts from a known quantity of throughput. All of the ancillary equipment will then be designed and selected based on achieving that production value plus some percentage for future expansion.
But often the extrapolation from the known to the anticipated is when things can get out of hand, as projected values will often be hedged higher and higher again.
For instance, say we are dealing with a refinery that is cracking crude oil into the various hydrocarbons such as Bunker C Oil, gasoline or diesel.
Again the throughput is known or decided in advance, in other words how much crude oil will be processed at this unit.
From this, coupled with the knowledge of similar operating units, the ancillary equipment such as boiler fans etc will then be sized. We will look at fans as the example, but it can just as easily apply to any piece of process equipment.
The boiler operators who are involved with the operation are the closest on a day-to-day basis and are often the first people asked. They, in conjunction with the process engineering group, will look at historical data and based on their experience and using mass calculation equations, will give an opinion on the required fan volume at various temperatures and densities of air.
Because there is a lot of variability on the process from fluctuating densities of air to fluctuating varieties of crude oil and what cracking strategy can deliver the highest profit for the raw material they will want to err on the safe side which can be summed up as “more is better than less”.
This may then go to the overall area supervisor for final approval who looks at the results and will generally apply a contingency factor. From there it goes to the consulting engineering group who put together a myriad of components. When it comes to the fan they will generally apply another safety factor. Finally the bid request is issued and a fan engineer will size it often with a small over-sizing percentage for further contingency – not to mention that within the software program they use there is a hidden contingency allowance.
All want to make sure there is a bit of margin to cover any unknowns in the fan performance or in the data they have because the thinking is you can always throttle back but if the fan is too small then it gets expensive if you need a larger fan.
That is why if you walk through a boiler house in many industrial plants and look at the ID or FD fan’s variable-inlet-vanes or (inlet or outlet) louver dampers are choked or throttled way back because the fan although designed for some larger future flow is running at a significantly lower rating point..
When this operating condition exists then control of the process is difficult as the control actuator is cycling within a small band — say between 85% to 100% closed, rather than between a larger range such as 0% to 50% closed.
In terms of energy consumption the fan running throttled way back is using a lot more than required as the operating point does not line up near its maximum efficiency point – not to mention the added air pressure that must be artificially knocked down by the dampers. Many plants will run like this because it is time consuming to go back and resize for the present consumption.. There are many reasons but one main reason is lack of known data. If one can’t access data from a computer historically tracking the percentage open of the dampers people are reluctant to make a change in case it requires a lot more air volume under certain circumstances that they haven’t witnessed.
This is one reason why historically tracking parameters is a useful and cost saving tool.
Without good tracking but even with it is a justification to explore speed control. Using variable speed with a feedback loop based on a key parameter such as pressure or flow the fan will automatically slow down to deliver only the required amount of air.
There are issues to observe of course. Every situation is unique. Some processes will use speed control with vane control to hit multiple points of ratings because sometimes the system will need high pressure with lower flows.
Nevertheless with the cubed relationship between speed and power for fans and pumps (the Affinity Laws) reducing the speed by half will result in consumption of power being reduced to 1/8 of the power — (1/2) 3 or ½ x ½ x ½ = 1/8 th of original power.
So a powerful incentive exists to take those real offenders that are operating almost completely choked off and slowing them down to a speed capable of delivering maximum control and optimum efficiency.
Alternatives do exist as well to reduce to a fixed speed on fans with vane control. Adjusting belt drive sheave ratios will allow a fan to run slower and allow the vanes to operate in a less throttled back position. Similar strategies exist for direct drives fans without having to use VFDs. This is a bit more complex but options do exist.
As energy has a direct line to pollution doing our part only makes sense and as a bonus can have a very quick energy payback.
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