## Posts Tagged ‘blower’

### A Pulley Speed Ratio Formula Application

Friday, April 21st, 2017
 Last time we saw how pulley diameter governs speed in engineering scenarios which make use of a belt and pulley system.   Today we’ll see how this phenomenon is defined mathematically through application of the Pulley Speed Ratio Formula, which enables precise pulley diameters to be calculated to achieve specific rotational speeds.   Today we’ll apply this Formula to a scenario involving a building’s ventilating system.     The Pulley Speed Ratio Formula is,                                                 D1 × N1 =  D2 × N2                             (1) where, D1 is the diameter of the driving pulley and D2 the diameter of the driven pulley. A Pulley Speed Ratio Formula Application     The pulleys’ rotational speeds are represented by N1 and N2,  and are measured in revolutions per minute (RPM).     Now, let’s apply Equation (1) to an example in which a blower must deliver a specific air flow to a building’s ventilating system.   This is accomplished by manipulating the ratios between the driven pulley’s diameter, D2, with respect to the driving pulley’s diameter, D1.   If you’ll recall from our discussion last time, when both the driving and driven  pulleys have the same diameter, the entire assembly moves at the same speed, and this would be bad for our scenario.     An electric motor and blower impeller moving at the same speed is problematic because electric motors are designed to spin at much faster speeds than typical blower impellers in order to produce desired air flow.   If their pulleys’ diameters were the same size, it would result in an improperly working ventilating system in which air passes through the furnace heat exchanger and air conditioner cooling coils far too quickly to do an efficient job of heating or cooling.     To bear this out, let’s suppose we have an electric motor turning at a fixed speed of 3600 RPM and a belt-driven blower with an impeller that must turn at 1500 RPM to deliver the required air flow according to the blower manufacturer’s data sheet.   The motor shaft is fitted with a pulley 3 inches in diameter.   What pulley diameter do we need for the blower to turn at the manufacturer’s required 1500 RPM?     In this example known variables are D1 = 3 inches, N1 = 3600 RPM, and N2 = 1500 RPM.   The diameter D2 is unknown.   Inserting the known values into equation (1), we can solve for D2,                                (3 inches) × (3600 RPM) = D2 × (1500 RPM)         (2) Simplified, this becomes,                                             D2 = 7.2 inches                                      (3)     Next time we’ll see how friction affects our scenario.     Copyright 2017 – Philip J. O’Keefe, PE Engineering Expert Witness Blog ____________________________________

### Simple Pulleys

Tuesday, June 28th, 2016
 Pulleys are simple devices with many uses, and as an engineering expert, I’ve often incorporated them into mechanical designs.   They’re used in machinery to transmit mechanical power from electric motors and engines to devices like blowers and pumps.   Another common usage for pulleys is to aid in lifting.   There are two types of pulleys for this purpose, simple or compound. We’ll start our discussion off by looking at the simple type today.     The simple pulley may have been an advanced application of the wheel.   It consists of a furrowed wheel on a shaft with some device for pulling threaded through it.   The pulley wheel supports and guides the movement of a rope, cable, or other pulling device around its circumference.   The pulling device runs between a pull-ee and pull-er, that is, the object to be moved and the source of pulling power, with the pulley itself situated somewhere between them. Simple Pulley     Pulleys are believed to have first been used by the Greeks as early as the 9th Century BC.   We’ll look into how they put them to use next time. Copyright 2016 – Philip J. O’Keefe, PE Engineering Expert Witness Blog ____________________________________

### Industrial Ventilation – Local Exhaust Ventilation Fans

Sunday, May 22nd, 2011

### Industrial Ventilation – Local Exhaust Ventilation Filters and Air Cleaners III

Sunday, May 15th, 2011

### Industrial Ventilation – Local Exhaust Ventilation Filters and Air Cleaners II

Monday, May 9th, 2011
 We’ve been talking about mechanical filtration, like the type used by fish tanks.  Now we’ll consider another type, the “cyclone.”  It’s something which most of us have become very familiar with, thanks to a British bloke and his awesome vacuum that “…won’t lose suction!”  His invention makes use of the principles of cyclone technology, and as effective as it is used in vacuums, it’s equally impressive used in local exhaust ventilation system applications.  A cyclone that has been incorporated within this type of system is shown in Figure 1. Figure 1 – Local Exhaust Ventilation System With Cyclone      Here’s how it works.  A local exhaust ventilation system draws in corrupted air by means of a strategically placed hood, and its fan pulls the captured air and dust mixture through ductwork and into the cyclone.  The cyclone is shaped like a cone standing upright on its small end.  A cutaway view is shown in Figure 2. Figure 2 – Cutaway View of a Cyclone      When a quickly-moving air and dust mixture gets drawn into the cyclone by the fan, the mixture is forced to spiral down into the cone by the shape of the inlet passage.  Because dust particles are heavier than air molecules, they tend to separate due to centrifugal force.  The heavier dust particles are sent crashing into the sloping sides of the cone.  They then slide down to the bottom of the cone, where they will eventually fall through the bottom and into a waiting trash bin.  The lighter air tends to stay in the center of the cyclone and is eventually drawn out by the fan through the outlet passage.      Unfortunately, cyclones are not 100% efficient when it comes to removing dust from the air.  Their efficiency depends on many factors, including the shape of the cyclone, the speed of the flow going through it, and the weight of the dust particles.  In any case, there’s always going to be some dust that will escape along with the air that’s being exhausted to the building’s exterior through the exhaust stack.  If necessary, this air can be cleaned further before being released into the atmosphere by the use of additional filtration located within the ductwork between the cyclone and the fan.      That wraps up our discussion on dust removal through mechanical filtration.  Next time we’ll look at systems capable of removing chemical vapors. _____________________________________________

### Industrial Ventilation – Local Exhaust Ventilation Filters and Air Cleaners

Sunday, May 1st, 2011
 My wife is an aquarist, meaning she keeps aquariums.  Three of them.  Each contains a different variety of fish housed within its own unique liquid environment.  One of these is a 35 gallon tank containing three goldfish.  These fish have two unique characteristics that make them especially noteworthy, they are extremely hardy and extremely dirty.  Hardly a week can go by between tank changes before the water quality starts to deteriorate, evidenced by cloudy, stinky water.  It’s the kind of stink that makes a passerby in the area exclaim, “Who used the bathroom and didn’t turn on the exhaust fan!”  Thank goodness for activated carbon.  With its proper placement inside the aquarium’s filtration system a cleaner, fresher environment is delivered, both to fish inside the tank and the humans who watch them from outside.  Put the carbon in the wrong compartment, however, and the water quality plummets back to its original fetid state within a matter of days.      As is true with the proper care of goldfish, it is often necessary within an industrial environment to remove contaminants before the air that contains them is once again dispersed into the general environment.  This is where filters and air cleaners come in.  They’re generally placed inside the ductwork, somewhere between the hood and fan.  Their job is to ensure a good, clean outcome, usually through an external exhaust of some sort.  Local exhaust ventilation systems begin with a precisely positioned hood at the source of contamination and end with an exhaust stack located outside the building.  Some airborne contaminants being released from the stack are deemed unsafe for the environment, and outdoor air quality standards promulgated by state and federal Environmental Protection Agencies limit their release back into the atmosphere.  For this reason the proper use of filtration and air cleaners is crucial.      Airborne contaminants are in the form of dusts and vapors.  If the issue to be addressed comes in the form of dust, then filters and mechanical separators are commonly used.  Filters, like the atmospheric conditions they are meant to address, come in many configurations.  They are typically positioned within the local exhaust ventilation system ductwork, as shown in Figure 1 below. Figure 1 – Local Exhaust Ventilation System With Filter      The fan draws in air and dust through the strategically positioned hood, located at the source of contamination, then follows a course through ductwork, passing through a filter along the way.  The filter contains media with holes tiny enough to allow for air to pass through, but small enough to stop dust particles.  The cleaned air is then drawn out of the filter by a fan, which finally exhausts it into an externally positioned stack.       Next time we’ll continue our discussion on filtration devices by examining a cyclone.  And no, I don’t mean the famous vacuum cleaner, although the methodology is similar. _____________________________________________

### Industrial Ventilation – Local Exhaust Ventilation

Sunday, April 10th, 2011
 My wife often says I’m the worst cook she knows.  This doesn’t really bother me too much, because she’s the best cook I know, and she keeps me well fed.  But there are times that I have to fend for myself in the kitchen, and this sometimes results in a foul smelling mess plastered all over the stove.  Lucky for me we have a nice exhaust hood, and it usually manages to suck out the odor before my wife gets home.      Local exhaust ventilation systems, like the vent over my stove, work much the same way in an industrial setting, albeit on a larger scale.  This type of ventilation system gets its name because its action is quite specific, localized to contain exhaust air from a particular area.  They’re routinely placed as close as possible to the source of contaminants, and they are able to work quickly to capture and expel chemical vapors, dusts, and fumes, before they spread.  This type of ventilation is effective for other reasons, too, because it helps keep down heating and cooling costs.  Instead of treating an entire building for ventilation issues, the problem can be nipped in the bud at its source.  In many situations, local exhaust ventilation is preferred over dilution ventilation systems for these very reasons.      A basic local exhaust ventilation system is comprised of a duct, a fan, and a hood as shown in Figure 1 below. One end of the duct is attached to the intake of the fan.  The other end of the duct is attached to the hood.  The duct can be rigid or flexible.  The hood is positioned in the workplace near the source of contaminants. Figure 1 – Basic Local Exhaust Ventilation System      This local exhaust ventilation system operates by much the same principle as the one generally governing the movement of liquids and gases.  If you’re a regular reader of this blog, you’ll remember me writing that liquids and gases always flow from areas of higher pressure to those of lower pressure.  Well, the air within the room has pressure principles at play as well, and the air within a given work area is at atmospheric pressure.  When the fan is introduced into the scenario, a vortex is created within the duct which is less than that of the atmospheric pressure in the room.  This difference in pressures causes the room air to flow into the ventilation duct along with its contaminants. The room air and contaminants flow out through the ventilation system, where they are then exhausted outside of the building.       But because room air is being drawn into the ventilation system, provisions must be made to supply enough replacement air.  Without the proper ratio of air moving in to that moving out, a ventilation system will not work properly.  In other words, the suction created by the local ventilation system could cause the pressure in the room to drop below atmospheric pressure.  This could cause the higher atmospheric pressure outside the room to bear down on doors, making them difficult to open.  Worse yet, contaminants could back up into the room, causing workers to get sick.      Now that we’ve covered the basics of local exhaust ventilation, we can move on to its five constituent elements and a discussion of their design.  We’ll do that next week.  _____________________________________________