Wind Pressure... a question that needs answering

[soundboarddude]

Hey there... I posted this question in a post in the building section, but no one really has responded, and it's been a while.

I've got a rank of 4' principals that are voiced on 6" to 7" of WP. What does this equal roughly in CFM? How would I convert "inches" of WP to CFM anyways?

muchos gracias

[Guest]

Re: Wind Pressure... a question that needs answering

The term Inches refers to a water column and is a measure of air or gas pressure. A pressure of 6 inches above ambient will raise a water column by 6 inches. CFM cubic feet per minute is the volume of air a blower is capable of delivering at some specific pressure, usually at least 3 inches above what the pipe needs. A regulator is then used to reduce and stabilize the pressure at 6 inches or whatever pressure is needed. The pressure is a function of the regulator area, spring tension and added weights. These are also adjusted for trem response. Hope this helps a bit !

[Guest]

Re: Wind Pressure... a question that needs answering

Hi Al,

Correct me if I am wrong, but the CFM of a blower cannot be measured in inches of pressure, unless there is a regulator in the line.

For example, the way I have seen inches measured, though it is somewhat crude, is with a clear rubber hose, mostly filled with water so that a the hose has at least 12 inches of tubing on each end empty, if all the pressures are less than 12 inches. One end is attached to a ruler, so that the line of water is at the zero inch mark. When air is blown into the other end of the hose, the water rises so many inches along the ruler and thus, a measurement in inches of pressure. The regulator "bleeds" of access pressure, thus keeping the water from just blowing out the end of the tube.

If you were to attach the end to an air or wind supply line without a regulator in place, the blower would just continue building pressure until the water was blown out of the tubing, given that the CFM was great enough.

There are formulas in physics that will help you to determine CFM or the volume of an air blower needed to supply the wind pressure you need going into a regulator without "hosing" down your building shop, although I have no idea what they are. I just buy from the guy that sells me what I tell him I want to accomplish, haha.

I realize that the measuring of inches can be determined by devices that are not so crude, but for an amateur/hobby builder, hopefully this idea will help.

As a side note for the novice builder, regulators must have a material that permits the excess air to breath or exhale from it (the regulator). Most builders use a special "leather" to accomplish this. Further, this "leather" acts like an accordian. As the air pressure builds, the leather allows the top to rise to the full cubic capacity of the regulator, or to the regulated pressure. Then it exhales the balance through this "leather". However, as demand is put on the reserve, the regulator collapses to maintain constant pressure (the function of the springs and weights you mentioned in your response). The CFM of the blower must be capable of producing enough air to the regulator to supply the demand of at least 10 to 12 pipes that speak from all ranks attached to it at the highest inches requirement. To put that into English, if I have 4 ranks of pipes regulated at 10 inches of pressure, and 3 ranks regulated at 6 inches of pressure, then I must be able to put the demand of 10 to 12 notes from each rank at 10 inches (4X12X10) + (3X12X6) or 696 inches of pressure from my blower. Now, before you panic, 1 CFM is equal to 1728 Cubic Inches per Minute, if my math has not failed me. However, 1728 CIM is not 1728 inches of pressure, since there is no way to bleed off the pressure. That is just the volume of air that is produced. Now, you can see why I leave that part to the sales people. They can determine not only the volume but the actual pressure that a blower will need to produce to maintain the regulation of inches.

Regards,

Mark G

[Guest]

Re: Wind Pressure... a question that needs answering

Hi, Mark
I was refering to the general description of CFM as it applies to volume of gas or air. For instance most industrial fans are rated at CFM at a certain pressure. You always have to specify CFM at a certain pressure. It might be zero or .1 inch for an ordinary fan. If you reduce the CFM on a blower to zero, by throttling the outlet, the pressure goes up to the maximum cut off pressure. So the CFMs of a blower depends on the pressure. It's also not necessarily a linear function (straight line) The max pressure and min CFM would be with the organ idle with no leaks. Hah! Never happens but its close. In my case it's 8.5 inches. The minimum pressure really is zero but you cant get there as the motor will be overloaded but there is a minimum allowable pressure which equates to maximum motor load. I suppose you could tell by measuring the motor current with a clip-on ammeter. An organ blower should never be run with the duct disconnected. Now really I'm not as worried about the CFM as the pressure. Mine is .9Hp and should be good for 10-15 low pressure ranks. I'm only running 6. Really the pressure is more important than CFM unless you have many ranks that take a lot of air. Obviously my blower would not be suitable for a 6 rank theatre organ. The CFMs might be the same as a 3hp 2stage Spencer but the pressure certainly wouldnt. I use a Magnehelic gauge which is quite accurate and has a very fast response. In fact I can tune the trem by watching the gauge. Kinda neat. These gauges plus water column are widely used in the heating and air conditioning industry so these are readily available.

The only relationship I can see between Inches of Water and CFM or CIM is a graph plotted for a specific blower with a specific motor. I'm not a mathematician but I dont think you can equate 1728 CIM with 1728 inches wp. Maybe you can calculate the CFMs at the chest pressure with say 10 Diapason pipes playing then calculate the CFMs required at the regulator inlet using the gas law P/V=T. Maybe some program is available that will do this.

Also my curtain regulator simply throttles the inlet as the top of the regulator raises. When it's at the high limit the curtain is across the airflow from the blower completely closing it off. Most Theatre organs use several cone valves to do the same thing. Just you can get a wider trem pressure swing with those regulators. I'm sure there is discussion as to which is better.

Regards,

Al Johnson

[davidecasteel]

Re: Wind Pressure... a question that needs answering

Mark, such a pressure measuring device should provide a U shape with the upper end of one side connected to the pressure source and the other end open to the air, with a suitable quantity of water in the lower part of the U. When measuring, the water in the side connected to the pressure will go down and the water in the open side will go up; the pressure is measured by the DIFFERENCE in the levels of the water in the two legs of the U.

David

[Guest]

Re: Wind Pressure... a question that needs answering

I agree whole-heartedly. However, the original question appeared to ask the relationship between CFM and the WP of a specific rank. Unfortunately, the only responses to the question that seem to address that issue, including my own that did not, seem to come from those who related that the blower's manufacturer could provide that information. But again, would that information necessarily take into account such factors as the number of ranks, the regulated pressure for each of the chests. I know that regulators check the flow of air to the wind chests and maintain that pressure when pipes are speaking. But the maximum CFM capabilities of the blower would determine the ability of the blower to keep up with ALL the demand placed on the system. For example, the needed CFM would be different for one rank playing one note and full organ playing eight notes. Then take into account the CFM needed to sound a 15 inch Tuba verses a 6 inch flute. Now, it would appear that the CFM requirement to meet the greatest of the 15 inch Tuba speaking would be 18 to 20 inches. The next question would regard both speaking at the same time. Would the demand require a blower capable of CFM to provide 24 to 25 inches. Is the math that simple? I would think not. Especially as the organ grows to 50 or 60 ranks. What if you had two 15 inch ranks? Would the blower then be sized for CFM to produce 39 to 40 inches of pressure?

I wish I knew the answers to these questions. Reason, when adding ranks to an existing organ, at what point do I have to consider adding an assist blower, or replacing the existing one to facilitate the additions?

[Guest]

Re: Wind Pressure... a question that needs answering

Hi

Tell you what I'd do: I'd look at other organs and see approximately what CFM's or HP their blower is. For a TO you typically need 20 inches from the blower as I recall a 3-5hp Spencer with 2 fan stages will do that for maybe 8-10 ranks. Now if your Tibias and other wind hogs are on 15 inches that's about the limit. Remember the following, rarely would you have all ranks playing at the same time. I'm sure one of the Wurlitzer experts would know the typical CFM for a rank at 8 or 10 or 15 inches. A way to tell if your organ is satisfactory is to measure the blower current consumption as you play the organ heavily. You can do this with a Clip-On-Ammeter. You don't need to disconnect anything, just clip the probe around one of the motor wires. Compare this with the rating on the motor. If it comes close to the maximum then it's time to get a larger blower or add a booster fan. As a note the Roosevelt Memorial Park near Los Angeles Wurly as I recall is 50 inches on several ranks and the blower is 100hp. It's a monster. Remember also rather than changing blowers, just put the new stuff on a second blower. That way you don't have to change what already works. I'll ask on one of the other lists whether someone has developed a computer model of the wind supply.

Al

[VaPipeorgantuner]

CFM measurement and Inches of water column (wind pressure) are inter-related...a blower that puts out a certain CFM at a specific wind pressure. There are typical consumption allowances for ranks of pipes at a given pressure (1 rank being a 61 note set of pipes at 8' pitch, for example). A blower operating into a reservoir/regulator might have a useable output of 600 CFM at 5 inch water column, but regulate that blower to have a useable pressure of 3.5 water column, the CFM rating might be 800 CFM. these are HYPOTHETICAL numbers, not real ones, but given to show the relationship between regulated wind pressure and cubic feet per minute capacity.
A rank that is voice on 3.5 inch wind pressure (water column - same thing) will require about 50 CFM in the grand scheme of things, where as that same rank voiced on 6 inch pressure needs an allowance of 65 CFM..in other words, the higher the wind pressure the pipes are voiced on, the more cubic feet of regulated wind the pipes will 'consume' in operation. Hope this is what you are looking for.
Rick in VA

[Guest]

6 ranke of Wurlitzer pipe work on 10" pressure uses a blower rated at 500 cfm at 15" pressure at the blower outlet using a 3 hp motor. Low pressure pipe work will not use at much air and can use a lower power blower.

CFM is a measure of the volume of air passing a given point. Inches of water is a measure of the pressure of air whether moving or static.

[jkrusel]

Look at it from the standpoint of electricity.

Electrical POWER is measured in WATTS. WATTS are the product of VOLTAGE and CURRENT. VOLTAGE is identical to the air pressure in PSI or pressure, and CURRENT is identical to the CFM or flow. POWER measured over a period of time is known as WORK.

PSI measures the FORCE of the POWER, CFM measures the FLOW of POWER.

It takes a certain amount of POWER to sound a pipe; it takes a certain amount of WORK to keep it sounding over time.

Fluids (like air) flow exactly the same way that electricity flows and are governed by the same types of laws. (Some of the math is just a little more complicated.)

[jkrusel]

CFM is a measure of the volume of air passing a given point. Inches of water is a measure of the pressure of air whether moving or static.

Very close!
"CFM is a measure of the volume of air passing a given point." - DURING A GIVEN PERIOD OF TIME.

[PeteKnobloch]

The way that the original question was asked has a problem. He knows the pressure but wants to know now much air (CSM) is needed to blow the rank of pipes. Just knowing that it is a 4' principal at 6” WP is not enough. You have to know the scale (size) of the pipes. Larger scaled pipes just take more air. I should say that I don't have much experience in what the values would be, just that some of the information in this thread bothers me even thou most of it is correct.

What I do know is that if you have a fixed size hole in a box, and inject a pressure of 6” WP into the box, then you can calculate what the CFM flow of air passing thru the hole. And the air passing thru the hole is equal to the flow thru the box. Cutting the area of the hole in half with the same 6” WP reduces the CFM flow in half. This is where the scale of the pipes come in.

When it comes to organ blowers, you buy a blower that will supply a given wind pressure at a specific CFM value. It is very common to get the reading of the WP at 0 CFM since this is easy to test. Just place a board over the outlet of the blower with a small hole for your WP gauge, sit on the board and have someone else turn on the blower. If the board (and you) doesn't hit the floor or ceiling, take a reading. Its just that simple. What you have to worry about is if the WP goes below the required pressure at the pipes when a large number of pipes are being played at the same time which is why this question was asked for in the first place.

The pressure in a blower will never increase uncontrollably until something blows. Inside the blower cage are fins that rotate inside a housing. There are gaps between the fins and housing and areas between the air inlet and output on the blades for the air will go when the output line is completely closed. All that happens is that the air starts taking alternate paths in the blower cage and the pressure stops going up. The only problem is that this churning of air creates friction between the air molecules which heats up the air and blower cage and detunes the organ.

I know that none of this talk has answered the real question. You just have to talk to someone who has a similar pipe scale, pressure and number of ranks. And don't get a 500CFM blower for 1 rank of pipes. 100CFM sounds better to me.

Pete Knobloch (Tempe AZ)

[jkrusel]

I brought up the electricity anology because it demonstrates that it takes WORK to sound a pipe. And WORK is a combination of PRESSURE and FLOW. You have to know what both of them are.

Only one is not enough to specify the needs of a pipe..

A lightbulb needs voltage and current to light it up, add more lights, you need more current.
A pipe needs pressure and flow to sound it, add more pipes, you need more CFM.

[doppleflute]

Uf you wish to get VERY techical, trythis web site
www.efunda.com/formulae/fluids/manometer.cfm

[doppleflute]

Want to get very technical try this site
www.efunda.com/formulae/fluids/manometer.cfm