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I want to select a screw compressor for my application wherein I need a pressure of about 18 psig, and 300 cfm at the aforementioned pressure. The compressors I'm finding suitable for the system have their pressure ranges listed in the catalogues starting at 116 psi. So, I'd need to design a system (duct with changing area) to decrease the pressure first. What's confusing me is that the catalogues specify the flow rate as Free Air Delivery, whereas, in my system, since I have a duct, it would incorporate an impediment, and thus change the flow rate of the compressor. Also, the change in pressure through the ducting (due to changing area) is going to alter the flow rate due to density changes as well, I suppose.

My question is this:

  1. How do I find out which compressor would fulfill my volume flow rate requirements based on Free Air Delivery as the only volume flow rate given in the catalogue?
  2. What would be the best system to change the pressure of the compressor (116 psi) to the required pressure (18 psig). How can I control the pressure while achieving the required flow rate in the system. (Is the varying cross sectional duct with pressure regulation valves the best option)?
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  • $\begingroup$ Use a pressure regulator. $\endgroup$ – Solar Mike May 19 at 12:17
  • $\begingroup$ There is no point in doing all the work to compress to > 100 psi only to depressurise it to 18 psi. It's a waste of energy and a lot of heat.Can you not just set the compressor output cut-off, or whatever, to 20 psi or so? $\endgroup$ – Transistor May 19 at 12:24
  • $\begingroup$ When you only need 18 psi / 1.2 bar, why do you use a screw compressor? I'd go with a radial or even axial fan. $\endgroup$ – mart May 19 at 12:24
  • $\begingroup$ I couldnt find a centrifugal blower to fit the 18psig as per my requirement. I was thinking of improvising, using a Garrett automotive turbocharger, driving it via a gearbox/belt through a motor. Would that be advisable? The only thing that worries me is the constant back pressure but that could be alleviated using a BOV/recirculation. $\endgroup$ – afterburner77 May 19 at 13:20
  • $\begingroup$ @Transistor yes, but the question remains, how would I know if a specific compressor is providing the required flow rate. Since there is performance curves/maps are not given in the catalogues I'm checking, and air flow rate is only given as FAD. $\endgroup$ – afterburner77 May 19 at 13:36
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That is on the high pressure end of normal pneumatic conveying blowers, but they are out there. See the notice at the bottom of the attached chart.

enter image description here http://www.hsiblowers.com/products/high-speed-turbo-blowers.html

You don't want a high pressure compressor, you just need to find the right compressor that will deliver at 18 psi. You need to specify if the 300CFM is at 18 psi and 70 degrees, or 18 psi and 270 degrees, which is about what the outlet temp will be from an uncooled blower. If this thing is going to run 5000 hrs a year, you may want to look at a two stage system with intercooling.

The usual way you sort this all out is to contact the vendor after you have done the preliminary work. They will have you fill out a worksheet for your project and then they can check their application guides and engineering sheets to properly configure the system and give you a price. Realize if you are spending $15,000 per year on electricity to run it, efficiency is worth looking at.

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How do I find out which compressor would fulfill my volume flow rate requirements based on Free Air Delivery as the only volume flow rate given in the catalogue?

  1. Calculate the mass flow rate of air given your required duty. You can do this by looking up or calculating the density of air, $\rho_\text{out}$, at your outlet conditions ($300 \space \frac{\text{ft}^3}{\text{min}}$ at $18 \space \text{psig}$ and $T_\text{out}$) and then dividing your volumetric flow rate by this density.

$$\dot{m}=\dot{V}_\text{out} \cdot \rho_\text{out}$$

  1. Convert mass flow rate $\dot{m}$ to standard volumetric flowrate, $\dot{V}_\text{std}$, (probably $\text{SCFM}$, or Standard Cubic Feet per Minute in your case) since this is the number vendors use to rate their compressors. You can do this by dividing the mass flow rate by air density at standard conditions, $\rho_\text{std}$.

$$\dot{V}_\text{std}=\frac{\dot{m}}{\rho_\text{std}}$$

$$\dot{V}_\text{std}=\dot{V}_\text{out} \cdot \left( \frac{\rho_\text{out}}{\rho_\text{std}} \right) $$

  1. Provide this standard volumetric flowrate along with:

    • available suction pressure and temperature
    • required discharge pressure and temperature (note: often air compressor packages come with cooling options since compressing heats up air)
    • utility power details (3-phase 240 VAC? continuous or intermittent duty?)

Here's an example setup worked out:

My process simulator (DWSIM 5.8) gives $\rho_\text{out}=0.163758 \space \frac{\text{lbm}}{\text{ft}^3}$ for $T_\text{out}=80^{\circ}F$ (?) and $P_\text{out}=32.7 \space \text{psi}$ (this is $18\space\text{psig}$ assuming atmospheric pressure is $14.7\space\text{psig}$)

It gives $\rho_\text{std}=0.0763781 \space \frac{\text{lbm}}{\text{ft}^3}$ for $T=60^{\circ}F$ and $P_\text{out}=14.696 \space \text{psi}$ (probably standard conditions in United States)

Plug it all together and you have:

$$\dot{V}_\text{std}=\left( 300 \space \frac{\text{ft}^3}{\text{min}} \right) \cdot \left( \frac{0.163758 \space \frac{\text{lbm}}{\text{ft}^3}}{0.0763781 \space \frac{\text{lbm}}{\text{ft}^3}} \right)=643.21317 \space\frac{\text{ft}^3}{\text{min}}$$

$$\dot{V}_\text{std}=650\space\text{SCFM}$$

Note: This value is only valid for the exact pressures and temperatures I assumed. You didn't provide an outlet temperature so I filled one in. Also, I didn't take pressure drop across the ducting into account (I simply used $18 \space\text{psig}$).

What would be the best system to change the pressure of the compressor (116 psi) to the required pressure (18 psig). How can I control the pressure while achieving the required flow rate in the system. (Is the varying cross sectional duct with pressure regulation valves the best option)?

If you purchase a compressor that has a much higher maximum operating discharge pressure (ex: the $118 \space\text{psig}$ screw compressor you mentioned), then you can use a pressure regulator (which also would have to be sized for your mass flow rate and compressor outlet conditions) to reduce the pressure to that needed by your duct system. You'll also want to make sure a pressure relief valve (sometimes included with compressor packages if you request one) capable of relieving the maximum flow rate of air back to atmosphere in order to prevent equipment damage or injury in case accidental block-in causes dangerous pressure release (especially important with positive displacement compressors like a screw compressor).

Some air compressor packages offer a recycle system that automatically sends air from the discharge directly back to the compressor suction in order to reduce flow; it's less efficient but cheaper than a "VFD" option (variable frequency drive) which lowers the compressor rate of rotation to something less than what the nature of AC power imposes on induction motors typically used in small air compressors.

What's confusing me is that the catalogues specify the flow rate as Free Air Delivery, whereas, in my system, since I have a duct, it would incorporate an impediment, and thus change the flow rate of the compressor.

You're correct in thinking that restricting flow from a compressor will decrease the flow rate from the compressor. All fluid compressors have what is called a "pump curve" and any given outlet piping and ducting system will have a "system curve". Both curves are plotted with "head" ("pressure" but in a more generalized form) versus flow rate.

Operating Curves of Positive Displacement pump Image by Baltakatei derived from work by Ryan Toomey, licensed CC BY-SA 4.0

The point at which the pump and system curves cross determines the flow rate through your system (the "operating point"). Adding a restriction (such as narrowing the duct) steepens the yellow system curve, driving the actual flow rate towards the left on the plot. Using a VFD or recycle line permits shifting the pump curve down or to the left.

A screw compressor is a form of "positive displacement" pump. Positive displacement pumps have a pump curve that is very steep (as opposed to those of "centrifugal pumps"), meaning they do not respond strongly to discharge pressure if their mechanical configuration doesn't change. That said, screw compressors can achieve lower flowrates (in other words: achieve higher "turn down") without the use of a VFD or recycle line by utilizing adjustable suction and discharge port apertures ("slide valves") which change the timing for exactly when gas in a flute begins and ends compression. However, all these are bells and whistles that cost money and increase complexity.

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  • $\begingroup$ Thank you for the detailed answer. That clarified my confusions. $\endgroup$ – afterburner77 Jun 3 at 11:09

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