Screw Air Compressor "FAD" Capacity at High Altitude 2

[Cookiepuss]

I am sizing air-cooled, oil-flooded, rotary screw compressors for a project located at 5,200 ft above msl. The site atmospheric pressure is 12.14 psia. The air compressors will be located in equipment rooms ventilated to 95 deg F.

I understand screw compressors are positive displacement machines and most manufacturer “FAD” capacity values are stated for standard intake conditions of 14.5 psia/68 deg F. I further understand the rated “FAD” capacity values must be adjusted for intake pressure variations due to altitude. (The effects of increased temperature are generally minimal due to the oil temperature?)

Below, I have outlined a process for utilizing manufacturer’s literature to select an air compressor at high altitudes. Does anyone find errors in this approach?

1.) Determine the volume flowrate the compressor must deliver to the compressed air system in scfm (14.5 psia/68 deg F). Add 20% margin for future air users. Add an additional 15% - 20% for desiccant dryer regeneration to obtain the total compressed air requirement.

2.) Convert the scfm value to acfm at the actual compressor intake conditions. For this project, the intake conditions are 12.14 psia/95 deg F. This "free air" volume flowrate is utilized to select an air compressor from vendor data.

3.) Reduce the “FAD” capacity values stated in manufacturer’s literature for operation at high altitude. As per Mark’s Handbook Section 14.3, the capacity multiplier is ~0.99 at 5,200 ft altitude.

Thanks in advance for your help!

 

[zdas04]

Your technique looks ok (where are you at that your standard conditions are 14.5 psia and 68F, never seen those numbers before, more common to use 14.73 or 14.696 psia and 60F, but whatever works for your site).

If you are using a Gardner Denver or Frick screw then you can use their programs (GD's is on their web page, Frick wants you to sign up for theirs) and avoid the whole thing (they both account for elevation changes). Even if you're using someone else's screw, you can download Rotosize from GD and get pretty close.

I think your Dessicant losses are pretty high for that elevation (air at altitude can't hold nearly as much water vapor as it can at sea level, and you rarely see over 25% of sea level RH at this elevation).

David

 

[Cookiepuss]

zdas04, thanks for your fast and informative response.

The standard conditions of 1 bara (14.5 psia) and 20 degC (68 degF) stem from ISO 1217 Annex F and ISO 8573-1. I believe CAGI and PNEUROP have adopted these values.

This client has utilized Atlas Copco machines on previous projects. (However, the equipment will be competitively bid.) I'll check out Gardner Denver's RotoSize based on your suggestion.

Good point about the humidity at this elevation (5,200 ft). However, I wonder if the desiccant dryer will see less than 100% humidity coming from the air compressor package. I expect the water separator will collect less. I will further investigate.

Thanks again.

 

[Cookiepuss]

One aspect of this issue that I should have emphasized in my initial post is the very small reduction in "free air" capacity. (~0.99 at 5,200 ft altitude as per Mark's.) I don't have access to CAGI publications.

I expected altitude would have a greater impact on compressor performance. However, I suppose it can be said that screw air compressors are constant ACTUAL volume machines.

 

[ARenko]

Your approach seems fine. The important thing is what standard conditions the manufacturer used - you need to ask each manufacturer (or it should be in their literature).

I probably wouldn't go higher than 15% for the dryer purge assuming it is a heaterless dryer (or heat of compression dryer). Coming from the air compressor will be saturated air - the amount of water vapor depends on your inlet conditions and aftercooler temperature.

Do you have a design spec for the relative humidity of the compressor room? I'm not sure you can say there will be less water at 5000 ft than sea level. For a given temperature and relative humidity altitude will have no effect on the amount of water the air can hold. I wonder if in the compressor room you could have similar relative humidity to what you might see at sea level.

 

[ARenko]

I'm not sure about this .99 factor. Your step 2 should give you the correction for altitude. You can expect to lose 3% capacity for every 1000' of elevation. Also about 2% for every 10F over 60F. So, for 5000' and 95F you can expect a 1000 FAD compressor to give you about 780 scfm. This includes water vapor, so you have to extract the vapor dropped out as liquid water in the separator after cooling.

 

[ARenko]

I should add the numbers above are based on STP of 60F, 14.696 psia, and 0%RH.

 

[zdas04]

The water content of a gas is a function of both temperature and pressure. I don't have detailed saturation calcs for air, but for methane the change between 12.1 psia and 14.5 psia for 95F is that at 12.1 psia the air can hold 19.5% more water vapor. Which to someone who lives at 5,400 ft and talks about evaporation a lot is really interesting. Our RH is never higher than 50% and is rarely greater then 10%. I understand that 10% RH at 5400 ft is the same as 12% RH at sea level, but so what? Why is 80+% common in Houston, and unheard of in Denver?

At elevation the air can hold more water vapor, but it never does. I'm going to have to think about this and see if I can explain it to myself. Right now I don't have a clue.

David

 

[Cookiepuss]

The compressor room will be ventilated via outside air. The site outside air conditions are 89 degF dry bulb and 60 degF mean coincident wet bulb (RH ~ 20%, W ~ 0.0068 lb/lb).

I'm searching for a reference to determine the humidity ratio at saturation based on pressure and temperature. Typically, the air exiting the aftercooler is saturated. I am compressing to 145 psig.

 

[ARenko]

zdas,

I'm sure the outside air is dryer up there. My main point was is if the room is maintained at a specific %RH (or if a design requirement specifies it at some level) then cookieplus has to consider that. 95F/ 40% relative humidity at 5200 ft can hold the same water as 95F/ 40% RH at sea level. From Coolware...

95F/ 40%RH/ 14.7 psia has 99.2 grains/ lbm and a density of .0699 lbm/ft^3 = 6.93 grains/ ft^3.

95F/ 40%RH/ 12.14 psia has 120.7 grains/ lbm and a density of .0573 lbm/ft^3 = 6.92 grains/ ft^3.

 

[ARenko]

Cookie,
You answered while I was typing my message. I've seen inside conditions much different than outside air conditions depending on what's happening in the room where the process equipment is. Sounds like you've got a handle on it though. The following link gives humidity ratio at saturation.

http://www.engineeringtoolbox.com/humidity-ratio-air-d_686.html

 

[zdas04]

I've never seen an industrial air compressor take its suction within a humidified, temperature controlled space. I've seen them pull inside air, but in those cases there was a bunch of blower capacity to keep from sucking the building inside out. Seems like a lot of cost that then imposes additional costs to remove the water you just put in.

David

 

[dcasto]

I agree with zadas, why waste all the energy to control the air going into the compressor when the discharge temp and humity will be a function of the after cooler and outside temperature.

add a dehydrator to the compressed air to acieve your goals, it'll be chaeper and more operable.

 

[Cookiepuss]

My biggest concern was to determine the reduction in screw compressor "free air" capacity at high altitude. The reduced inlet pressure increases the compression ratio of the screw compressor which, in turn, impacts the efficiency. However, it appears the ACTUAL volume flowrate through the compressor will change very little. I was concerned I could be making an error because I do not have CAGI or other compressed air-specific texts available.

With regards to the water content of the compressed air...
The compressor room is ventilated with outside air to maintain a maximum temperature of 95 degF. However, there are no sources of moisture in the compressor room. Therefore, the compressor inlet moisture content is based solely on outside air conditions. These conditions are 89 degF dry bulb and 60 degF mean coincident wet bulb which yields a humidity ratio of W = 0.0068 lb/lb at 5,200 ft (12.14 psia).

The compressor package outlet conditions will be follows:
p = 145 psig = 157.14 psia
T = 95 + 20 = 115 degF (20 degF aftercooler approach)
p-sat(115 degF) = 1.47286 psia
Based on ASHRAE equations, the saturation humidity ratio is caculated to be W = 0.0059 lb/lb.

Therefore, I conclude the compressed air entering the desiccant dryer will be at 100% humidity. The water removed by the water separator will be less than a typical application. However, the dryer will see no benefit of the comparably lower ambient humidity.

Thanks for everyone's help! This was my first post!