9 replies to this topic

[shan]

Hi Everybody with Air Compression Experience,

 

Per Hysys, if air is compressed from 0 psig, 80 F to 120 psig with a single stage 75% adiabatic efficiency compressor, the discharge temperature is 703.1 F.  How can an air compressor stand this high temperature? 

 

Thanks

 

Shan

[thorium90]

I would figure choosing the right materials?

The following will help in making a better simulation and show that your simulation doesnt tell the whole story.

 

http://www.thermokin...es Of Thumb.pdf

 

http://www.ecompress...tercoolers.aspx

Edited by thorium90, 27 February 2013 - 08:35 AM.

[Art Montemayor]

A single-stage, reciprocating compressor made of conventional cast iron cylinders and forged steel rods, valves, and other gear cannot operate under steady-state conditions compressing atmospheric air to a pressure of 125 psig -- PERIOD.  At least, it will only operate long enough to self-destruct.

 

What HySys doesn't know (because it has no knowledge or expertise in compressing air) is that some single stage compressors are used to produce 125 psig air - either because the owners/operators are too ignorant or cheap to do the correct, economical, and safe operation of doing the operation in a 2-stage machine.   What is being referred to here is obviously the ordinary, cheap, car garage where the operator is only interested in producing the 125 air under unsteady state operation - not continous operation.   So what is happening is that the machine is stopped and started as the air needs call for it.  This may allow the whole system to heat up - but under unsteady state heat transfer:  the entire system never reaches the 700 ^oF (Thank God!).   It is certainly not a proper, well-designed and safe system - but it may seem to work to the layperson who only worries about the immediate capital costs.   These type of compressor applications don't last very long; they auto-consume themselves with excess heat and wear-tear operation.

 

HySys will only produce garbage if you input garbage.  If you don't advise HySys that this is an unsteady state operation, it can't figure it out because it is too stupid and can't engineer.  It depends on YOU to do the engineering.

[ankur2061]

Shan,

 

When I do a calculation using my spreadsheet for a reciprocating compressor I get a discharge temperature of 557 deg F for your conditions. Your compression ratio is 9.16 when compressing from 14.7 psia to 134.7 psia.

 

No way you can use a single-stage compressor for such a large compression ratio. The discharge temperature is simply unmanageable. You will have to have a compressor built with exotic metallurgy to withstand such high temperatures. This is simply impractical. That is why multistage machines are recommended for high compression ratios where intercoolers are provided to cool the gas entering successive stages. If you refer API STD 618 for reciprocating compressors Section 2.3 "Allowable Discharge Temperature" it clearly states that the stage discharge temperature shall not exceed 300 deg F. Note that they have used the term shall and not should, which makes it mandatory.

 

Hope the above clarification helps.

 

Regards,

Ankur.

[shan]

The purpose of compressed are is for the plant instrument air and utility air.

I run a two stage air compression with inter stage cooler case with Hysys. The summaries are listed as the following.

1st Stage Inlet P=0 psig Inlet T=80 F, Outlet P=40 psig Outlet T=407 F
2nd Stage Inlet P=35 psig Inlet T=100 F, Outlet P=125 psig Outlet T=356 F

The outlet temperature values are still way higher than the desired 300 F.

Do we have to use 3 stages air compressor? Could we use a single stage screw compressor to let lube oil circulation to carry away the compression heat?

[thorium90]

Shan,

 

Do read this part of the website I post. It explains alot even though its made like marketing material... Ankur has explained that if reciprocating compressors are used, the discharge temperature will be high, which is what you originally simulated in hysys, the results of which can be validated in the first link in post 2 as well as Ankur's spreadsheet. Art has also explained that garbage in garbage out, which in this case means that hysys cant simulate a compressor that is cooled while the gas is being compressed. Hysys also cant simulate the unsteady state heating up of its own compressor parts. However, you likely do not have the expertise to design such a unit and it would be better to engage a contractor like Ingersoll etc..

http://www.ecompress...ompressors.aspx

Edited by thorium90, 27 February 2013 - 10:59 AM.

[ankur2061]

Shan,

 

Some company standards recommend that the compression ratio per stage should be less than 3 (2.7 to 2.8). Going by that guideline, a  minimum of 3-stage reciprocating compressor would be required.

 

One important question that I need to ask is why is so much research required on air compressors which are generally standard off-the-shelf skid mounted package items for different capacities and compression ratios from reputed manufacturers such as Atlas-Copco, Kaiser and Compair. What stops you from approaching the manufacturers and asking the right configuration?

 

Oil-flooded screw compressors could be an option for reducing heat due to high compression ratios. But then these are special in design and only the manufacturers can confirm the machine configuration in terms of number of stages.

 

Have you considered getting in touch with an air compressor manufacturer.

 

Regards,

Ankur.

Edited by ankur2061, 27 February 2013 - 11:14 AM.

[Art Montemayor]

 

Shan:

As I stated to you previously, if you feed garbage to a simulation program, it spits garbage right back at you.  It does this because it is stupid.  It needs you and your intelligence to correctly input your basic data.

Your HySys results are garbage.  You can prove this by doing the common, practical manual calculation which is just as fast.   No compressor manufacturer that I have known has used a simulation program to calculate the expected discharge temperatures of a reciprocating compressor.  There are good reasons for not doing this.
 

I get 263 ^oF and 313 ^oF for the discharge temperatures of the first and second stage.

 

You are not balancing your two stages as a normal compressor manufacturer would.  You should read the GPSA Engineering Databook for knowledge on how a reciprocating compressor is designed for multistage operation.

[neel_avi]

Dear Mr. Shan,

 

for typical instrument and plant air requirement (of similar discharge conditions) - in two refineries we have been using three stage centrifugal compressors with two intermediate intercoolers and an aftercooler - suppliers Atlas Copco and Elliot compressors. Like any other compressor they all had trip value of high interstage discharge temperature.    

 

Regards

 

Avijit

[shan]

Hi Ankur,

Yes, we usually order air compressor package with specified flow and discharge pressure. Vendors provide us with P&ID's of compressor (single stage), coolers, filters, dryers and receivers. I just wonder why they can handle high discharge temperature.

Regards,

Shan