32 replies to this topic

[Snorm]

Hi all I’m a well driller and I recently acquired a rig with a le roi reciprocating compressor. It is a two stage compressor capable of 450cfm at 250psi. I have 8 years prior experience in be ing an industrial millwright in cement manufacturing, I have been commissioning the rig and all is working as it should, except I’m seeing high discharge temperatures from the compressor, I did orifice testing to check output and it is producing it rated output, the first stage makes 35 psi feeding the second stage and runs about 350f before the intercooler, after the cooler it’s about 120. The second stage runs at 340f at 90 psi and up when I build more pressure. I cannot get any information on what the pipe temp should be directly after the heads I’m looking for help generating a ballpark estimate of what I should expect. My temperature readings are all done with an infrared gun and have checked with other guns it is accurate. I know jumping 35psi to 250 psi will generate a lot of heat but it makes me nervous seeing temps this high. Any help would be appreciated thanks

[Pilesar]

Are you compressing air? I would not be surprised at over 300 F temperatures. I found a manual https://ndsdrillings...compressors.pdf  I did not see temperature limits listed. For expected temperatures, you might want to explore this online calculation page: https://checalc.com/calc/compress.html Change the units of measure at the gear symbol at the top.

[Snorm]

Thank you for responding, I am compressing air. I have been looking for a calculator like you provided but I’m not savvy enough to fill in all the units in the second portion. It is 2 stages with an intercooler . The first stage feeds the second stage 35psi the temperature of air at the suction of the second stage is about 120 I would like to know what temp would be generated at 160 psi and 250 psi. The intake cfm is 450. the first stage stays at a constant 35 even when building more or les pressure at the second stage.

[Snorm]

I also have that manual you found and have reached out to the dealers and the are unable to provide any useful information on discharge temps,saying they are not provided that by the manufacturer.

[latexman]

Google is your friend.  I found air properties here:

 

Google "air critical properties" - https://www.concoa.c...properties.html

 

Google "air accentric factor" - http://www.coolprop....fluids/Air.html

 

Google "air Cp/Cv" - http://www.coolprop....fluids/Air.html

 

Intercooler pressure drop of 1-5 psi is reasonable.

 

You can check the sensitivity of different equations of state.  I doubt it matters much.

[Bobby Strain]

Try this company in Odessa.

https://www.sanjuanc...roi-compressors

 

There should be some thermowells to check temperatures with a dial thermometer. And, I recommend that you not increase the discharge pressure beyond 90 psig where discharge temperature is already 350 F.

 

Bobby

Edited by Bobby Strain, 21 May 2022 - 01:20 PM.

[Snorm]

One of my questions would be is surface temp of pipe a good reading as far as discharge air temp

[latexman]

If insulated, yes. If not insulated, not so good.

If not insulated, temporarily use some insulation and wrap the line and probe.

[Bobby Strain]

Something seems strange about the interstage pressure. How are you controlling the discharge pressure? Is this a variable speed driver? I would expect to see a higher interstage pressure than 35 psig. This is a compression ratio of ~3.3. I would expect the interstage pressure to be about 45 psig if this compressor is built to achieve 250 psig discharge pressure. But maybe it has been modified for a lower pressure operation. These machines are made to a standard capacity, and, if your flow measurement is correct the compressor appears to be mechanically sound. When I get time, I will check your information. If I can get physical dimensions of cylinders, speed and stroke I can model it. Also whether cylinders are single or double acting. And rod diameter if double acting cylinder.

 

Bobby

[latexman]

When I was playing around with it, I saw Stage 1 ~ 35 psi when Stage 2 = 160 psi.

[Bobby Strain]

What does that mean; to"play around"? Do you have access to some details? I couldn't find anything other than the manual.

 

Bobby

[latexman]

Pilesar’s checalc link

[Bobby Strain]

Here are a couple cases. Don't pay any attention to the capacity or power. Just look at pressures and temperatures. You will see that the compressor exhibits quite high discharge temperature for the data given compared to a calculated value. The design case shows operation at high pressure with approximately equal compression ratios, which is the minimum power design. The measured interstage pressure suggests that the compressor is not delivering design air flow. And something is amiss.

 

The calculations use an adiabatic efficiency of 98%. This small compressor is likely to be somewhat lower. Countering that is cooling the cylinders.

 

 

Bobby

ps The interstage pressure loss is %, not psi. I have had this for 40 years. First in Basic.

Attached Files

•  CHEAir.pdf   38.08KB   15 downloads
•  CHEAirDesign.pdf   38.16KB   5 downloads

Edited by Bobby Strain, 22 May 2022 - 05:34 PM.

[Snorm]

This is an old compressor rebuilt in the late 90s first stage should be producing 45-50 psi I believe.It is belt driven by a Diesel engine. rpm is 1200 at full speed. first stage has 4 7” cylinders not dual acting I have had all the valves out and they are in decent shape they hold solvent quite well. So I’m guessing lack of pressure is due to blow by in the cylinders. if the first stage is running low at 35 psi could this cause a significant change in discharge temp of the second stage?

[Bobby Strain]

It is difficult to assess the problem without reliable temperature and pressure measurement of inlet and discharge. I am only guessing, but I suspect that the inlet temperature to the second stage is much higher than you report. Or the inlet pressure to the second stage is lower than estimated from the first stage discharge pressure.

   And, yes, the discharge temperature of the second stage increases at a given discharge pressure if the first stage discharge pressure is reduced. If you can give me more information for the cylinders, I can be more accurate in my assessment. I need piston diameters for the second stage as well as the first stage, speed of the compressor (or driver along with pulley diameters), piston stroke, and piston to head clearance if available.

 

Bobby

[Snorm]

First stage 4 cylinders 8 1/8” diameter, second stage 2 cylinders 6 1/8” diameter, stroke is 5 1/4” on all cylinders, piston speed .875’ per rev, 1200 rpm turning speed, head clearance is unknown approximately .040”
Thanks for helping on this

[Snorm]

Intake temp is outdoor ambient 50f first stage discharge at 35psi 320f before cooler 120f after cooler. 340f discharge temp on second stage at 90psi

[Bobby Strain]

I'll have to look at this data on a process simulator. My simple software doesn't have capability to change efficiency. Looks like you are in Canada or northern US. I'll check it tomorrow, and look at higher discharge pressure.

I expected to see a lower intercooler outlet temperature with 50 F air. Is it clean? To match your pressures I have to use a significant clearance for the first stage. This indicates that the first stage cylinders are somewhat deficient in air delivery. If you are not observing much blowby this suggests that perhaps the suction valves on a cylinder are not fully closing.

Also, is the compressor rpm the same as the driver? Which means the pulley diameters are the same. I'll post tomorrow the results from a better analysis.

 

Bobby

[Snorm]

Yes the turning speed of the compressor itself is1200 rpm when driver is at 1800rpm

[Snorm]

My dealer said first stage should be providing 40-55psi

[Bobby Strain]

I forgot to ask the elevation of your site.

 

Bobby

[Snorm]

Elevation 4500’

[Bobby Strain]

I did a bit of work to try to estimate discharge temperature at higher pressure. I estimate the second stage discharge temperature will be about 375 F with 105 psig discharge. The first stage discharge pressure will change about 2 psi. Unless you have conflicting information I recommend that you limit operation to 380 F. Since you are measuring skin temperature, the gas temperature will be higher. More so with air from the cooler blowing around the equipment.

 

What you may observe as you raise the discharge pressure is a lower temperature than I predict. Why? These high speed compressors typically have an increase in efficiency as the compression ratio increases. And this produces a lower temperature rise.

 

I suggest that you provide more accurate temperature measurement of the gas by some means. If there is no way to install a thermowell, the next best is to use a thermocouple attached to the pipe with heat transfer cement. And provide at least 2" of insulation over the area for a couple feet.

 

The temperatures you are experiencing seem to be not unreasonable for this compressor at the given operating conditions. But I have no experience whatsoever with this machine. You should take a look at the cooler. Sometimes operators spray water to improve cooling. But it gets worse soon due to salt deposits.

 

Let us know what you discover.

 

Bobby

Edited by Bobby Strain, 25 May 2022 - 10:03 PM.

[Snorm]

Thank you very much Bobby I will keep you updated

[Bobby Strain]

Snorm,

     At your site elevation, the compressor will deliver about 85% of the 450 cfm. Which means that the first stage discharge pressure can only reach less than 40 psig. Thus, you can't expect to achieve 250 psig because the discharge temperature would be too high.

 

  I failed to mention that the first stage shows good efficiency; around 90%. The second stage efficiency is less than 65%. So, there is hope that you can achieve higher pressure without temperature imitation. This exercise has stimulated some dormant grey matter.

 

The second stage shows a significant capacity loss (contrary to my initial estimate before I accounted for your site elevation). The first stage looks OK. So the second stage discharge valves might be leaking. Or the suction valves may not be closing leak tight. This could explain the low efficiency and high temperature. When you raise the discharge pressure this will get worse. Thus discharge temperature may be above my estimation.

 

Bobby.