New gas compressor with existing coolers

[bicycledisciple]

Hello everyone,

This maybe a heat transfer related question as opposed to gas compression but context is within a gas compression application. Requesting some insights from process and/or air cooler specialists.

Consider a new replacement centrifugal machine being installed in an existing plant with all interstage equipment remaining the same. There is an intercooler and a separate aftercoler. Aftercooler has a design limit of 250 F with no tolerance.

Looking for a qualitative understanding of impact to cooler - flows, pressure drop, temperature out etc should the compressor discharge T exceed 250 F. Would there be severe impact to the existing equipment? Process conditions below :

First stage inlet P = 15.7psia
First stage inlet T = 120 F
First stage inlet V = 51535 cfm
First stage discharge P = 164.9psia
First stage discharge T = 324 F
Second stage inlet P = 154.9psia
Second stage inlet T = 140 F
Second stage inlet V = 4329 cfm
Second stage discharge P = 484.7psia
Second stage discharge T = 255.7 F

Thanks.

 

[LittleInch]

I don't really think 0.7F in 250 is that much of an exceedance.

However I would question the tolerance and input values for the data provided, especially the cooling air temp. If all are based on max or min design values then in reality you're probably within the temperature limits.

The key factors would be exit temperature of the gas out of the after cooler, official pressure rating perhaps of different flanges and internal stress calculations within the cooler.

The key will be controlling flow based of inlet temperature to the cooler if it does actually exceed 250F.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[bicycledisciple]

@LittleInch,

Thank you, I share the view point on the magnitude of the exceedance. The cooler is out of our scope and the concern is continuous operation with the higher temperatures. Meeting the discharge temp limit means having to move closer to surge on the first stage compressor map.

Do you think a small excursion could do with little modification like re-size the fans or re-spec'ing the Motor? I can't see the bundles and their area changing as those are fixed. Thanks.

 

[LittleInch]

In terms of duty you really need to talk to the HX vendor, but depending on the fouling factor they use or the current cleanliness of the cooler, in reality I can't see 0,7F or even 7F making any real difference. I wouldn't touch the fans or motor if I was you....

Any chance you could e.g. add some cooling fins to the new pipe going from the compressor to the HX?

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[georgeverghese]

Not clear what you mean by design limit of 250degF for the stage 2 cooler - is this is the process design temp limit or max permissible operating cooler inlet temp or the mechanical design limit ? 250degF as a mechanical design limit for an air cooler seems unusually low.

If the mechanical design limit is indeed 250degF, ask the mechanical engineers if it can be re rated to 300degF or so. You need some room for the operating fluctuations / compressor fouling / TAH / TAHH alarms and trips. Operating modes to be covered should include full recycle also with no fresh feed.

If there is a bottleneck in the cooler duty to cover all these cases, agree one way would be to put in bigger fans or increase fan speed.

 

[rotw]

I would say if the design temperature is 250 deg.F with - no tolerance - as stated, then this must be complied with. If something goes wrong then the vendor is not liable for the equipment performance or other guarantee term under the contract. Otherwise, I guess a "feasibility study" should be ordered (maybe just an email from the heat exchanger vendor that says yes...). I agree the exceedance seems relatively low but contractually it can be sometimes tricky...not sure what is your context.