8 replies to this topic

[renat547]

Hello Everybody!

 

Here is a problem:

There are three oil flooded screw compressors with design capacity of 1800 Nm3/h each, used for instrument air supply. Compressor discharge pressure and temperature is 7-7.5 barg and 65- 70ᵒC respectively. (Compressor's integrated aftercoolers are not working properly) Compressed Air to be cooled from 70ᵒC to 35ᵒC. This will improve air quality entering air dryers and prevent moisture going into dry air system.

Equipment to be installed outside. Ambient temperature is -36⁰C to 44⁰C. Relative humidity is 68%. 

 

Some solution:

Install chiller unit & HEX (no matter what type of HEX). I conducted many suppliers & manufacturers, but no one has answered that they can supply such chilling unit.

 

I run two cases for that system in HYSYS & here are its results/my conclusion:

1. Chiller & heat exchanger for cooling capacity of 18.72 kW.
2. Chiller& heat exchanger for cooling capacity of 56.16 kW.

NOW IS MY QUESTION:

1) Could anyone advise another solution that will help to reduce air temperature?

2) In simulations I took 5C as a Delta T for chillers. I have to include that information to my report. There are many info on that topic (delta T = 5C) at suppliers web-sites, but I want use more official data. Does anyone have it?

 

Thanks in advance.

[PaoloPemi]

air density at normal cond. of 20C 1 atm is about 1.2 Kg/m3

 

then your (mass) flow  is about 1800 Nm3/h * 1.2 = 2160 Kg/h

 

supposing you have a pressure drop of 0.5 bar in exchanger

 

and Tin 70 C tout 35 C

dH (difference in enthalpy from 7.5 Bar 70 C, 7 bar 35 C) is about 35 Kj/Kg

 

from these numbers you can calculate the required exchanger duty

 

it is not clear how you have calculated the reported 18.72 kW. and 56.16 kW.

[Art Montemayor]

 

Renat:

 

I’ve never been to Kazakhstan, but I know a little about its geography and location.  Its continental weather has a broad range – as you indicate it.  However, this should normally be no problem in obtaining a reliable and efficient compressed air cooling system.  Your major problem is probably during the hot summer months.  I have compressed and dried a lot of air in my past engineering assignments – most of these projects involved what you have described and I resolved my problem by using a variety of solutions.  I have to assume that you have no reliable cooling water source nearby, so you are reliant on atmospheric air cooling.  In that case, you can use one of two methods:

1. A mechanical refrigeration “chiller” unit; or
2. An evaporative cooling system that uses a minimal of water consumption.

If you are having trouble getting professional help from engineering suppliers on this type of equipment, you can simply design and build your own cooling system.  The evaporative cooler is the simplest to design and build.  The mechanical refrigeration unit takes more effort and capital, but is more robust and yields a very dry, cool air product.

 

You do not mention it, but I assume that you take the cooled air discharge from the screw air compressors and put it through an adsorption dryer to bring down the air dew point to approximately – 60 ^oC in order to protect the instrument air from forming water ice during the cold Kazakhstan winter months.

 

I don’t have to use HySys to calculate the cooling capacity when I cool the 5,400 Nm3/h of air from 70 ^oC to 35 ^oC.  My calculations show that your heat load on the cooler would be approximately 70 kW.  See the attached workbook.

 

My recommendation for you is to buy or build an ammonia mechanical refrigeration system that will cool the 5,400 Nm³/h from 70 ^oC to 2 ^oC – and thereby condense out the greatest majority of the air’s water content and drain it out in a vapor-liquid separator before introducing the air into an adsorption dryer bed.  I would use two heat exchangers in the “chiller”: a heat exchanger between the incoming air and the exiting air as well as a refrigerant evaporator for the “chilling” of the air down to 2 ^oC.  This type of equipment is very common and should be readily available in your marketplace.  If you have any problems in specifying or obtaining proposals on this equipment, let us know here in the Forum and our members will gladly help you out.

 

My normal specifications for instrument air in continental weather conditions are 150 psig with an atmospheric dew point of -60 ^oF (10 barg at -50 ^oC).

I don’t know what you mean by a “Delta T of 5 ^oC”.  Is that the approach of the compressed air temperature to the cooling air temperature?

Attached Files

•  Instrument Air Compressor Cooling.xlsx   12.01KB   720 downloads

[renat547]

Paolo, thank you for your reply.

 

I used HYSYS V7.1 (23.0.0.7119). But using your number of density & formula Q=m*c*dT, then,

2160 kg/h*1.0035j/gK*(70-35)K we get 21.0735, which is approximately same for cooling air from 1 compressor.

 

I made mistake in my first post, as numbers 18.72 & 56.16 were for the cooling to 40C not 35C. Now I re-run their cases, and new duties are as follows: 21.84 kW & 62.52 kW respectively.

[PaoloPemi]

your corrected values for duty now agree,

 

with my previous data (from air properties tables)

Pin 7.5 Bar.a Tin 70 C

Pout 7 Bar.a Tout 35 C

 

duty = 3 * 2160 * 35 / 3600 -> 63 KW

 

if I solve with a software (PRODE PROPERTIES) I get 63.883 KW

similar values with NIST REFPROP

 

as commented by Art there are different options to cool the air,

the selection may depend from utilities available in your plant (water etc.)

or favourable ambient conditions which may allow direct heat exchange with air, water etc..

If you decide for mechanical refrigeration (whch is usually expensive compared to other options) 

make sure to set an average saturation temperature (evaporation pressure in modern dry evaporators is not constant) in evaporator above or close to 0 degrees, to avoid constant formation of ice.

[renat547]

 

Art, thank you for your reply!

 

Please see attached PDF file. Is that is what you are saying?

 

My initial guess (and still no other options) was the same, as described in attached file. Grey color - is existing piping & equipment. I deleted unnecessary Instrumentation, bypass, etc.

 

I also want to be sure, that we are talking about same "chiller" units...For that chiller units, delta T of 5C is approach for refrigerant temperature entering & leaving evaporator.

 

I didn't understand your opinion on reducing air temperature down to 2C. It is OK, that most water content will be drained at air receiver with lower temperature of compressed air, but as I was informed, air dryers are designed for such loads of moisture.

 

I think, suppliers, which I conducted, are working on HVAC systems only & they are afraid to work with compressed air at such temperatures. As you suggested in your post, I will post topic on required equipment here (at this forum).

 

Also, at plant, that we work on, additional chilled water consumers are not allowed. So, only separate unit or air could be used in solving my problem.

Attached Files

•  Cooling model.pdf   30.02KB   491 downloads

[Art Montemayor]

Renat:

 

The attached pdf describes what I mean as a standard method of refrigerating air in order to dry it.  The additional Word document explains how the dryer works and how to size one.  In the past, I often used 35 ^oF as my design and operating point for the chilled air exiting the refrigerated evaporator.  Today, the ISO has fixed 38 ^oF as their standard.  Close enough.  Nevertheless, the basis of design is to produce compressed air at ambient temperature (or close to the incoming, wet air) but with a pressure dew point that is depressed below what it goes in as.  To do this, a heat exchanger is used – as illustrated in the brochure – to exchange the heat of the incoming air with the cold of the exiting refrigerated air.  This reduces the refrigerating load and eliminates the formation of condensed humidity on the surface of the produced air piping (and its resultant corrosion).  This is the standard, normal way of drying air by refrigerating it.  Everybody that I know uses it this way.  Where a lower “Pressure Dew Point” (the dew point of the air at the operating pressure) is required – as probably in your case - a downstream ADSORPTION dryer is used to “polish” the dew point down even further.  I have often used this system with excellent results.  I used Activated Alumina as the adsorbent and dropped the air atmospheric dew point down to as low as -103 ^oF (-75 ^oC).

 

With only a refrigerated dryer you can produce a product air that will not condense its water content (because it still has water vapor mixed with it) until it reaches 3 ^oC (38 ^oF).  That means if your instrument air piping and tubing is exposed to an ambient air temperature below 3 ^oC, the air inside it will start to condense and form ice particles in your piping.  That is why in cold regions I always used adsorption dryers (by themselves or as follow-ups to refrigeration dryers).  In your part of the world (Kazakhstan) I would expect to always use adsorption dryers – with or without prior treatment with refrigerated dryers.

 

I hope this helps you out.

 How does a Refrigerated Air Dryer Work.docx   25.59KB   2886 downloads

 IR Refrigerated Air Dryers_Nirvana.pdf   1.36MB   1291 downloads

[renat547]

Many thanks Art!

 

I will work on that option.

 

Last general question. Do I have to post my actual decision, which will be done in month or more?

[gegio1960]

Last general question. Do I have to post my actual decision, which will be done in month or more?

 

Yes, thanks. We'll be happy to see the end of the story and, more generally, how the suggestions from the forum members have been applied "on the job".

;-)))

Edited by gegio1960, 29 August 2013 - 10:08 AM.