6 replies to this topic

[Abid]

Dear members,
I need some input regarding the thermal design of new air cooled exchanger. we normally work for Saudi Aramco projects and our client have customized their standards from industrial standards. Saudi Aramco standard for air cooled exchanger suggests that airside fouling resistance shall be 0.002 ft2.h.F/Btu. I just want take reference from API STD 661 which does not give such reference for airside fouling resistance. Please let me know if you know about some industry practice airside fouling resistance values. My concern is how much conservative this value is if it is on higher side and what would be its affect on exchanger design if we use this value.
The other thing is deign air inlet temperature, I understand this should be the design dry bulb temperature in summer. Client standard suggest this temp. should be increased due to location of the exchanger with respect to processing units and buildings and the number of bays. For example 2 F shall be added to basic design dry bulb temperature if the exchanger is located within industrial plants or shaded by buildings. If we account other temperature margins like for bays and layout effects then the final air side design temperature may be 10 F above the actual design dry bulb temperature. My question is whether these values give us more conservative design or industry practice also suggests about these values to be considered in the thermal design of the air cooled heat exchanger. If these marginal values are exceptions from the general industrial practice then what shall be the effect of these exceptions on the design of the exchanger. What is your expert opinion on this. Your feed back on this will be highly regarded.

Abid

[ankur2061]

Abid,

ACHE design air temperatures specially in the middle-east (I worked 4-1/2 yrs in the ME) are considered arbitrarily based on operating experience due to the extreme climate changes. I prepared some company standards for a ME Oil & Gas company called ADMA-OPCO. One of the standards titled "Specification for Process Design Basis" has a dedicated section for ACHEs. I am reproducing the section for your benefit:

Air Cooled Heat Exchanger (ACHE)
a. Induced draft type ACHEs are not recommended for effluent air temperatures above 95°C. This is to prevent potential damage to fan belts, bearings,
V-belts, or other mechanical components in the hot air stream. Reference is made to GPSA Engineering Data Book, 11^th Edition.
b. A straight tube length of 9.2 m (30 ft) or 12.2 m (40 ft) is preferred. If required by specific design considerations the use of other lengths within the maximum length specified, may be proposed.
c. Finned tubes with fins serrated on outside edge shall not be used.
d. Reference is made to BP GS 126-2 and API standard 661 / ISO 13706 for design of ACHE’s.
e. The design air temperature for ACHE’s shall be taken as 45°C (113°F) for all ADMA-OPCO requirements.
f. One vibration cut-out switch shall be provided for each fan driver unit. These shall be of the manually, externally reset type not requiring dismantling of the switch for resetting and shall have sensitivity adjustment.
g. When aluminium alloy fan blades are selected the fan ring shall be manufactured from glass fiber reinforced plastic. If glass fiber reinforced plastic fan blades are selected, precautions must be taken to prevent build-up of static electricity.
h. Partition plates with holes shall be provided to allow complete draining and venting of header compartments.
i. Generally a slope of at least 1:100 should be provided towards outlet to ensure complete emptying of tube bundle.
j. Fin spacing for tubes shall be such that, dust/sand does not settle in between fins. This is particularly relevant to Gulf region (sandy conditions).
k.Temperature control shall be done either by automatic louvers or using variable speed drive for the fans.
l. At least two fans per bay for each ACHE shall be provided in order to maintain operability during breakdown/maintenance of one fan.
m. Direct drive (fan shaft directly coupled or through gear box to electric motor) is preferred over V-belt drive.
n. All electrical equipment shall be selected to suit the specified area classification.

Hope this helps.

Regards,
Ankur

[Zauberberg]

You should be following the standard from your client since they probably know very well what kind of fouling factors and ambient air temperatures have to be considered for air cooler design. They may be over-conservative, but if you chose to follow your own line of thinking - you will save some of the client's money if you are right, but you will also put your company in trouble if your assumptions prove to be incorrect and you end up with an undersized cooling/condensing equipment. You certainly want to avoid that.

Fouling factors and ambient air temperatures have mostly been derived from operating experience, since even in the same geographic areas (e.g. Middle East) there can be several quite different climate conditions. As an example, sandstorms are quite frequent in Qatar but you will not see them so much in Oman, and that can affect the rate of air-side fouling up to a great extent. Do you wish to challenge SA experience and guidelines? I know I wouldn't.

As for the ambient air temperature, remember that the plant has to be designed to run safely and efficiently at design throughput (nameplate capacity) during entire year, which includes peak summer heat as well. Now, if you set your contract agreement that the plant swill meet this throughput on average yearly basis (not very likely that any Operator will accept) or, more likely, on a daily basis, then you realize that you certainly cannot go well below maximum air temperature - it all depends on how much extra production you can get during other periods of the day, and if all other equipment in the plant will be capable of meeting those extra production rates in a safe way.

Best regards,

[Art Montemayor]

Abid:

Ankur has furnished you with some excellent, experienced insight into how practical and real field engineering is carried out in practice. He is addressing your query in a round-about way.

The Middle East is not an exception; it is merely one of many specific and unique engineering environments that have been empirically found to demand special and unique solutions to ordinary engineering problems. Ankur’s information reveals a lot of hard-earned “know-how” in applying air-cooled units to that part of the world. I am certain that Aramco is displaying that same kind of in-house experience when they assume what you or I initially see as an ultra-conservative stance. But it is refreshing and a relief to find that degree of client interest and knowledge in specifying what they need and what they want to fulfill that need. This is a mark of a very knowledgeable, sophisticated and experienced client – one to be admired and grateful for. Usually, the project picture is reversed: the client expects the engineering firm to resolve all of its problems without furnishing any basic data or details of its expectations and requirements.

My response to your specific question is that a conservative design is not necessarily what it seems to you. A client knows more about how his facilities work – and under what conditions – than you or I ever will. Don’t ever forget the basics: an engineering firm only designs and builds and then walks away to another project; the client has to remain behind and make the whole facility work and work continuously while generating a set and pre-defined profit margin. The engineering firm normally knows next to nothing about operating a facility. It is in Operations that the client should be the expert. And it is because of this operations expertise that the client has the inherent responsibility to be very specific and detailed about what it requires and to what detail. The Dupont Company used to be the ultimate in following this business principle and I hope they still follow these basic concepts that they helped to establish nearly a century ago.

What I am referring to is that the client should know exactly what he/she requires from you and when they specify it out, you should be all ears because it saves you a ton of work in guessing what it is that you should be quoting on. And this type of business arrangement almost always ends up in a “win-win” situation instead of a finger-pointing contest of placing the blame for a fiasco or error on someone else.

Of course the air-side fouling factor and an inflated dry bulb temperature make for a larger air-cooled exchanger. That is basic engineering. How much is not possible to tell, but the final design will certainly be more costly. Rest assured that Armco knows this up-front, regardless of what their purchasing agent says. If they specified it, they should be willing to pay for it.

[Abid]

Dear Members,
Thank you very much for your timley and very insightful feed back in this matter.

Regards

Abid

[sundutt]

yes your & ankur's information is very useful, furthermore for operational stability & maximising production how we can optimise performance of finfan coolers, as ambient temperature is very high & due to dirt deposition heat transfer affects a lot, variable speed motors reaches maximum speed,fixed speed motors also limiting the production, how we can consider these point while designing as well as in operation

[chemsac2]

Shell DEP has following statement for airside fouling resistance:

"Airside fouling factor is generally not used as it does not affect heat transfer, but may add resistance to air flow".

Proposed airside fouling resistance of 0.002 F-hr-ft2/Btu is much lower compared to typical airside heat transfer resistance of 0.12 F-hr-ft2/Btu (typical airside HTC of 40 kcal/hr-m2-C). I remember reading an HTRI presentation explaining significant effect airside fouling layer thickness has on air flowrate. It is this decrease in flowrate, increase in air temperature and decrease in LMTD that affects ACHE performance.

Air recirculation studies can be conducted to check if such high margin above design air temperature is required.

Regards,

Sachin