You Can’t Cheat Physics 101

We are starting a new category of postings called You –Can’t-Cheat-Physics-101

These are things we see time and again where the requester is basically looking for something that is physically impossible.

Small caveat. It is entirely possible that in the future a new scientific principle will be discovered (or some alien dude from Alpha-Centauri will deliver it) to accomplish what is currently not possible. But it would have to turn everything on its head what we currently know about physics and thermodynamics including concepts around entropy and enthalpy.

If that occurs I’m sure you will hear it on the news. Especially the part about the dude from A-C.

So in the meantime as all we can deal with is our knowledge and understanding of physics, as it is currently, we present the first entry on you-cant-cheat-physics-101.

First up we will talk about air cooled heat exchangers. The same thing applies for shell and tube heat exchangers but we will confine the discussion to air-cooled heat exchangers for now.

Often we get requests for an air cooled heat exchanger (ACHE) and we need to start educating a bit.

Basically the concept is dead simple. You bring a liquid or gas through the cooler and using the ambient air the ACHE will cool down (lower the temperature) of the contents within the tubes to a temperature close to the existing ambient air temperature.

If unlimited funds and real estate were available you might not care what it cost or how big it was. But as that isn’t the case our design must be realistic in commercial terms (i.e. price) or space (footprint considerations).

Now as we don’t all live in the tropics (don’t get me started) we generally will see some significant swings in ambient temperature. In fact -40F to 100F is not unheard of in some parts of North America (really, don’t get me started).

We have to land on an ambient temperature. That’s what is referred to as the ambient design temperature. When the temperature is below freezing the likelihood of not having enough cooling is remote so we need to look at the upper end. For instance, let’s use 100F again.

How often does it get that hot out? What if it does get that hot? Would the process just limp along until sun set (with reduced production) or would you have to consider shutting the process down or providing other measures of cooling.

Because basically to meet objectives of cost effective we don’t really want to design at the far upper end of the temperature range as that temperature may only be seen once a year or perhaps once every several years.

So we pick a design point that reflects a reasonable value where most of the time it wont be exceeded but is not so high as to MAKE THE UNIT TOO EXPENSIVE. As my friend Steve will point out,” the last few degrees of cooling are the expensive ones”.

Now the hot gas or hot liquid side is already determine d. What is not yet defined but must be selected is the outgoing gas temperature from the exchanger that is returned back to the process.

And generally it seems everyone wants it as low as possible.

I would have to say in general we see process engineers generally have a very good idea on what the outgoing or cooler exit temperature is but only have a vague idea on what the ambient design should be. Basically because it is “outside the process” so to speak and besides they have no control over the weather.* They may not even care. They will attach a spec with annual temperature data and basically throw it at the heat exchanger company to figure it out.

The problem is we often and I’m talking, really often, see the temperature range be (say) -20 to 100F (sensible heat) but the gas leaving the air cooler to be (say) 75F.

Well here’s where you can’t cheat physics. If your outdoor temperature is 100F there is no way you can cool that gas to 75F using the ambient air. I don’t care how fast you turn the fan. It just ain’t gonna happen. The gas temperature in the tubes will approach but never reach the outside ambient temperature. Also note the key word is “approach” which is a jargon word ‘cooler guys’ use. Typically on a cost effective scale a reasonable approach is about 7 to 10 degrees F so in our example — 107 to 110F.

Now when we get into these discussions that you can’t cool below the ambient air it often takes a while for the realization to sink in. Likely because there is a range. And everyone sees that range and perhaps they are doing the math in their head to figure out how the process will need to change when temperatures creep up. Or whatever. Not really sure.

All I know is cooler guys generally have to go around this pole a few times. Generally there is a reluctance to have to decide. The process engineer has to explain to someone who might not be classically trained in physics that they can’t cool below ambient (and they don’t have money in the budget for a chiller) nevertheless that person wants his output at 75F because that is where the process runs real well.

At the end of the day the technical people who know the process well at the plant must decide on an ambient design temperature and the outgoing temperature.

The air cooled heat exchanger people can’t do this for them. They know the air coolers. They know the metallurgy. What it can do. The energy it will use. How it will stand up to the gas or the liquids. What the average life will be of the tubes. How to guard against plugging. Whether it is best to use induced or forced draft, and much more. In other words, all in all, a fair amount of complexity to consider.

But the intimate process knowledge belongs to the plant purchasing the equipment and they need to specify those two temperatures – temperature of of gas or liquid leaving the heat exchanger and the ambient design temperature (and not a range).

And the only other thing is to make sure the design ambient temperature is 7 to 10F lower than the expected cooled gas or liquid temperature returning back to the process. Coz we can’t cheat physics – yet.

*Editor note: this might not apply in Russia


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