Cooling a Packed Bed with an A/C

Is the air recirculated?

The air is not recirculated. It is outside air passed through an HVAC once.

I'm guessing you should feed it from the bottom. The A/C air is being heated by the contents of the vessel causing the air to rise. If you feed it from the top, you are opposing the convective flow.

I wonder...

At what air(?) speed will there be enough turbulent friction for these "packing peanut" pellets to ignite in spontaneous combustion?

At what air(?) speed will these packing peanuts be lifted up or packed down to block any air flow?

Not enough velocity to fluidize the bed. Think blowing through a straw into large cup of ice?

"Not enough velocity to fluidize the bed. Think blowing through a straw into large cup of ice"

Sounds like, in this description, you believe cold air should supplied from the bottom of the vessel.

I agree with you, Usbport, and Rixter.

Why once through and not recirculated?

"Why once through and not recirculated?"

I got the impressing there was no air outlet, no "through" just blowing into a closed tank.

"The air is not recirculated. It is outside air passed through an HVAC once."

Well, that could be, I suppose.

Seems like a static condition would soon develop if warmed air isn't exhausted. Prolly would work better if the cold inlet air is filtered through several slices of baloney.

Well that's at least one additional bit of information. Trouble is I used two of the twenty questions to find that bit of information. You don't play by the rules of the game.

Without enough coolant flow this Thorium bed of pellets can possibly thermally run away to the point of fusing the pellets. Do you think it is wise to hide information from us?

Blowing in from the bottom gives you two advantages. The first is that under normal conditions hot air rises, so you making use of natural convective flow. The second is that the upwelling air will help lift and separate the pellets, allowing the air to flow between them better. These two processes work together to cool the pellets.

Conversely, blowing down means you are fighting the convective flow, and you are pushing the pellets together - which both hurt your scheme of trying to cool the pellets.

In addition to natural convection assisting total flow, natural convection when flowing from the bottom up will also help to distribute airflow where it is most needed, reducing hot spots and allowing more rapid heat transfer.

Flow paths that add the most heat will undergo the greatest expansion and experience the greatest bouyance. This works to bring higher flow rates to the hottest areas.

If there are two nozzles at the top one could be used to inject cold air which would want to sink.

The other open nozzles at the top And bottom would allow for convection removal of additional heat.

The location of nozzles is a factor needing consideration. As well as rater of flow through each nozzle.

.my guess

..

Here's another vote for "in at the bottom". That way, the bed is being lifted, though only slightly, and the incoming air is working with natural convection, and not against it.

I would think with small pellets as you stated, that an A/C wouldn't put enough air pressure out to ventilate through 9' of it. Seems like it would restrict it pretty well unless you have a very hefty blower. How big of an A/C are you talking about?

Depends on the packing factor which depends on the pellet size distribution. A bed of spherical pellets of uniform size ends up with about a 30% void. The differential pressure across depends on effective pore hydraulic diameter to the fourth power. So a tank full of basketballs will have much lower resistance than a tank full of b-b's. Then it is just a matter of matching the AC fan curve to the effective hydraulic diameter and length.

There are also two schools of thought on the nozzle entrance. Cold air added to a warm tank will want to sink, promoting air flow. Cold air placed in the bottom will warm and want to rise. Neither case is the full story and most likely a computational flow dynamic model would be needed to optimize it. I suspect that the natural circulation component is going to be much less contribution than the fan curve and compressible flow hydraulic circuit elements.

Question breakdown.

A. 10 foot diameter and 16 foot high vessel. Using the word, " high " would indicate that 10' is horizontal and 16' is vertical.

B. 9 feet of packing in the middle. Nothing to indicate what the packing is made of other than pellets. Pellets could range from pea gravel to styrofoam peanuts. As all mass has weight, a presumption would be that there is open space approximately 7 feet from the top of the vessel. Or the packing is equally divided between the top and bottom, leaving an open space of 3.5' on the top and the bottom.

C. Nozzles, the measurement of 20-24" is not clearly stated as to if this is the outside measurement or of the inside measurement. Also the configuration of the nozzle (s) , flat fan, adjustable, flood jet, hollow cone, etc. Length, etc.

D. Cool. Why do the pellets need to be cooled. If there are only nozzles entering into the vessel, how does air conditioned air enter the vessel. If the blower is placed on the top or bottom and air conditioning is placed on the top or bottom ( blower placed opposite of air conditioner ) , what conditions are required to create equivalency between top and bottom cooling effect.

E. Blower. As the blower creates positive pressure and the a/c and nozzles do also, under what means will air be extracted if all entries are into the vessel and none are provided for relief.

This is why I made the absurd proposal that the pellets were Thorium pellets. Thorium pellets would need cooling due to fission reactions.

We also have no idea what if any voids would nominally exist between any pellets. If the pellet geometry tessellates like rectangular or hexagonal bricks then no cooling fluid (air?) could possibly migrate through.

IMHO this is nonsense.

Yet another secret part of the instructor's scenario may be that the (presumed) waste heat needs to be harvested, perhaps with a peltier device as a thermoelectric generator, this producing enough power to operate the mechanical A/C units.

This process is also likely enhanced with a baloney filter of some sort.

Thorium mixed oxide pellet reactor research is happening. I doubt they are cooling this with an air conditioner but it does fit the stated conditions.

Clarifications:

The blower will be pulling air out of the reactor.

The pellets are metal oxide pellets that need to be cooled in order to be extracted from the vessel (no head being generated)

The pellets are about 75-95 lbs/ft^3 and individual pellet is about the size of rabbit feed.

Why, then, chilled air and not ambient?

I still vote air inlet at the bottom, exhaust from the top.

Heat transfer is based on differential temperature, so just as a typical counter-flow heat exchanger, the cooler medium is introduced at the hotter end, which one might think could be at the top of the vessel.

Measurements could confirm this...

Measurements would probably refute.

There are some problems in your statement.

"... so just as a typical counter-flow heat exchanger, the cooler medium is introduced at the hotter end, which one might think could be at the top of the vessel. ..."

'Cooler medium is introduced at the hotter end' is not a true statement for counter-flow heat exchangers. The cooler medium is introduced at the cooler end where the hot medium is exiting and has already cooled substantially...hence counterflow.

Your statment would hold true for parallel flow and kind of true for cross flow.

"...the hotter end, which one might think could be at the top of the vessel...."

Why? Why would one think that? If the pellets were recently deposited, why would there be a likelihood of segregation of hot pellets to the top? If the hot pellets had been sitting for a period, why wouldn't a hotter region be more likely to be in the middle?

The vessel could be similar to a reactor, with static internal elements, (similar to a RTO, perhaps), where simple convection would place the warmer components at the top.

I may have my heat exchanger terminology mixed up, but you get the principle. It is common practice with steam to water tube and shells, if I'm remembering correctly?

I get the principle though I suspect if you think about it, the counterflow heat exchanger does not have the cool stream entering where the warm stream enters (where it will be at highest temp) but where it exits (lowest temp).

Moreover, I think the analogy to heat exchangers with physical separation of hot and cold streams is misleading when considering direct contact heat exchangers.

Consider that if cool air is fed in the top, the air pulled from the bottom will be more likely to be denser cooler less boyant air. This means the air that has removed the least heat will be the most rapidly removed. The air that has already taken the most heat will be more likely to stick around in counter currents and pockets as it will be more buoyant.

The point is the differential temperature dictates the heat transfer, so the cooler air in at the hotter end should move more heat. It is likely the specific heat of the fill/ packing is higher than the cooling medium, so get the heat out while you can is the philosophy I’m going with, short of an actual thermal modeling exercise...

So once your cooler air at the hotter end has removed more heat and become warmer, where does it go?

back to its source, the great enthalpy of our planet, from what I understand...

A one pass heat exchange in the microcosm of the vessel