Low temperature air-air heat exchenger

[kjocis]

I am having a difficult time in figuring out the best way to recover heat from drying process.
The basical idea is - heat is supplied to drying line, where gras is dried. Thhe hot and moist air from grass is collected and through heat exchanger cooled down. The fresh air for burning process (biomass burner) is pre heated with the exhaust air from grass drying (HE mentioned above).
Air flow volumes are quite large, ~30 000 m3/h. Temepratures are Fresh air ~15C, air from drying process ~80C.
What would be the best way to do this?
This probably ir the most efficient way to do this

http://www.energydepot.com/RPUcom/library/images/Misc002_1.gif

, but then I am thiking about the tubes... finned.. dimpled.. other?
Any suggestions would help, thanks!

 

[IRstuff]

Wouldn't that really depend on how much tubing you have? The bottomline should be what your total exchanger surface area is.

TTFN
faq731-376

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[kjocis]

Okay, true, but ''tubing'' is a wide term. I mean, there are ways to increase the turbulence of air flow - finns and different heat transfer augumentation devives. How to choose the best one..? Are there any comparative studies I could check out? Or should I just get 10m of different types and try to check what the temperature and pressure drop is..?
And of course I would like to keep the HE as small as possible, but the size isn't the main argument here.

 

[speco]

kjocis,

These types of heat recovery units have a few problems. First, the operating pressures are near atmospheric on both sides, allowing for very small pressure drops on both sides of an exchanger. Low pressure drops, generally translate to low heat transfer coefficients. The second problem is that the available temperature difference is very small. These factors combined result in realatively huge exchangers for the amount of heat recovered. The payback for this type of exchanger is generally extremely long.

Since the heat transfer in both fluids is low, fins are of no help. The better choice is either bare tubes or plate-type exchangers. In either case the econimics of these are pretty awful.

Regards,

Speco

 

[kjocis]

I guess that this is the harsh reality Speco.., but what would be the solution then? Heat pump-something (+ energy costs, maintenance etc)? Or increasing the pressure (sounds awfull)..

 

[speco]

kjocis,

I wish I knew. I've never seen a project of this type actually go forward. Here are a few ideas to get you going. Let's assume an overall U value of 6 btu/(hr-deg F-Ft^2). This is approximately 34 W/(m^2 deg K) Calculate the heat you are trying to recapture. Calculate a log mean temperature difference using the resulting four terminal temperatures. From that, you can obtain a required surface. Assume some value per surface area (maybe on the order of $50/ft^2). That's just a guess. The larger the surface the smaller the cost per unit area. You will quickly get an idea of how much it costs to recover any quantity of heat. The exchanger will only be a portion (maybe half) of the installed cost.

As an alternative, you might try contacting a few vendors of this type of exchanger.

Regards,

Speco

 

[NoHtPp]

Perhaps throw some numbers at the 2-row 30 sq meter face area of a coil at an OEM/ (their software- coil design or equal) of air coil HX's and see what fluid temps at 3 GPM (?l/m) that you arrive at per 10,000 btuh to 15,000 btuh at selected water temps and DX refrigerant conditions to get a close idea.

I only put it that way because with ground-loop NO Heat Pump process cooling in 50f soils, I have circulation numbers and differences for both pre-heating and pre-cooling air +/- 7f or more different than of those 50-52f soil temps I have.
This is a ground loop of tubing is 1" dia PE3408 dr-11 and circulated fluid from water to 34% p-glycol is used. Earth-Coupled to Air exchanges, near 12 months a year, like a solar collector-
just an Earth-Collector / Rejector (ECL: Earth Coupled Loop)

Some 7ft deep x 2ft wide to 32" wide ditches in damp clay transfer with three 3/4" PE lines [over at 5ft + under at the 7ft, of 6 pipes in a cross-section view], about 100 btuh per foot, rejected heat as a sink, and absorbing from those same ditches as then a "source" at about 70 to 80 btuh/ lin-ft [ 500ft piping is over-under straight in up to 240ft ditch length, per. ]

 

[moltenmetal]

What you're looking for here is a gigantic version of a heat recovery ventilator exchanger used in HVAC. These are generally flat plate type units, giving very cheap surface area without fins and with low pressure drop, but with very, very low pressure handling capability (which will suit your needs). Don't have a mfg to suggest but if you search for this type of unit you may find what you're looking for. The key is to be able to transfer this heat without expending too much energy running blowers/fans to overcome the pressure drop across the exchanger on either side, otherwise the opex of the blowers will eat your heat energy savings completely.