It looks that the fluid flow channel size of heat exchangers is becoming smaller and smaller. The widely used air coil heat exchangers for refrigeration and air-condioning, for example, use copper tube OD from common 19mm, 16mm, 12mm, 10mm, now to 8mm, 7mm, 6mm, 5mm and even down to 4mm and 3mm. In addition, more and more micro channel heat exchangers have been used for commercial and household products. Two questions:
1) Is it theoretically reasonable with thermodynamics to have the saying "the smaller, the better"?
2) Are we going to use micro channel HXs only or use normal/small/mini/micro channels combined HXs in a fairly large thermal system in the distant future?
Dear
the way of using channel HX is related of the process needing of heat exchange in the other hand of the HX location and material. a micro channel HX is request of the micro-material but itsn't benefited to us it of the big plant.
Hello, it's a interesting question and I'd like to share my opinion on it.
I don't think "the smaller, the better" holds for the generic heat exchanger.
The smaller size will increase the surface-to-volume ration, which helps increase the heat transfer surface area A. However, the heat transferred is determined by Q=hAΔT. Once you decrease the tube size too much, the flow inside the tube will turn from turbulent to laminar flow, and the heat transfer coefficient h will suffer from a huge loss. As a result, the final performance is the determined by the trade-off between h and A. So it's hard to conclude "the smaller, the better".
The mini/micro channel HXs are used mainly for reducing the medium/refrigerant charge quantity and improving system compact. Optimizing the channel sizes of a HX network is a good topic for R&D. Pinch analysis might be a good start. Is there any other method for the heat exchanger network analysis based on the second law of thermodynamics?
Whilst all your answers are correct, you evade the crux of the matter, if I may say so, forgive me.
The tube walls need to resist the pressure differential between the two media. The true purpose of thinner walls is to provide a thinner membrane between the participants.
The essence of good design is then to remove the pressure differential between the media, even if only temporarily for the HX.
It is now almost common knowledge that this is best done by a device called a Heatgate. https://youtu.be/phGaFavF5GQ .
Ali in your case, imagine if the participants are at the same pressure, large thin sheets/membranes, might provide separation, whilst porous structure might afford support. The whole HX would sit in a vessel at common pressure. Your approach temperatures would practically be eliminated. The cost of running a Heatgate, in terms of remedying decompression, say 100 Bar differential, could be as low as 25 joules per liter processed, or 50 joules, there are two methods.
Liquid decompresses approx. 50 parts per million per bar.
Hi, Derek
The use of micro channel HXs is mainly for the compact of the unit dimensions, the minimization of flow fluid charge, saving channel materials, and etc.
For some high operating pressure media, such as carbon dioxide, the pressure difference is about 100 bar, but most copper tubes commercially available like 1/4" with wall thickness >0.6mm are able to cope with the pressure difference.
The Heatgate that you mentioned can help the channel wall thickness; however, the focus here is on the channel size, meaning the cross-sectional area of the fluid flow channel, often called normal, small, mini, micro depending on the channel cross-sectional area sizes.
Take a transcritical co2 refrigeration unit as an example, gas cooler pressure is about 100-120bar, evaporator about 30-40bar. Probably, you mean, for example, a heatgate can help reduce the CO2 gas cooler tube/channel wall thickness from current 6.35mmOD/0.7mm to a thinner wall tube/channel, so saving some copper tube material, right?
Just for your information and comments.
Recently, my focus has been put on minimising the charge quantity and optimising the energy efficiency of flammable refrigerant hydrocarbon refrigeration systems ( www.refriglab.com ). Using mini or micro channel heat exchangers are helpful; however, the challenge is the even distribution of refrigerant among the evaporator channels, and the other is the relatively high costs of micro channel heat exchangers for refrigeration manufacturers. Any comments?
Hi Jeff.
In regard to your answer ...... /0.7mm to a thinner wall tube/channel, so saving some copper tube material, right? ....... the real function is not to save copper. According to my research, going from a .7mm wall thickness to a .1mm wall thickness increases the heat flux 7 fold, because resistance to conduction through wall is dependent on wall thickness in exact ratio, amoung other factors granted.
Hi, Derek
I am not familiar with the heatgate that you described. Can you help briefly show how to make 0.1mm thick 5-8mm outer diameter copper tubes working under 100-120bar.a of carbon dioxide gas cooling pressure?
Also, tube wall thickness change from 0.7mm to 0.1mm to increase the conduction heat flux, that is the fact. To my understanding, the overall heat transfer is mainly depending on the heat fluxes of tube inner and outer surfaces as the copper tube wall conductive thermal resistance is relative small. Any misunderstanding here?
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