Yes there is a limit for flow rate in the heat exchanger. When the inlet and
outlet temperature become the same then higher flow rates will not help.
However with inlet and outlet temperatures near the same that is the more
efficient operation of the heat exchanger. That affords maximum surface area
temperature differential to the outside.
Increasing flow too much for a given system you come up against pressure
drop problems. This is a huge problem in cooling engines as the boiling
point can be exceeded where there are low pressure points. That is the
reason the system is run pressurized, to increase the boiling point.
Also flow is regulated in engines to keep temperatures more even.
Baffles are used in heat exchangers not to slow the flow but to enhance
surface area and liquid contact to the tubing. They force liquid to flow
into the bends and create turbulence that breaks up surface tension between
the coolant and the tubes. Same applies for air flow.
Cooling a tube is a little different in that there are no tubing channels in
this case. However if this was a tube with a water or oil jacket then
pressure drop would come into play more so.
> -----Original Message-----
> From: Amps [mailto:firstname.lastname@example.org] On Behalf Of Jim Thomson
> Sent: Wednesday, April 01, 2015 12:35 PM
> To: email@example.com
> Subject: [Amps] more - oil bath GS-35b
> Date: Wed, 1 Apr 2015 16:37:16 +0100
> From: Chris Wilson <firstname.lastname@example.org>
> To: Bill Turner <email@example.com>, firstname.lastname@example.org
> Subject: Re: [Amps] more - oil bath GS-35b
> Message-ID: <email@example.com>
> Content-Type: text/plain; charset=us-ascii
> There are optimum flow rates based on heat exchanger coolant medium,
> be it gaseous, or fluid, the type of liquid, and its viscosity. It
> also depends on the surface area of the inside and outside elements of
> the exchanger and the device being cooled. For sure faster flow is
> *NOT* necessarily better. In automotive racing water (or water mix)
> coolant is NOT flowed as fast as possible through the engine, but very
> careful design has water flow going at different rates in different
> parts of the engine. Those who have replaced a cylinder head gasket
> may have noticed the flow restriction hoses of varying sizes in the
> head and gasket. Different areas of the engine run at different
> temperatures, and flow rate is varied to optimize heat extraction.
> Water to air radiators have internal turbulators in high end coolers
> to increase internal surface areas, thermostats have restrictor plates
> to control water flow rates. Current cooling systems use electronic
> water pump drives and electronic thermostats to optimize flow
> according to load, blah blah.
> Air flow through heat exchangers is similarly regulated for optimum
> heat exchange speeds.
> I am sure in an oil to air heat exchanger with a submerged valve there
> will be an optimal flow rate for the oil and the air, that is far away
> from the fastest flow rate, and there are big differences between
> convection, conduction and radiation. I am most definitely *NOT* a
> heat transfer engineer, but I am a race car engineer and I know a lot
> of science and experimentation goes on in the engine world to maximize
> heat exchanger performance, and engine coolant flow is a lot slower
> than you perhaps imagine.
> Best regards,
> Chris mailto:firstname.lastname@example.org
> ### Us hot rodders run into this exact same issue with superchargers on
> V8 engines.
> The intercooler is just a small radiator, typ bar + fin type... placed
> on the hot output
> of the supercharger. Its job is to EXTRACT heat. A small pump passes
> the distilled
> water /glycol mix to the HE, aka heat exchanger..aka rad # 2. The HEs
> job is to DUMP heat.
> Mine doesn't have a fan on the HE,but simply relies on airflow due to
> vehicle speed.
> Some will use dual fans on the HE, to keep things cooled down a bit at
> red lights,
> and also between runs at the drag strip etc. For road course folks, the
> fans don't provide
> any benefit once above 20-30 mph. They impede airflow.
> ## faster pump speeds will make the intercooler work better..and
> extract more heat,
> BUT the entire assy is one big loop. Pump the hot water/glycol coolant
> through the
> intercooler-HE loop too fast, and all you end up doing is spinning hot
> water around,
> with no increase in eff at all. Unless the HE (external radiator in the
> case of the GS35B)
> can dump the heat as fast as its being generated, you are wasting your
> time with faster pump speeds.
> ## 1 watt = 3.15 BTU. 1 kw = 3150 BTU In the case of the proposed
> setup, 1-2 kw CCS of heat to extract..then dump is not a big issue.
> And with
> ssb /cw modes, the heat generated is going to be a lot less. Beware of
> advertised GPM
> flow rates on pumps. They are typ for no load. IE: fire hose connected
> from 55 gallon
> drum of water, to input of pump.... then another big diam hose from
> output to the ground.
> ## Once actually hooked up to the tight bends and puny size fittings,
> small tubing, and
> the restrictions inside the GS35B tube and the external radiator, the
> actual measured flow rates
> drop like a rock. Install a much bigger pump, and flow rates typ
> increase very little
> if any.
> ## check out the BTU specs for small radiators...like automatic
> transmission coolers.
> They are typ 8000-20,000 BTU rated..provided airflow of XXX CFM is put
> through em.
> I had to install a 2nd auto tranny cooler in series with my oem cooler,
> since the tranny temps
> climbed way too fast in just a few secs when super charger on..and gas
> pedal mashed. Like
> 170-200 F in just 7 secs.
> ## Old style radiators were tube + fin type. Better types are bar +
> plate. You can also get em
> in single, dual or triple core..and also single and dual pass. My bar
> + plate Tranny cooler uses a unique
> T stat......operates on viscosity. The hotter the synthetic tranny
> fluid gets, the higher it rides up either side
> of the rad. At that point it makes more and more parallel passes from
> one side to the other.
> ## The GS35B, with its low dissipation, will not be an issue.
> Perhaps 2-5000 btu at most.
> Ultimately... ALL the heat has to be eventually transferred to the air !
> BTW, you will have to provide
> some cooling to the base seals of the tube, where the fil connections
> are made. With Eimac water cooled
> tubes, or vapor phase cooled tubes, a small air pump, is used to cool
> the fil pins.
> Same deal with a normal air cooled tube which uses anode to base
> flow.... instead of the usual base to anode flow,
> typ 3-5 cfm on a 3CX-3000A7. But uses anode to base flow ...nobody..its
> fubar and requires way more cfm.
> The additional air flow to the base will not be required if the entire
> tube is immersed in oil, like you proposed.
> ## A 2-4 GPM pump will more than meet your requirements. Then
> perhaps a pair of 120 mm square
> fans, like the 65 cfm rotron whisper types.. placed in front of the
> external rad. They are very quiet, and can be
> further lowered in speed with a simple metal finned resistor, variac,
> variable dc supply etc. Shroud the front of the
> rad, where the fans are... other wise you are wasting your time... air
> will leak like a sieve out the sides.
> Un shrouded rads don't work too well. IMO, you are better off pushing
> air through the rad, vs sucking
> it through the back end of the rad. Then you don't have hot air passing
> in and out past the fan bearings.
> later.... Jim VE7RF
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