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Author Topic: How to calculate water cooling systems?  (Read 12634 times)
CT2IRY
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Posts: 8




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« Reply #75 on: October 03, 2017, 06:07:38 AM »

Very interesting replies here on this topic and also very useful links.

I've been looking at R3KBO work on water cooling with is 2xBLF188 and 2.4kW amplifiers, and also DJ0ABR, OZ1BXM, OZ1CT.


R3KBO is using for is amplifiers water system an 300L/h (79.25 gallons/h) pc pump with an 150mL water deposit attach and an dual 120mm fan radiator with 10.7"x4.73"x1.18" (272x120x30mm).

R3KBO water cool material:
Radiator - < http://www.ebay.com/itm/Useful-240mm-Dual-Radiator-Copper-Water-Cooling-Heat-Exchanger-2-X-120mm-/282092489004?epid=508307708&hash=item41ae05c92c:g:yVcAAOSwLN5Wij8M >

Pump - < http://www.ebay.com/itm/12V-DC-ultra-quiet-ceramic-bearing-pump-pump-tank-water-Cooling-System-kit-/231904733621?hash=item35fe9969b5:g:2uoAAOSwsN9XBR4q >

I saw that the math is asking for an radiator of at least 8500 BTU for the 2xMRFX1K80 at 2.5kW 24/7/365 for RTTY use in contest.

I've found this Tru-Cool L7B with 15000 BTU for $76 here < http://www.ebay.com/itm/Tru-Cool-L7B-15-000-BTU-Engine-Oil-Transmission-Fluid-Cooler-Long-Manufacturing/202050755123?_trkparms=aid%3D222007%26algo%3DSIM.MBE%26ao%3D1%26asc%3D41451%26meid%3D52a34a2efc274dfbb6adbab0712643d5%26pid%3D100011%26rk%3D6%26rkt%3D12%26sd%3D172816311002&_trksid=p2047675.c100011.m1850 >

The L7B radiator is 11''x5.75''x1.5'' (280x146x38mm), this is almost the same size of the radiator use by R3KBO so could be that the radiator use by R3KBO be around 9000 BTU?

Any of this radiators could be use together with a 600L/h (158.5 gallons/h) pc pump, now it's just missing the tank for the water, the ones from pc's that I've seen are only 300mL witch are just insufficient for this task (I think), maybe something with 1L could be use.

Any ideas on this.

Nuno
CT2IRY   
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W9IQ
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Posts: 1705




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« Reply #76 on: October 03, 2017, 06:41:26 AM »

A reservoir of water does nothing for the condition of continuous operation which is the basis of this thread. If the cooling system cannot remove all of the heat generated from the amplifier, eventually the temperature of the reservoir will reach the point will it can no longer function as a source of cooling. Essentially, the reservoir just delays the inevitable - the cooling system is inadequate for the application.

Where a reservoir could prove useful is when the amp is designed for intermittent use. In this case, the reservoir can store heat until it can be dissipated during transmitting times.  My math in the earlier part of this thread can be used to support this case.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
W9IQ
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« Reply #77 on: October 03, 2017, 07:13:47 AM »

Quote
In this case, the reservoir can store heat until it can be dissipated during transmitting times.

That should have been "non transmitting" times.

- Glenn W9IQ
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- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
CT2IRY
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« Reply #78 on: October 03, 2017, 08:10:20 AM »

Hi Glenn,

Thanks for the reply, so this L7B 15000BTU radiator and the 600l/h pump will still not cut it to remove the heat?

I know that what was ask was for full power 24/7/365, but if we think even RTTY ctest is not 100% duty all the 48 hours, with short TX and some RX time to grab the exchange then CQing again so maybe 30s TX 30s RX this for the top guys, the Short & Pounce guys will have more RX time then TX, so for Running station we can have ~80% duty and less for the S&P guy.

Here in EU we've a good source of heatsinks from Guash in Spain, see here the air type < https://www.poweralia.com/combi-disipadorventilador > this are 87mm height but they have ones with 127mm, and here the water type < https://www.poweralia.com/refrigeracion-liquida >.

I'm thinking on one of this air type heatsink 300x300x127mm and a copper plate of the same size but with 12mm thick and another copper plate of around 200x200x12mm to attach the board for air cooled, in case of 2xMRFX or maybe would be better two single boards, to spread the heat on the heatsink, so I'm looking at the water cool projects to see what kind of material they are using and if is feasible using modest material.

I'm not looking at 24/7/365 full power like the OP, but at full power on an RTTY ctest and on chasing a DXpedition.

Nuno
CT2IRY
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W9IQ
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Posts: 1705




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« Reply #79 on: October 03, 2017, 08:37:43 AM »

Nuno,

I presented all of the necessary math and design processes throughout this post. You should be able to apply this to your specific requirements.

If you are using a water cooled solution, a one piece copper heat sink will provide the least thermal resistance. Adding an aluminum to copper junction only makes things worse thermally and chemically.

- Glenn W9IQ
« Last Edit: October 03, 2017, 08:40:40 AM by W9IQ » Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
CT2IRY
Member

Posts: 8




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« Reply #80 on: October 03, 2017, 12:32:49 PM »

Glenn,

I'm thinking more and more about water cooling the amp, but in my case the water cooling system must fit the enclosure, if that is not possible I'll steer to air cool type.

Grabbing your math and correct me if I do this the wrong way:
Let's say 2xMRFX1K80 full output for RTTY ctest (here maybe we can assume ~60 to 90%, depending if one is a running station or short & pounce station) and day to day operation.

Let's take 1500W per LDMOS (don't know if it's possible, there is one guy in US with two on one PCB with 2.800W+++ after the amp completed).

1500W per LDMOS at 60VDC at 65% eff.
ID = 1500/60VDC/0.65% = 38.46Amp

Power dissipation formula;  PD = (Pinput + (ID*VD)) - Pout
Taking the power dissipation formula we got;
(1+(38.46*60))-1500 = 808.6Watts dissipation by each LDMOS or 1617.2W by the 2xMRFX1K80.

Now the thermal part;
Assuming the 808.6Watts of dissipation by each LDMOS
Thermal formula; Rθtotal = (TJmax - Tliquid) / Watts
With an TJ limited to 175ºC and the water to 50ºC.
Rθtotal = (175ºC-50ºC)/808.6W = 0.154ºC/W

The RθJC of the LDMOS is 0.09ºC/W, so 0.154ºC/W - 0.09ºC/W = 0.064ºC/W; so now he got 0.064ºC/W for the "cold plate".

Now the water cooling part;
If we keep the water temp below 50ºC at an ambient temp in my case the worst is in the summer around 30ºC, in winter time is around 15ºC temperature in the radio room.
 RθHE = (50°C - 30°C) / 808.6W = 0.0247 °C/W
So the cooling capacity of the heat exchange would be the minimum of 0.0247 °C/W

Maybe this would be possible with two copper plates of 300x300x12mm with water channels inside and two PCB's, each with it's only LDMOS, instead of single PCB with two LDMOS.

I can't understand well enough your explanation on the temp rise of water in replay #37, about the circulation need of water trough de radiator
« Last Edit: October 03, 2017, 12:42:25 PM by CT2IRY » Logged
CT2IRY
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Posts: 8




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« Reply #81 on: October 03, 2017, 01:57:42 PM »

Glenn,

After reading some more times your replay #37, I've this let's see if I get it correct;

An 600L/h (158.5gpm) pump is capable of circulating 10.000gm (10Kg) of water per second.

The specific heat of water at 22ºC is 4186 J/kg °K.

After here I'm lost in the math.
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M0HCN
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« Reply #82 on: October 03, 2017, 04:38:51 PM »

Guys, you are missing something...

RTTY is a continuous carrier mode, you can run the things in fully saturated class C (Possibly even with slight negative bias!), it does not need to be a linear amp for this application, and that makes a big difference to the potential drain efficiency.

1500W from a 1.8kW device @ 100% duty in a "linear" is pushing it IMHO, I would run those parts at more like 1, maybe 1.2kW RF out each, sure they will do more, but the lower power level allows a reasonable drain match at slightly lower DC voltage and a bit less current, and less heat which means that you can get the device/heatsink interface hotter for a given junction temp, and that is helpful. 

Running with your 65% assumption, at 1KW RF I make the PD 540W near enough at which point I make the final capacity 0.141 K/W, a much nicer number then 0.025 K/W...

One other warning, that 65% only applies to the amp when run at full snout, you will find the power dissipation actually goes up a bit as you back off the power before falling back.

BTW: 600L/H is not 10L per second, it is 10L per minute, 0.16L/s, which is more then required, but not by that much.

73 Dan.
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CT2IRY
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« Reply #83 on: October 04, 2017, 02:54:32 AM »

Hi Dan,

For class C you need to have lower voltage on the bias, like you said perhaps even a negative voltage, could here be useful a button with SSB/RTTY to change the bias voltage, like the tube amps have?

Let's assume 3V of bias for each LDMOS, with the SSB/RTTY button you can vary the bias voltage from 3V to negative, this can be a way.

This way maybe we can vary between the 35% duty cycle needed for SSB/CW at 1500W each LDMOS and the 100% duty cycle class C in RTTY, would this be feasible?

I've two 300x300x12mm copper plates to attach the PCB's and LDMOS, one or two LDMOS for PCB not made my mind yet, if one LDMOS in each PCB the heat is more distribute on the copper plate since the LDMOS are more apart, if two LDMOS in each PCB the LDMOS are more close so the heat is more concentrate in one spot of the copper plate, the best option can be the first choice, I think.

About the pump it can be a little more efficient one, like a Liang D5 pump that can move 1500L/h (25L/m or 0.416L/s), the radiators maybe two triple fan radiators with 60mm thick, like the EK-CoolStream XE360.

You're right about the pump, I should not write at later hours in the night. :-)

Nuno
CT2IRY
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AH7I
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« Reply #84 on: October 07, 2017, 01:35:22 PM »


For 1250W dissipation/part, the part temp can not exceed 90 degrees C per the data sheet.
If you want the part to operate at 40 degrees C ambient air temp, you've got 50 degrees of temp gradient to work with.

Your coupler to the water will have some gradient dependent on the power. If it's a well designed commercial product, they should be able to tell you and will be your best resource for sizing the system.
If they don't know, you'll have to measure.
 
You can now calculate the maximum water temperature corresponding to 90 degrees C at the part when you have the full 2500W. This will tell you how much water you need as a function of input water temp.

Now you have to figure out how to maintain that input water at or below it's maximum temp.

You might want to consider mean time to failure (for the part) as a function of operating temperature. The part maker should provide this info.


73, -Bob ah7i/w4


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VE3VEE
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« Reply #85 on: November 01, 2017, 05:47:51 AM »

Hi Guys,

I have a silly question. Instead of water, could one use car radiator coolant? The reason I'm asking, all my equipment is housed in a metal outdoor enclosure attached to the base of my antenna tower (remote). There is no heat. During winter, the temperature drops way below the freezing point. Water would freeze, but radiator coolant for cars would not.

Marvin VE3VEE
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W9IQ
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Posts: 1705




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« Reply #86 on: November 01, 2017, 06:42:26 AM »

Marvin,

You can use an anti-freeze but make certain it is compatible with all of the metals and plastics in the cooling loop. Also be aware that the specific heat  of the anti-freeze solution will be lower than that of water (~20% lower for a 50/50 mix of water and glycol for example). This may require that the liquid be circulated at a higher rate to achieve the same cooling capacity.

- Glenn W9IQ
« Last Edit: November 01, 2017, 06:44:37 AM by W9IQ » Logged

- Glenn W9IQ

I never make a mistake. I thought I did once but I was wrong.
VE3VEE
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Posts: 1158




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« Reply #87 on: November 01, 2017, 06:45:41 AM »

Glenn,

Thanks for your prompt reply! I've been looking at the R3KBO amps, they use WC systems.

Marvin VE3VEE
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KM1H
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Posts: 2436




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« Reply #88 on: November 01, 2017, 07:46:51 AM »

For what its worth... Royal Purple has a product called Purple Ice which increases the surface tension of water a bit.

I have used it for years in the stock radiators of vehicles with more V8 HP than stock and it has provided enough of an edge to keep temperatures in the safe zone even stalled in summer traffic.

There are several other brands that claim similar performance.

Carl
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N1CZ
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Posts: 23




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« Reply #89 on: November 01, 2017, 08:23:22 AM »

To get that extra edge in cooling my computers, I have used Red Line Water Wetter which is a pink-ish red. I actually have no idea if it helps heat transfer or not. Huh  It does permanently tint the (formerly) clear tubing. The color ranges from an almost opaque dark red to a light pink. There is no degradation of the tubing other than things are not as purty as they were.
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