How to decide heater power?

Well, you could run the SS tube through a pot of boiling water, and just heat the water. You can use a smaller pot if you're allowed to coil the SS tube.

The power you need is at least whatever power you lose to the apparatus' surroundings by radiation and convection and conduction. You will need a bit more than that to ramp the temperature up to 100C. You will need a lot more to ramp it up quickly.

-Mike-

Your data are incomplete. This is not unusual though.

You completely specified the water heating load: mass times rise in temp times calorie to joule conversion = 4.2 J/cal.

This is 2 X 4.2 X 85 watts about 750 watts taking the inlet temp as 15 degrees C and the outlet as 100 degC with no latent contribution.

However, the tube will convect and radiate depending on its insulation. So a first estimate would be 1kW to 3 kW of electrical input.

Brian W

2cc /s down a 0.6 mm diameter pipe....

is the heating due to drag going to be significant ? there is going to be a large pressure loss.

is the 0.6 mm the OD or the I.D. of the tube ?

-- Jonathan

Barnes's theorem; for every foolproof device there is a fool greater than the proof.

To reply remove AT

the tube diameter is more or less irrelevant in this case, the mass flow and desired temperature difference is important. Drag heating also makes no big deal here. You need 4.2 Watts per K*cc/s, assumed perfect insulation.

If you want to heat 2cc water/sec water from 20°C to 100°C you need approx.

670W. So a 1kW heater and a PID controller should do the work in any case.

"Jonathan Barnes" schrieb im Newsbeitrag news:c9adtl$hmk$ snipped-for-privacy@taliesin2.netcom.net.uk...

Hi,

The tube has outer diameter of 6mm and I.D. of 4mm. I know it needs very large power (more than 500W) to make the water reaching 100C. But the water runs only once or twice per hour(about 60cc/run), so mostly, the tube and the water stay in hot equalibrium.(I think my heat isolation is not bad.:) ) The power consumption is pretty low most of the time. So if I choose large-power heating system, I'm afraid the heating coil will be burn out with such large power even with a PID. (heat from the min. PID heating pulse maybe larger than the dissipation of the system)

It becomes a dilemma. :( maybe I should get up my plan to let outlet temperature to reach 90+ degree. If I have a 200W heating coil wound around the 3-meter tube and the system stays in thermal equalibrium at 100C. How hot the water is at the outlet if the flow is 2cc/sec and 60cc in total?

Stanley C.

"Stanley" schrieb im Newsbeitrag news: snipped-for-privacy@posting.google.com...

if you calculate the amount of stainless steel in the tube you get 47ccm or

376g of stainless steel (assuming 6/4mm tubing, not 0.6 mm as stated in original post).

comparing 0.5 J/g*K for the heat capacity of steel to 4 J/g*K of Water, you can see that 60cc of water correspond to about 480cc of steel in heat capacity.

if you feed your tube with 60ccm 0°C Water while it is at 100°C, the average outlet temperature will be below 50°C , while the Temperature drops from

100°C at the beginning to low values at end.

Add the power of the 200W heater, you get additional 20K increase in average temperature, if the heater is nicely coupled to the tube.

As you can see from this, a low-power heated tube alone is not sufficient to solve the problem. maybe you fill the space around the thin tube with several liters of hot water, just below boiling point. This gives you a lot of heat storage capacity. This way your system may run without a high power heating system.

may I ask what application you have in mind?

Volume of tube is

300 X 0.4 X 0.4 X pi /4 = 38 cc

Hence, the first 38 cc will be at the 'rest' temperature of 100 degC

The next 22 cc is heated at a rate of 200 watt, so it rises in temp from 15 degC (say) by 200 X time /{4.2 X 22} where time is the dwell time in tube of the make up cold water:

19 sec min, 30 sec max, so use 24.5 sec average dwell time = 53 degC to 15 + 53 = 68 degC

The final result?

38 * (100 - X) = 22 * ( X - 68) 3800 - 38X = 22X - 1496 60X = 5296 X = 88 degC

The result of holding the tube at 100 deg C is to produce 60 cc of water at a mixed temp of 88 degC.

If I may say: you are severely mistaken to suppose that a 750 watt heating wire will burn out if controlled by a PID If the wire is rated at 750 watt, it will take 750 watt continuously. Period.

Your concern should be flashing the water in the tube. That would cause a big over pressure.

You might set a pressure switch to cut power to the heater on

4 psi overpressure say. [like an auomobile oil pressure sender switch)

A pressure switch and power relay might be cheaper than a PID, if you can handle a minor steam spurt in the eflux.

But perhaps the best advice would be for you to refer to an engineer.

Brian W

The only solution I can do, is heating incoming water to 80+ even 90+ degree, and lowering the temp variance to 1 degree. (I wish I could) A bigger boiler indeed solves all the problems. :)

There are what I may do:

1. Use a 5M tube, and a 100W heating coil. -> I don't have so much space inside the machine. SS tube is expensive too. :~
2. Use a 1M tube and a 500W heater. -> maybe too large in thermal equalibrium?
3. Use a 3M tube. heat the tube with 100W in static and heat the first 1M with another 250W coil. power on the 250W heater when the water is flowing.

hmm.. No.3 seems to be feasible. :)

Stanley C.

hi Brian,

Thanks for your advisement. My target temperature is around 95 degree C. I said 100 degree to simplify the calculation. At first, I want to do in the same way, store 60cc hot water inside the tube, and fresh cold water won't go into the boiler in the brewing cycle. It's quite hard to put a 5M tube inside the small espresso machine and $$$$$ So I need alternative solutions.

The PID consideration comes from someone on the internet. He use thermostates only, and his heater burn out due to too-large power. I am a electronic engr but I am not so good at mechanics. so....

I've already had a PID to heat my boiler. the incoming tube and coil is wound around the boiler to help keeping it warm and to save the space. If I have another PID to control the tube temp. I don't know if these 2 PIDs will interact and effect each other.

maybe I should heat the coil at full 300W during the 25sec brewing cycle. and use thermostats or a PID to keep the temp at 95 degree in thermal equalibirum. God bless my boiler. :-)

Stanley C.

"Stanley C" schrieb im Newsbeitrag news:c9fkb3$ snipped-for-privacy@netnews.hinet.net...

aah you want to build a espresso machine that generates 60cc of hot water, every cycle... :-)

Why don't you just use a bigger diameter tube that holds 60cc in a useful length?

Yep yep yep There is a boiler already. and it's only 300cc. so the temperature vibrate during brewing. It affects the taste. I have only one solution - feed hot water to the boiler. A seperated boiler may work great. But it looks so dumb. And the boiler's inlet has a diameter of 6mm. I'll have a problem to connect a bigger tube. That's why I have to stick with the 6mm tube. :)

The heat transfer capability of s.s. tube is my only concern now.

Stanley C.

"Stanley C" schrieb im Newsbeitrag news:c9i6eh$ snipped-for-privacy@netnews.hinet.net...

What if you feed the fresh water after brewing?

Hmm? What so you exactly mean with heat transfer capability?

If the tube is hot and insulated, you won't lose any heat of the water while pumping it through. If it's cold then you will lose some degrees in the beginning.