View Full Version : Water flow in evaporators

reggie

08-05-2004, 02:07 AM

Can anyone give me some advice on water pumps in chillers. I vaguely remember learning the formula Q=mcp dt at college.

Has anyone got any formulae for calculating the flow rate through evaporators and condensers per KW of refrigerating capacity??

Also what difference in pressure should we be getting either side of the pump and across our heat exchangers??

Much obliged

Reggie

Peter_1

08-05-2004, 07:15 AM

You will need to calculate the total pressure drop of the whole circuit and then select a pump that will give the needed capacity at that pressure drop.

Take in account the water flow through the pipes (the smaller the pipes, the faster the water will flow, the more pressure drop you will have)

Most manufacturers will give you for their condensers the needed waterflow at prescribed conditions.

Or they will give you the pressure drop over the condensers at a certain waterflow. You then need to add the pressure drop over the lines and other components.

Water flow for an evaporator? :confused:

Pressure drop over HE? The less the better but that's already decided or made up in the design of the product.

A good practice is to install to pressure indicators over the pump - together with the AMP's - so that you can check if it gives the rated capacity.

There is no recommended pressure drop over a pump. You can only plot it in your pump graph and see what it needs to be.

You also need some equilibrium start pressure of 1 bar to avoid cavitation on the impellers in the pump.

frank

08-05-2004, 06:22 PM

Hi Reggie

You are nearly right with your formula. You need to determine what the delta t will be across the machine, normally 6C leaving and 12C return - this gives a dt of 6K. If you want to know the correct flow rate then you must know the chiller's duty. If you know the flow rate then you can find the chiller's duty. Use the formula Q = m(dot) c dt.

So for a chiller with a duty of 35kw and a dt of 6K you can find the flow rate thus: m(dot) = Q/c dt or m(dot) = 35/4.19*6 so the correct flow rate for this chiller would be 1.392kg/s.

If you had a chiller where you could measure the flow rate at say 4.86kg/s with a leaving temp of 7C and a return of 11C then you could determine the amount of refrigerating duty thus: Q = 4.86 * 4.19 * 4 = 81.45kw.

Hope this helps

Frank

Peter_1

08-05-2004, 06:45 PM

Think you need to convert first your kW to kcal or divide the kW with 1.16 and then use that number.

Anyway, that's how I do it.

I learned in cal which was very easy: you need 1 cal to raise 1 gr. water with 1°C or 1 kcal is the amount for 1 l of water

reggie

09-05-2004, 03:06 AM

Thanks for the quick response. I see what your saying Peter in regards to selecting a pump that can overcome the building height/head and the extra resistance caused by the fittings etc.

As regards to the flow through the condenser are we using the same formula that works for the evaporator calculation?? Q=m(dot) dt??

In Franks example where he calculated the evaporator conditions reverting to the condenser side water is entering our condenser at 30c and leaving at 40c then our dt is 10k and the Kw rating is already known then the mass flow rate can be calculated identical to the evaporator.

Is this correct Frank??

Regards Reggie

Peter_1

09-05-2004, 07:50 AM

If I may Frank, sure,.. it's exactly the same.

Once you made the calculation, you will need to recheck everything with the selected pumpcharacteristic to see if the pump will meet the needed flow.

That's more in the case you calculated first your tubes. You will find by calculation theoretical diameters then won't match with the real world and you need to chose different excisting diameters. So your circuit will be different as theoretically calculated.

Pressure drops can be found via caculation (fluid mechanica) or special-purpose chart.

In your case, you only have to take in account a 100% fluid but in an refrigerant circulation system, you will encounter fluid, gass and a mix of the 2.

Never install fluid pipes horizontally. Install an air purger (don't know the correct English word for it) on the highest points.

Friction loss will dependant on used pipe material.

If it's a closed circuit , you don't need to calculate the pressure loss due to climbing lines. If it goes up somewhere in a closed circuit, then somewhere it will go down again for the same height.

Hey Reggie, maybe this link can help you.

http://www.engineeringtoolbox.com/6.html

Look around it and see what can help you.

frank

09-05-2004, 07:04 PM

If the system has a water cooled condenser then you should check to see if there is a 3 port valve fitted. On water cooled condensers it is normal to have the head pressure controlled in this way. The valve will have a capillary directly connected to the discharge pipe. You can only take a water flow reading reliably when the machine is working flat out.

Frank

Peter_1

09-05-2004, 09:43 PM

We use a control on the fan speed and/or regulate the speed of the waterpump.

This is in my opinion a less power consuming solution then a three way valve.

never saw 3-way valves, even not on those of colleagues of me.

frank

10-05-2004, 08:50 PM

Peter

With a water cooled condenser then you do not have a fan. All of the condenser is cooled by water flow. :D

Peter_1

10-05-2004, 09:35 PM

Of course Frank, I was thinking only on a drycooler. :o

Sorry.

3 GPM per TR @ 15 ft head... better still look up the pump curves for the pump manufacturer you are using

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