Pump limited 4

[drodrig]

Hi there!

We have a cooling machine (it operates at -25ºC) which is cooled with water. According to the specifications we need cooling water between 3 and 10 bars (the machine has a valve opening and closing depending of the need) and it consumes 900 L/h at 3 bars. We will pump it from a deposit with the following pump: Lowara Scuba SC 211C

http://doc.lowara.com/lowdata/doc/EN/borehole-td-en.pdf

One can see the curve in the page 11, It has a maximum presume of 8 bars (1200 L/h) and minimum of 3 (4500 L/h).

My question is if this setup will work. The water pump will send water let's say at 7 bars (direct connection with with cooling machine, plastic tube), if the cooling machine in a moment needs only 3, what happens?

I understand one of the following options:
a) the cooling machine valve breaks (think it is strong enough)
b) the pipe breaks (we will use strong pipes)
c) the water pump breaks
d) nothing, the pump respects the cooling machine

What do you think?

A sketch of the setup

http://files.engineering.com/getfil...71-462d-96c2-db8ac553d7d0&file=water_pump.png

Thank you
regards,

 

[LittleInch]

To find out what actual pressure your system will run at you need to construct a system curve. This might be quite difficult unless you know how the cooling machine works. Does it literally "consume" the water? Does it control the flow rate internally?

If it does then your system seems to be over powered. Why do you have a pump which has a MINIMUM flow rate of 1200 l/h to feed something which only needs /consumes 900l/h??

You will damage your pump if you operate long term at less than minimum flow and you will use lots more energy than you need to as it is operating very inefficiently.

So it will "work", but after a few months the pump might fail. Everything else seems OK as your pump max pressure is lower than the machine and hopefully your pipe which should be rated at least as high as 10 bar.

If the machine does not control the flow then you could flow much more, but given that the spec seems to say any water pressure is OK between 3 and 10 bar it would seem that the flow control is internal to the machine.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[drodrig]

The pump will be very close to the cooling machine, so there is no big head load losses, so we could say the pump will work at 7 bars.

The water will be returned to the deposit (and this will be cooled with another small cooling machine)

The flow is too big because it increases as the the head capacity of the pump increases, and we need to be between 3 and 10 bars.

I guess the cooling machine will control also the flow (it takes whatever it needs) so I understand this will damage the water pump as you say.

I'll have to look for a pump with less flow, right?

cheers,

 

[bimr]

You need to install a backpass valve and line to recirculate back to the water tank. The valve will ensure the backpressure in the piping to the cooling machine. See the attached sketch.

 

[LittleInch]

You could do that, but if you haven't bought the pump yet, get a smaller / lower flow pump or you're just throwing money away in energy costs. However, you do need to work out if the machine controls the flow or not. If the return flow is also close to the tank then you could end up with very little back pressure and have a huge flow through the machine.

Figure out the load (flow / pressure) first then design the pump to match, not the other way round.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[JJPellin]

If I am reading the curves correctly, you are proposing purchase of a pump with a best efficiency point flow of 50 l/min and are planning to run it at 15 l/min or 30 percent of BEP. I assume that the valve you are describing is a control valve that will pinch back to limit the flow based on some other parameter (temperature perhaps). Running a pump like this at 30% of BEP would be below the manufacturer’s recommended minimum flow. This would likely result in poor pump reliability. You have two options which have already been described above. Add a spill-back line to spill-back the additional flow needed to keep the pump above the minimum flow requirement. Or, purchase a pump sized for lower flow. The first option would waste some energy recirculating flow. The second option would reduce your installation cost and consume less energy. Purchasing a pump designed for a lower flow seems like the wise choice.

Johnny Pellin

 

[gerhardl]

JJPellin: Thank you, I was just about to write something similar to your answer to try to pinpoint the problem.

To summarize:

1. Check that the necessary flow figure is correct. AS JJPellin says this will probably correspond to a given flow, at a givin temperature measured at a given point.

2. Sensible to check necessary flow at max, minimum and normal operationoal conditions of the complete system. (Cooling amount for machine corresponding to cooling effect given byi a given amount of water at a given temperature).

3. Check necessary pipeline diameter.

4. Returncircuit to tank probably necessary, to open if pressure is somewhat higer than preset for inlet to cooling system.

5. Select pump to suite capacities.

5. Total circuit will probably feed cooling water into the machine with a temperature controlled valve, wile the pump (soft start/stop) should be operated to upkeep a given pressure. 3-7 bar is to wide and probably upper/lower limit. Select something in between, and safe with retur as in point 4.

 

[drodrig]

Hi all,

It seems the pump adjusts its pressure automatically depending on the needs.

In any case a by-pass seems a good idea to be in the safe side. I will follow your advise.

Thanks for all

regards,

 

[LittleInch]

Errr, in a sense it does, but only within a certain range. Your pump is quite rare in having quite a large head range (75 to 35m) though the last one will be at a high flow. Most centrifugal pumps are more like 25% from max to min head before it runs off the curve.

From everything we've seen so far you need a different (smaller) pump once you've worked your system losses out and how flow is controlled. Just putting in any old pump that looks ok will not end well.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way