Water cooled condenser higher than open type cooling tower issue

[François]

Hello Everybody,

I'm new on this forum and I'd like to get some advices and explanations.

I've worked on a water cooled chiller operating with high pressure problems (capacity limitation due to abnormally high pressure and loss of refrigerant due to safety relief valve opening).
It seems that there is a low water flow at condenser side and our problems would come from this situation.

The chiller plant uses open type cooling tower. The cooling tower are located around 5 meters below the condenser. The condenser water pump is located around 0,5 meter above the water level in the water basin.

The condenser capacity should be around 1000 kW, with a 5°C delta °T. This means the water flow should be around 170 m3/h. The condenser pump is a Wilo Cronobloc BL100/250. The pressure drop accross the condenser should be 0,6 bar at full design water flow.

At the suction of the pump, we've measured -0,3bar. At the discharge the pressure is 1,2 bar. With these measurements we aren't inside the limits of the pump.
At the inlet of the condenser, the pressure is around 0,5 bar. At the outlet, the pressure is negative.

The total piping distance is around 35 meter (stainless stell pipes, 6 inch).

The cooling tower manufacturer requires 0,4 bar at the spray nozzle inlet to properly distribute the water.

First of all, I don't understand why the pressure at the condenser outlet is negative.

I've opened the water pump to inspect the impeller, nothing special found. We've also opened the condenser water boxes to inspect the tubes, no abnormal fouling found.

We are out of the limit of the pump's curve with having a low water flow.

May be after understanding the origin of the problem, I'll be able to understand how to solve this bad situation.

Your advice, support and feedback will be highly appreciated.

Regards,

François

[The Viking]

Without actually seeing the system what stands out in your description of it is the location of the pump, normally the pump would be below the water level in the tower.
The negative water pressure after the condenser is generated by the weight of the water flowing back to the tower, it could indicate a blockage in the condenser but not necessary so.

From your perspective it would be interesting to know the actual water flow instead of the pressures, if there isn't metering devices fitted to the system then there are ultrasonic ones that just strap on to the outside of the pipe.

Inside the tower, is the water exiting to the pump through the side or the bottom of the tower? And if through the side, is there a suction hood inside?





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[install monkey]

make sure the float valve isnt jammed,check no airlocks , check the strainer,check the outlet from the tower- had i once where a well known supermarket bag was causing a restriction, also any non return valves that arent fully opening

[Tesla]

Hi Francois and welcome to the forum
This sounds like a lack of NSP? (negative suction pressure?)/(can't remember exactly) but some one will. Basically not enough head at the pump inlet for the system parameters. This causes cavitation - is there a good swirling in the basin which is sucking in air? Had this on a job before and the pump guru came out and closed off the discharge valve on the pump about half way which fixed it. It was causing the rubber bellows to suck in, reducing the flow and heat transfer. It should be stated on the pump data. Perhaps with the negative head at the outlet of condenser it could be wise to close off the condenser outlet valve instead of the pump outlet. You could simply get a hold of the pump manufacturer to visit site and advise as there could be other issues.

[DEVITG]

Could you , give us, a schematich DWG , with real and true measures ??

[François]

Dear all,

First, I'd like to thank you for your quick replies.

At 170m³/h, the pump curve indicates a NPSH of 2 meter (0,2bar, absolute pressure). Regarding the measured suction pressure (-0,3 bar relative = 0,7 bar absolute pressure), we are still in the range and it should be possible to operate the pump under these conditions.
The water temperature in the basin is maintained around 25°C and the altitude of the plant is around 100 meter.

I'd like to send you the pump curve but i'm having some problem to attach the file (2Mo). If someone can help .....

What sound strange for me is the next. On the pump curve, a low water flow should be equal to a high pump head. Regarding our measurements, it is even not possible to put the measured pump head on the curve. We have called the pump's representative and he has visited the site. He is proposing to install a bigger pump. May be he is right. However, if the actual one was too small, I should measure a high pressure head and I should be able to draw the head on the curve ?

The water level in the basin is OK. The pump suction is composed by a non return valve in the bottom of the basin, a short vertical straight pipe and a 90°C elbow connected to the suction flange of the pump. We've inspected the non return valve and it seems ok. The water is exiting the basin through the top.

Air in the circuit can be the origin of the problem because when the pump stops there is nothing to maintain the piping full of water (pipes between tower and condenser) so the water will be drained through the tower. However, we've once tried to fully fill the circuit with water (valve at the condenser outlet closed, filling line connected on the condenser inlet, so piping from non return valve at the suction to outlet of the condenser was full of water). Then, we've started the pump with the valve closed and opened the valve slowly. There were no improvement. May be, we forgot something ?

You're also right, it should be easier with a drawing. At the moment I'm outside of the country and not able to make a drawing with accurate measurements. I'll be back mid of March and prepare it at that time (sorry for the delay).

I'll search again to send you more information.

Thanks again,

François

[DEVITG]

Dear all,

First, I'd like to thank you for your quick replies.

At 170m³/h, the pump curve indicates a NPSH of 2 meter (0,2bar, absolute pressure). Regarding the measured suction pressure (-0,3 bar relative = 0,7 bar absolute pressure), we are still in the range and it should be possible to operate the pump under these conditions.
The water temperature in the basin is maintained around 25°C and the altitude of the plant is around 100 meter.

I'd like to send you the pump curve but i'm having some problem to attach the file (2Mo). If someone can help .....

What sound strange for me is the next. On the pump curve, a low water flow should be equal to a high pump head. Regarding our measurements, it is even not possible to put the measured pump head on the curve. We have called the pump's representative and he has visited the site. He is proposing to install a bigger pump. May be he is right. However, if the actual one was too small, I should measure a high pressure head and I should be able to draw the head on the curve ?

The water level in the basin is OK. The pump suction is composed by a non return valve in the bottom of the basin, a short vertical straight pipe and a 90°C elbow connected to the suction flange of the pump. We've inspected the non return valve and it seems ok. The water is exiting the basin through the top.

Air in the circuit can be the origin of the problem because when the pump stops there is nothing to maintain the piping full of water (pipes between tower and condenser) so the water will be drained through the tower. However, we've once tried to fully fill the circuit with water (valve at the condenser outlet closed, filling line connected on the condenser inlet, so piping from non return valve at the suction to outlet of the condenser was full of water). Then, we've started the pump with the valve closed and opened the valve slowly. There were no improvement. May be, we forgot something ?

You're also right, it should be easier with a drawing. At the moment I'm outside of the country and not able to make a drawing with accurate measurements. I'll be back mid of March and prepare it at that time (sorry for the delay).

I'll search again to send you more information.

Thanks again,

François

NPSH shall , be read as GAUGE or Relative to atmospheric pressure , you are far below the HPSH value required.

See it please

http://en.wikipedia.org/wiki/NPSH

If your schematics are so big you can send me it offline.
Find attached my e-mail address

[The Viking]

Pump curves and theories are good when you design things, however, when the proverbial hits the fan then real life measured values are much better...

One thing we have missed so far is the obvious question, has this system ever run properly?

[Magoo]

With the pump above the cooling tower basin, NPSH ( net positive suction head ) is below zero, a seriously good foot valve at pump is required and the pump needs to be primed before hand, or a self priming pump. Basically a disaster waiting to happen and has by low water flow and gas loss.
Move the pump to below cooling tower basin water level.

[RANGER1]

Might sound basic & stupid, but have you double checked direction of water pump.