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Centrifugal pumps find open cooling tower applications are the toughest duty there is in hydronic cooling systems. Pumps do best with piping systems that provide the most suction pressure, water that is free of air, and the absence of swirling motions in the pump suction. Today, the R. L. Deppmann Company’s Monday Morning Minutes offer tips for engineers and contractors to protect the cooling tower pumps.
Pressure, Air, and Flow Pattern, OH MY!
In the Wizard of Oz, Dorothy chanted the three concerns she had as she walked through the haunted forest. Well, the haunted forest of centrifugal HVAC pumps is the open cooling tower system. The chant that your Bell & Gossett (B&G) tower pump is chanting starts with not enough pressure in the eye of the impeller. The lament continues with the large amount of air found in cooling tower water. Finally, the flow pattern also becomes a challenge due to piping space concerns.
We introduced the Hydronic Centrifugal Pumps – Closed System Suction Piping earlier in our basic pump series. A quick review of that topic will help with the understanding of the cooling tower pump suction piping concerns covered in this blog post. Let’s take these concerns one at a time.
Cooling Tower Pump Static Suction Pressure
The cooling tower basin is an open tub of water. The pressure is zero PSIG at the surface. The pump should be located below the water level. When the pump is off, the pressure at the suction of the pump will be equal to the elevation of the water level above the pump. This is the static pressure.
The sketch above shows two simple piping systems. The one on the left will have a pump off suction pressure caused by the 8” of water. In feet of water the suction pressure will be 0.67 feet. Not very much. The sketch on the right has the same 8” plus three feet or 3.67 feet. The pump likes suction pressure, so the one on the right is the better option.
Cooling Tower Pump Operating Suction Pressure
Friction pressure drops begin once the pump is operating and flowing water. Anything in the suction line will reduce the pressure at the inlet of the pump. The goal is to keep that suction pressure drop to the very minimum. Look at the sketch above. A strainer has a pressure drop. A plugged strainer has a deadly pressure drop. The check and control valve have immense pressure drop. They have no place in this suction piping.
Pump Suction Pipe Valves and Fittings
Strainers should be 20 mesh or larger. The B&G suction diffuser is a combination strainer, elbow, and straightening vane and it uses a strainer with 1/8” holes. If you have equipment, such as certain chillers, that require much finer strainers, put a separate fine mesh strainer in front of the condenser. Do not put it in the tower pump suction.
The only other thing we should see in that piping is a line size shutoff valve, such as a butterfly valve. The use of throttling valves or check valves in the suction should be avoided. Visit the B&G technical training manual on cooling towers for more information.
Pump Suction Piping Velocity
The Hydraulic Institute (HI) section 9.6.6.3 recommends the suction pipe velocity be limited to 8 FPS when pumping from open tanks, such as a tower basin. But wait, there is more to know.
There are two reasons for limiting the velocity. One is the pressure drop which we will address in the next article. The second is the concern of creating an air vortex in the tower pan.
As the velocity rises, the minimum submergence or water level in the “tank” or tower pan above the water outlet location increases. The HI covers submergence in their section 9.8.6 and includes a formula. There is also a nice article by Jim Elsey in Guidelines for Submergence & Air Entrainment.
There is a “rule of thumb” out there that indicates, when pumping from an open tank, that the water level should be 5 feet above the opening. We have never seen a 60” high tower pan. The only way we have seen, to get 5 feet of water above the outlet to the pump, is with a drop-down or basin storage tank, a system used to prevent freezing in the tower basin by using a tank at a lower level inside the building. You can learn more in the Description 2024 ASHRAE Handbook—HVAC Systems and Equipment.
We have learned that a helpful solution is to keep the pipe velocity down around 5 to 6 FPS and to specify an anti-vortex baffling option also referred to as a doghouse from the tower manufacturer. This is a good thing to add to standard specifications. At 5 FPS velocity, the submergence required will drop to around 3 feet. This is still higher than most tower water levels, but the anti-vortex plate helps reduce the issue.
This pipe sizing will mean the pipe is normally one size larger than the ASHRAE 90.1 energy standard suggests for worst conditions. More about the cooling tower pump suction piping can be learned when you go to classes at the B&G Little Red Schoolhouse training program.
The screen shot above shows 6” pipe at 680 GPM has a velocity of 7.55 FPS. This meets the maximum velocity of 8 FPS, but more attention must be given to submergence.
Here is that same 680 GPM, but now we selected 8” pipe, and the velocity is more comfortable for an open cooling tower pan outlet pipe using an anti-vortex baffle.
Next week, we will continue the cooling tower pump suction piping with more on the pump inlet pressure and concern about air.
