Parallel Pumping in Condenser Applications: Part 5 of 5
/By Chad Edmondson (JMP) and Norman Hall (RLD)
Over the last few weeks the we’ve looked at pumping issues with multiple condensers and pumps. We have received questions about the topic, including these two:
Why do I need cooling tower outlet On-Off valves if I have an equalizing line?
Why do we see outlet On-Off valves on almost all jobs in North Carolina and only half the jobs in Michigan and Northern Ohio?
Let’s start with the need for the On/Off control valves on the tower outlet and continue to use the example of the condenser water pumping system we’ve used over the last several weeks. The example is a condenser water pumping system with a design flow of 800 GPM per tower for a total of 1600 GPM, with the third tower as standby.
In a part load condition, some tower inlets will be closed and one will be open. In our example the open or active tower will have 800 GPM flowing to it and the inactive tower will have “0” GPM since the On-Off 2-position inlet control valve is closed.
The tower basins, without control valves, may be modeled as follows:
The pump is attempting to pull 800 GPM from a tank or basin and both basins are open to atmosphere. With the exception of slight piping differences, the flow out of each basin will be approximately equal. The flow through the tower and into the basin will not be equal since one inlet is closed. The result is that the tower basin in the inactive tower will start using make-up water very quickly but the make-up is not 400 GPM. This drop in water level could cause the inactive basin to empty and possibly suck air into the pump, causing the system pump to air lock and stop pumping. Meanwhile the tower in operation overflows.
Sizing the Equalizing Line Between Cooling Tower Basins
Does the equalizing line solve the problem mentioned above? We checked with four tower manufacturers and they all size the equalizing lines the same way. The equalizing line is a gravity flow pipe between all of the tower basins. It is sized based on a flow rate equal to 15% of the largest tower design flow rate. In our example the total flow rate of each cell is 800 GPM so the equalizing line flow rate is:
800 GPM x 0.15 = 120 GPM
The pipe size is based on the calculated flow rate with the motive force being the gravity flow caused by 1” differential and 1” differential is .0833 feet. We could use the Bell & Gossett System Syzer for this calculation. Let’s assume in our example we have 20 feet of piping, two elbows, one tee, and the entrance and exit opening pressure drop. What pipe size would we need to flow 120 GPM of water through all of that with 1” pressure drop?
The Bell & Gossett System Syzer is a great tool helping us determine the correct pipe size.
We start by selecting a pipe size and entering 120 GPM in the Flow/Pressure Drop tab. Next go to the Length/Pressure Drop tab and click on the Help with TEL button and enter the pipe length and fittings. Change the pipe size until the pressure drop is less than .0833 feet.
When we performed this task, the result was an 8” pipe for the equalizing line. We were actually surprised by the amount of total equivalent length (TEL) that the entrance and exit openings caused. In the past, we have ignored this pressure drop when calculating cooling tower suction piping pressure drop. However, in the process of writing this article, we came across a note in some Baltimore Aircoil (BAC) cooling tower documentation that expressly reminded engineers not to forget the entrance and exit pressure drop. In our own example, these pressure drops added about 40 feet of TEL! We certainly won’t forget this in the future!
(Remember that the correct way to size the equalizing pipe is to contact the tower manufacturer. The pipe size should never be less than the opening size of the tower you select.)
Why Are Control Valves Often Used on Tower Outlets in North Carolina But Not in Michigan?
It does not take an engineer to know that if a basin is flowing 120 GPM in and 400 GPM out the level will drop and the make-up float will call for make-up water. For this reason, the tower manufacturers want 2-position On-Off control valves on the outlet of the tower basin when using multiple staged towers.
So why does our friend and fellow Bell & Gossett rep, Norm Hall of RL Depmann in Michigan, sees so few installations with 2-position outlet valves in his home state? One reason is weather! It gets well below zero in Michigan while such cold weather in North Carolina is rare. The engineers in Michigan put basin heaters in the specification but some engineers are still nervous about having a basin with no flow in the winter. What is different?
If they have the room, they could use a cold well tank mounted indoors which eliminates the need for the 2-position valves as well as the basin heaters.
The 8” pipe in the calculation above has a pressure drop of .28 ft./100 at 400 GPM so maybe there is a combination of less pipe length, different fittings, a drop in basin water level greater than 1”, and the required makeup water difference. (A lot of IFs!)
If they oversize the equalizing line to 10”, the friction loss is .09 ft./100 at 400 GPM.
Maybe they did not know better and there are problems and the tower manufacturer works with contractor during commissioning to solve the problem.
OR MAYBE we need to understand why the 2-position valves are there and follow the tower manufacturer’s instruction! Remember, this is all about cooling tower installations so we always defer to the manufacturer of the specified tower for specific details and requirements.
