Cooling tower parallel 3 pumps

[virk]

We are designing cooling tower cycle with similar 22 consumers. Each consumers requires max. flow of 50m3/h cooling water. Design flow of cooling tower cycle is 300m3/h. Considered worst case is 8 consumers in operation.

We want to install 2+1 cooling water pumps, each of them delivering 150m3/h at certain height. My question: What is the best way to determine whether the additional pump must be switched on or off. Should we maintain "constant" supply pressure, or how to manage it. Furtheron I must ensure the none of the pumps exceeds maximum allowable flow.

Any hint appreciated.

Kind regards

virk

 

[quark]

The standard logic is to control the flow by a dp transmitter which is placed at about 2/3 piping length and setting the dp when the 8 users are on(provided you have two way valves and the pumps are of variable speed type). Suppose, if only 4 users are on, then the pressure in the supply header increase and so your total dp. Pump slows down to match the set dp and likewise.

Variable speed drives come with a maximum frequency set parameter and you can restrict the maximum pump speed by this parameter. Maximum flow condition occurs when only one pump runs (as the system resistance is low). First check what flow rate a single pump gives and set the speed as per the affinity laws. If you always run two pumps then the maximum speed set parameter should be based on two pump running.

Both Danfoss and ITT-Bell & Gossett(there may be many others also) have developed good controllers to operate parallel pumping systems. These controllers can be fed with the pump characteristics so that you can run your pumps at the BEP. I highly recommend a discussion with them.

Regards,

 

[iken]

You should also keep in mind, that if you are reducing the flow through all three cooling towers, their efficiency will decrease, and you may not achieve the heat rejection required.

I had this situation once, and found the best cure was to install control valves at the cooling tower inlets, which closed depending on the quantity of pumps running. This not only ensured the nozzle pressure in the cooling towers remained near design, but also the water flow rate through the cooling towers.

You may find, that without the control valves installed, you may be operating all three cooling towers, when only one operasting at/near design conditions would meet the heat rejection required (waste of energy and inflated operating costs).

 

[cme]

I would also spec recessed sumps for the towers; motorized valves (butterfly) on each supply from the sump and an oversized equalizing line connecting them

 

[virk]

CME, what do you mean with "recessed sumps"? I am not familiar with this term

Thank you all, I think i got correct hints.

One additional question: If there are a lot of cooling water consumers, which are switched on independently, how do I cover these requirements best? Do I install

a) f.e. 3(+1) similar pumps in parallel
b) 1(+1) pumps for total flow with variable speed drive (pressure control)
c) 1(+1) pumps with throttle valve in discharge line (pressure control)
d) 1(+1) pumps without any throttle valve with the effect that the discharge pressure changes little bit due to changing flow

Any hints?

kind regards

virk

 

[tombmech]

There are a myriad of possibilities with the situation you present. Quark answered the fundamental question about control. How you select the number of pumps is a question of optimization, and the answer depends on your preferences with the specific application.

Taken in context, cme's statement about recessed sumps also mentions a control valve on the sump outlet, and an oversized equalization line. The equalization line assumes multiple towers or cells, which is not clear to me. You mention 3 pumps, but you never stated whether there is more than one tower or cell.

At any rate, your original premise includes a potential discrepancy that cme was trying to address:
Design Flow - 300m3/h, Max Pump Flow (8 users) - 400m3/h.

So, if your max case occurs, the tower(s) may empty. This is because a cooling tower presents an open circuit to the flow. All head is lost at the point the water is distributed through the fill. Suction head to the pump is only atmospheric pressure plus the physical height of the sump. Accordingly, the fill and distribution basin will only support a maximum flow. A larger sump (recessed), a valve to keep additional back pressure on the sump, and an oversized equalization line (more volume), may all be attempts to ensure that the tower sump does not empty - in the isolated case where 8 users may exceed design flow. However, these controls may only address the transition. Your pump may start to cavitate from decreased suction pressure even if the tower(s) does not empty.

In any event, the first thing I would address is the discrepancy in your design flow (300 m3/hr) vs. the expected max flow rate (400 m3/hr). A better solution may be to pick a maximum design flow, and then limit the system to that flow. Otherwise, the deliterious effects mentioned above will over-complicate the controls (if they work at all).

To answer your question about sizing the pumps - it's an optimization preference:
1. If you have a load that varies most often between 1/3 to 2/3 max, then 2+1 pumps may be a good solution.
2. If you have a load that runs most often around 50% max, then a 1+1 arrangement may be best.
3. If you have a load that varies all the time from 0% to 100% max, then a single pump with a VFD may be the best solution.
4. You may still want a "+1" pump just for simple redundancy.

These questions can only be answered with your knowledge of how the system may run, suggest some solution scenarios, then compare them with cost. Converting 300m3/hr to gpm = 1321 gpm. (I don't live and breathe metric, so "gpm" gives me a better idea of size.) That is not an insignificant size. Fewer pumps - but with VFD's - may be the best at that size. Discriminating among the load with several towers or cells will depend on your operational judgment. You have such an obviously large diversity, I would be concerned with whether you should increase the size of the system to 16 users or more, then incorporate optimum sizing and VFD's to limit the energy usage of reduced flow.

Actually, the size of the system may warrant decoupling the tower from the distribution circuit. In that case, you may achieve a closer scenario to that described in your first description: Size the tower for 300 m3/hr (primary), then size a separate, decoupled circulation circuit (secondary) at the higher flow of 400 m3/hr. The tower will not be able to maintain temperature at the 400 m3/hr flow rate, but that may be acceptable for short periods. Since the tower will be decoupled, it will not empty or overflow. This scenario could also be fairly inexpensive by placing rudimentary, limited controls on the tower circuit and pump, then applying the VFD to the distribution circuit.

Forgive me if all of this sounds complicated, but that may really be the case. It sounds as if you have a system of decent size that could be expensive with first cost and long-term cost from energy and maintenance. Yet, it also has a significant load variance and diversity. That is when Engineers earn their salaries.

 

[virk]

tombmech, this discrepancy in flow design arises from the following:

We agreed to design for 8 consumers being in operation, but started one after the other. As soon as f.e. the 5th consumer has started its operation, the first one has already decreased his flow demand. This is why the 300m3/h are the design flow and not the 8*50m3/h.

The cooling tower will be a two cell one, two fans, two separated bassins which will be connected by an (oversized) suction line. One suction line "goes" to the pump station. We already have considered the necessity of an equalization line and do not consider it to be necessary. In case of switching of one of the cells, inlet and outlet valves will be closed. Arising pressure losses in suction line will result in tolerable level differences in bassins. (...as far as we have calculated)

The pump installation will be +1 in any case. We asked ourselves (and you all) whether the job is already done, whether two 100% pumps equipped with variable speed drive are more or less expensive than three 50% pumps.
Rough calculations and discussions have shown that energy savings are expected to be "similar" in our case and customer accepts both options due to the fact that he already run both possibilities. I expect the two 100% pumps with VSD two be more expensive than the other solution. At the moment a colleague of mine is examining this.

Thank you, I will post any new investigations

virk

 

[tombmech]

Thank you for defining your system in much more specific detail with that post.

I still see several problems in your scenario.

Sometimes it's reasonable to reflect why a practice has been accepted for a long time. In this case, equalizer piping between cooling towers piped into the same circuit has been a traditional practice. Why? You state that you have calculated the pressure losses in suction and that it will result in tolerable basin level differences. With respect, these differences occur on a dynamic basis with many variables. It all depends on the closing and opening stroke speed of the valves, their effect on flow, and the pressures induced by the pump. The flow effects are not linear, and the pressures induced by the pump are also not linear. Both are impossible to predict on a simultaneous basis under all conditions. That's why the practice of equalization piping came about.

There are also concerns with variable speed pumps applied directly to cooling towers, period. The speed variance of the pumps cannot be coordinated with the flow in the tower between the fill and the basin, and transient overflows and/or exposed an suction outlet will result. This is practically impossible on a number of different levels, and VFD's on directly connected tower pumps are not used - except perhaps for soft start/stop.

 

[tombmech]

P.S. Variance in flow on a cooling tower will also occur with staged pumps. A VFD is not needed to create the problem. Tower cells are typically matched to a pump with constant flow, period.

Sorry I didn't mention this sooner - the decoupled scenario is best if you want variable flow on the users' end or simply use 3-way valves.

 

[virk]

tombmech, the tower we use are tower with nozzle distribution of water, not by gravity. Supplier allowes them to work between about 25% to 100% of course by increasing pressure drop accross the nozzles. Do you expect (larger) problems with this nozzle type two. We know about problems when using f.e. BAC gravity distributed ones. Too much flow causes overflow at distributor, too low flow causes maldistribution.

Additional. Tower will be highest equipment. The "remaining" cooling water cycle (pipes, pumps, consumers, etc.) will always be completely filled up. No uncontrolled draining, etc. will be allowed. We assure this by proper piping of the system.

I discussed equalization line with a colleague, who assured me, that no problem will arise concerning equalization line. Normally I believe him

Further hints and comments appreciated

virk

 

[tombmech]

Well, perhaps I'm revealing my ignorance here. I am familiar with the TowerTech rotary nozzle, but I believe that still only offers a 3 to 1 turndown, which is not a whole lot - perhaps giving a little flexibility in some situations. I think in TowerTech's case, it was a rationale to save money from buying separate tower cells. I'm not sure one could truly classify it as a "variable flow" cooling tower. The situation you first described is way beyond that.

As far as your statement,
"The "remaining" cooling water cycle (pipes, pumps, consumers, etc.) will always be completely filled up. No uncontrolled draining, etc. will be allowed. We assure this by proper piping of the system."

With respect, that's really the whole point. As Engineers, we are the ones that determine whether there will be "uncontrolled draining" or whether it has "proper piping." Honestly, I'm not trying to be flippant, but I thought that was the purpose of the discussion: whether a problem "will be allowed." The only ones that really determine that are Mother Nature and us (the Engineer).

 

[cme]

the 'std' connection is the suction off the side of the sump

recessed sumps are just what they imply ..... a recess in the sump so that they tap off the bottom and are continually flooded for the suction