So, let’s take a look at these potential troubles and see how to solve them.

Pump Head Requirements

The pumping head determination procedure for the “open” tower piping loop differs from the conventional “closed” loop piping circuit used for most hydronic (heat-cool) applications. The difference concerns consideration of “open” loop static heads.

The closed loop circuit has no need for consideration of static heads for pump selection because of a balance or cancellation of static heads between the supply and return risers. Static head lost by water flow to any height in the supply piping is cancelled by a static head “regain” as water flows down the return piping. The only pump head requirement for the “closed” loop is that due to flow-friction pressure drop, (Fig 1).

The “open” or tower circuit is different from the “closed” loop circuit in that all static heads are not cancelable. In the open piping circuit, the pump must raise fluid from a low reference level to a higher level; this requires pump work, and open statics becomes an important consideration for pump selection.

In Fig. 2, the required pump head will be the pipe flow-friction loss from A to D plus the energy head (Hs) required to raise water from the lower to higher level.

The cooling tower circuit differs slightly from the basic “open” circuit in that the discharge piping is seldom connected directly to a discharge tank. The tower discharge is generally to the atmosphere and then into a distribution pan.

For the tower piping circuit, the pump must overcome the piping flow friction loss: piping, condenser, valves, etc. It must also provide the energy head necessary to raise water from a low to a higher static head level.

Most discussions concerning tower and/or open piping circuits would simply define the required pump static energy head as Ho (see Fig. 3)—the “open” height of the piping circuit. This is, however, an over-simplified assumption which may or may not be true depending on whether or not a “siphon draw” is established in the downcomer return piping, DE. The nature of the downcomer siphon draw and its limitations should be evaluated.

Downcomer Siphon Draw

In Fig. 3, water is being discharged at E to the atmosphere. Pressure at D must be equal to exit loss plus flow-friction loss DE and minus the static pressure reduction caused by downcomer return static height Hr.

Pressure reduction to D as caused by static height Hr will generally, but not always, permit cancellation of height Hr as a part of the required pump head. This is because of a resultant siphon draw action in the downcomer.

Given that the “siphon draw” does indeed occur, the required pump head will become:

The pump head selection statement shown above is commonly accepted as a truism. It has limitations, however, and will not apply under certain circumstances. These circumstances should be understood if unnecessary cost and embarrassment are to be avoided by the consultant.

Exit loss and flow-friction loss in the downcomer will generally be less than the downcomer height Hr. For this circumstance, the downcomer must operate at sub-atmospheric pressure when the siphon draw is established. If the downcomer vacuum is broken, the expected siphon draw will not occur, and the estimated pump head may be inadequate.

The expected downcomer return siphon draw vacuum can be broken by any of three basic application circumstances: 1. Top-vented downcomer. 2. Inadequate downcomer flow rates; bottom-vented downcomer. 3. Fluid vapor pressure or flash considerations.

Top-Vented Downcomer

A downcomer vent will break the siphon draw vacuum. The vent may be a simple loose pipe connection, or it may be a mechanical vent purposefully applied at the downcomer return high point.

Vents are sometimes applied to establish known reference pumping conditions when downcomer return siphon draw conditions propose stability problems, as with a very high downcomer, when fluid boiling is a probability, or when start-up downcomer flow rates are anticipated as inadequate for the siphon draw.

Given a top-vented downcomer, the pump must raise water from the pump suction pan water level to the highest vented point in the downcomer.

Considering this point to occur at D in Fig. 3, the required pump static head will become Ho + Hr or Hs.

The total pumping head to point D will become Hs plus the flow-friction loss Delta h (AD). Separate consideration must now be given to the downcomer return.

Since the pump has raised water to level “D,” it provided a fluid head equal to Hr to overcome flow-friction loss in the downcomer. There are two different pumping possibilities; fluid head Hr greater than downcomer flow-friction loss (Delta h DE) and the reverse: Hr less than Delta h (DE).

The usual pumping circumstance will be the condition of Hr greater than Delta h (DE). This is because the available fluid head Hr is the equivalent of 100 ft./100 ft. pipe friction loss rate. Downcomer piping flow-friction loss will generally be to the order of 4 ft./100 ft. Since the pump has already provided the necessary fluid head to flow the downcomer, Hr > Delta h (DE); friction-flow loss in the downcomer is not a part of the required pump head, and total pump head becomes:

High downcomer pressure drops can be caused by control valves or tower spray nozzles. When this pressure drop plus the downcomer pipe flow-friction loss exceeds fluid head Hr, the pump head must be increased by the difference Delta h (DE) minus Hr. Total pump head then becomes:

Bottom-Vented Downcomer

Downcomer flow rates can be so low, relative to pipe size, as to allow air to enter at the pipe discharge, causing the downcomer to become vented and will prevent formation of the necessary siphon draw vacuum. Tests conducted at Bell & Gossett indicate the siphon draw will not be established when the actual flow-friction loss rate is less than the order of 1’/100’ based on clean pipe pressure drop evaluation.

Pump head requirements for the bottom-vented downcomer will be as previously noted for the top-vented circumstance.

An unfortunate operational sequence can occur during pump start-up when the pump energy head is devoted towards simply raising water from the low level pan to the highest part of the system.

During this start-up period, flow rates can be so low as to cause “bottom venting” and prevent (sometimes forever) formation of siphon draw circumstances and full design flow rates. A water-legged discharge or discharge reducer will provide automatic siphon draw establishment so long as minimum “start-up” flow velocity in the downcomer is to the order of 1 ft./sec.

In Fig. 4, air entry into the pipe discharge is prevented. The minimum flow velocity pulls air bubbles down the piping, finally evacuating the downcomer of air and establishing the siphon draw condition—downcomer pipe full of water and operating at sub-atmospheric pressure.

Unusual application circumstances will sometimes establish such a low start-up flow rate (less than l’/sec. velocity) that air bubbles are not carried down the piping. The downcomer cannot then be emptied of air and expected siphon draw will never occur.

For this circumstance, it is necessary to separately fill the downcomer with water. This can be accomplished by valve closure at the piping exit in combination with a top vent. During start-up, the exit valve is closed and the vent opened. After the piping is filled, the vent is closed and the exit valve opened.

Siphon Draw Limitation

Given sufficiently low sub-atmospheric pressure, any fluid will flash or boil. Fluid pressure in the downcomer piping cannot be less than the pressure at which the fluid boils. Fluid vapor pressure thus provides a siphon draw limitation.

Theoretical cancelable downcomer return static height (due to sub-atmospheric siphon draw) will vary dependent on fluid vapor or boiling pressure and on atmospheric pressure as this changes from sea level. The variation for water as affected by water temperature and height above sea level is shown in Table I.

Fig. 5 illustrates an example tower schematic for an installation located at 6,000 ft. elevation. The tower is to be used to dissipate heat from 180° water; what is the required pump head?

(The figures shown in Fig. 5 correspond to available fluid head over and above vapor pressure for the water temperature shown.)

Reference to Table I shows that the cancelable siphon draw height for 6,000 ft. elevation and 180° water is only 10’, while downcomer return static height is 30 ft.

If conventional pump selection practice were to be followed, the pump selection would be:

This pump selection provides a perfect example of low start-up flow rates; the pump head will just be enough to raise water to the system top. Start-up flow rate will be insignificant.

Even given the special application precautions previously stated, the pump selection won’t work because water flash in the downcomer will prevent establishment of the presumed 30’ siphon draw head. In this instance, water would flash because the downcomer return static height exceeds the cancelable siphon draw head (see Table I: 6,000 ft. at 180° = 10’).

When downcomer return height exceeds cancelable siphon draw head, it is necessary to separately evaluate downcomer needs. For these circumstances the summation of cancelable siphon draw static height plus downcomer return flow friction loss must exceed downcomer return height; the excess providing anti-flash pressurization.

The necessary downcomer flow-friction loss would generally be established by a balance valve positioned close to the outlet (E). This valve will now provide the necessary “back pressure” to maintain downcomer fluid pressure at above its boiling or vaporization point.

For the particular example, a valve pressure drop equal to the order of 23 ft. would establish an overall downcomer return flow-friction loss of 25 ft. (23 + 2 = 25).

A 25’ downcomer flow-friction loss added to the theoretical cancelable height of 10 ft. will establish a pressure over and above boiling of 5 ft. at “D.”

The correct pump head selection now becomes:

For this particular example, a simpler solution could apply an open vent at “D,”eliminating need for the downcomer balance valve and its setting. In this case, the pump provides an “available” head at D of 30’. This fluid head is available for downcomer flow and is greater than flow-friction loss in the downcomer (Delta h DE) of 2’. Downcomer return flow-friction loss can then be neglected since downcomer fluid will be in “free fall.”

Required pump head would then become:

Either correct solution will provide required design flow rates. Design flow rates would not and could not be established by the “conventional” head selection of 40 ft.

This is an abridged version of an article based on Bulletin No. TEH-1075, “Cooling Tower Pumping & Piping” that appeared in the November 1998 issue of Service & Contracting.

Reprinted from TechTalk January 1999
Copyright 1999 by ITT Industries