Everything You Need To Know To Find The Best Counterflow Fill

By Chad Edmondson

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Cooling towers for HVAC systems come in either crossflow or counterflow configurations. This is defined by the way the water meets the heat transfer surface, more commonly known as the “fill” surface. Crossflow cooling towers distribute the hot water perpendicularly to the air flow. Water flows from the top of the cooling tower through the hot water gravity distribution basin and into the fill while the cooling tower fan draws air horizontally across the fill. Counterflow cooling towers also distribute water from the top of the tower thru pressurized spray nozzles, however the air flow is parallel (but in opposing direction) to the flow of water. Air is drawn from the bottom of the cooling tower, passing over the fill surface and exiting out the top.

The obvious question is: Why would one specify one type of cooling tower configuration over the other?

Very simply, it typically comes down to footprint. In most cases, a crossflow tower is preferred for several reasons that include, most importantly, overall efficiency and serviceability. However, crossflow towers have a larger footprint so if you are limited in real estate, a counterflow tower, which is taller but has a smaller footprint, may be your only option.   A counterflow tower could also be a better option because of its air intake configurations if you are installing the tower next to one or more walls.

Now let’s explore the design characteristics of each type of tower.

Crossflow Towers

Serviceability is a big bonus when it comes to crossflow towers. Crossflow towers have an internal access plenum that can be fitted with an internal platform from which the drive system can be serviced, and the entire fill assembly inspected. These towers can also be fitted with an external service platform or a ladder with handrails that provides safe maintenance access to the hot water basin.

Water flow from the top of a crossflow tower is by gravity only. The spray nozzles do not require any additional pressurization, which saves pump energy. At reduced water flow rates, weir dams help to fully distribute the water across the fill surface.  Counterflow towers, however, require pressurized spray nozzles to ensure even distribution of water at part load.

This is an important distinction to remember since ASHRAE 90.1- requires that chiller systems be configured either with multiple or variable speed condenser water pumps so that the cooling tower can be operated:

(1)  With flow that is produced by the smallest pump at its minimum expected flow rate, or 

(2)  At 50% of design flow per cell.

The intent of this requirement is to improve energy efficiency by maximizing the number of towers operating for a given flow rate while also maintaining the minimum required flow rate through the tower.  This is because it is more efficient to run a single 500 ton chiller across two 500 ton towers. More tower tonnage means more heat transfer surface, which increases the amount of evaporative cooling. 

The challenge is reducing flow without risking insufficient water distribution across the fill. Dry areas across the heat transfer surface are bad for two reasons:

(1) Dry areas create less resistance to air flow, so more air will flow over dry areas than wet areas, which limits the potential for evaporative cooling

(2) Dry areas tend to increase the amount of mineral deposits that get left behind from evaporated water, increasing the potential for dry air disease. [Link to  Cooling Tower and Condenser Water Design Part 11: Avoiding Common Pitfalls.]

Counterflow Towers

In addition to being taller and more compact than crossflow towers, counterflow towers have pressurized hot water nozzles which increases the pump head requirement and total system operating costs. Counterflow towers also have limited internal accessibility for service and inspection.  An external service platform and ladder is usually required for access to the hot water spray distribution and drive system.

To operate a counterflow cooling tower at half flow you will probably need to modify the spray nozzle pattern to ensure a fully wetted fill.  Of course, before lowering the flow through any cooling tower, you should always check with the manufacturer to determine what the minimum required flow is. Otherwise, you run the risk of underflowing the tower and eventually building up scale on heat transfer surfaces.

(Note: All the above features and benefits primarily apply to factory assembled cooling towers, not field erected cooling towers.)

Fan Options for Cross and Counterflow Towers

Both cross and counterflow towers can be built with either axial or centrifugal fans. For indoor applications and/or those with extremely low height requirements, centrifugal fans can be used on counterflow towers to keep the vertical height of the overall unit to a minimum. This option, however, is typically limited to smaller applications that are 272 tons or less and under USGPM.

I built a counterflow wort chiller of the garden-hose variety, like many others out there. I mostly followed the typical pattern. I chose to use 1/2″ copper tubing fittings to make the ends, similar to other designs I’ve seen. The only interesting twist I added was an attempt to better-approximate the efficiency of the “convoluted copper” heat-exchangers like the Chillzilla or Convolutus.

And, as usual, I photographed the construction, thinking that maybe someday I’d make a web-page about it.

Parts:

• 30′ of 3/8″OD copper tubing.
• Garden hose, with an inside-diameter that can form a tight fit on 1/2″ copper tubing. I think mine was 5/8″.
• A long length of 14ga bare copper wire, from the electrical section. Doesn’t need to be one piece. I bought a shorter length of 6ga (TODO: check this) stranded grounding wire, and unwound the individual conductors.
• 1/2″ copper tube fittings, “sweat” type (ie, soldered):

• Two tee fittings.
• Two end caps.

Soldering Copper Tubing

The end fittings are assembled by soldering. Learn how. It’s not that hard, and is a useful skill to have. If you’ve never soldered copper tubing, this is a great way to learn, instead of inside the cabinet under your kitchen sink. Here’s a quick lesson.

You need a propane torch, plumbers’ solder, and acid paste flux. You need an acid brush for applying the paste flux. You need a steel wire-brush or sandpaper, for cleaning the inside and outside surfaces of the pieces to be soldered.

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Start by cleaning the mating surfaces with the wire-brush or sandpaper until they are bright and shiny. The idea is to remove surface oxide from the copper. Using the acid brush, apply acid paste to the mating surfaces, the inside of one, and the outside of the other. Push the parts together with a twisting motion. It should go in easily, and with most types of fitting the tubing will hit a stop when it’s been inserted to the correct depth (about 1/2″).

Fire up the propane torch, and adjust to a moderate flame. Apply the flame to one side of the fitting. The acid paste will melt and possibly drip out, so make sure there is nothing important below. The hottest part of the flame is at the tip of the inner cone, try to get that part of the flame right up to the copper. After 10 or 20 seconds (or before, it doesn’t matter), you can apply the solder the opposite side of the joint. The idea is that the copper should melt the solder, not the flame. If the copper can melt the solder, then it’s hot enough to form a proper joint. If the flame melts the solder, the copper may not be hot enough for the metals to alloy together, and you’ll get a “cold” solder joint.

In a proper joint, the solder is “wet” and flows all around. In a cold joint, the solder will be balled up, like water drops on waxed paper.

If the copper is hot enough, the molten solder will be sucked into the joint. Apply solder until the joint seems full, it doesn’t really take very much, just about 1/2″ of solder to do a 1/2″ pipe joint. It takes a bit of practice to get just the right amount of solder. I usually overdo it, and have blobs of excess solder hanging from the bottom of the joint. It still works, it just looks ugly.

End Fittings

Take a copper end cap, and drill a 3/8″ hole in the end.. You want the hole to accomodate the 3/8″OD copper tubing with a snug fit. Solder the three short pieces of tubing (about 3″) into each branch of the tee fitting. Onto one of the straight-through branches of the tee, solder on a drilled end cap. Onto the right-angle branch of the tee, solder the brass 5/8″ garden-hose to hose-barb adapter. The adapter isn’t really made to be soldered like this, but I found one that was a reasonably snug fit on the 1/2″ tubing, good enough that solder could seal it.

It should all look like this. Make two of them.

Inner Tube

The efficiency of counterflow heat exchangers is influenced by two things, the contact area between the hot and cold surfaces, and the ability to maintain a temperature differential between the hot wort and the cooling water. There are conflicting demands at work. Using a smaller diameter for the inner tube will make a more efficient heat-exchanger. The reason is that with a smaller diameter, a larger proportion of the hot wort is in contact with the tubing wall. But efficiency isn’t everything, we also need a decent flow-rate, or you’ll be cooling your wort all night (and that would completely defeat the purpose of a counterflow chiller.) The usual compromise most people go with is 3/8″OD copper tubing for the inner tube. I did the same.

Maintaining a temperature-differential is the whole raison d’etre of the counterflow design. If you had the wort and coolant flowing the same direction, you’d have a very large differential at the input side, but the differential would approach zero partway along the length of the chiller. At that point, the wort is lukewarm, and so is the coolant water, and the transfer of heat will stop. The counterflow approach is better because heat transfer continues along the entire length of the chiller. In fact, with good efficiency, the cooled wort can approach the temperature of the incoming cold water, and the cooling water comes out the other end almost boiling.

The efficiency of counterflow chillers is reduced by laminar flow and the skin effect. When liquid flows in a tube, the flow rate tends to be highest at the centre of the cross-section, but friction makes the flow slower as you move out toward the wall of the tube. The slower flow rate near the walls of the tube reduces the rate of heat-transfer. In addition, laminar flow can result in pockets where the flow-rate approaches zero, effectively eliminating heat transfer entirely in those areas.

The very best counterflow chillers, such as the Chillzilla, overcome laminar flow and skin effect using “convoluted copper” tubing for the inner tube. This is copper tubing with a square cross-section, twisted into a spiral. The result is more turbulence in the flow, breaking up laminar flow, and keeping the temperature of wort and coolant even over the entire cross-section.

I have no access to convoluted tubing, but I tried to approximate the idea by wrapping a spiral of copper wire around the outside of the inner tube, and securing it in place with solder. The wire is on the outer coolant side of the tubing, not the inner wort side, so I’m not worried about the effect on the beer of the solder or acid paste.

I uncoiled the tubing first, and straightened it on my basement floor as best I could. I only soldered in one place every half foot or so. Doing this to 30′ of tubing took quite a long time. Leave the ends of the tubing clear for six inches or so, so there won’t be wire inside the end fittings.

Don’t use too much solder here. It will form hanging blobs on the bottom side of the tube where you won’t see them, and those blobs will make the insertion of the inner tube into the garden hose a nightmare. Trust me on this. While the solder is hot, pluck the tubing like a guitar string to knock off the dangling solder blobs.

Assembly

Next cut the ends off the garden hose, and reduce the length to about 30′. If your hose is long enough, cut pieces on both ends that are long enough that maybe you can make use of it for somthing else, like filling carboys. You want the hose to be shorter than the inner tubing by about the length of both end-fittings, plus a couple inches more to have the inner tubing protruding from the end caps.

Inserting the inner tube into the garden hose is quite tricky. Try to get the inner tube as straight as you can. Try to do the same for the garden hose (good luck with that). To ease the insertion, I used a cable-pulling lubricant, the kind used by electricians to fish cables through walls. It’s basically a soapy water-based gel. Applying it liberally throughout this process will make it much easier. The lubricant is only in the coolant part of the chiller, don’t worry about its effect on your beer. When finished, you should have the inner tube protruding from each end at least a couple inches beyond the end caps.

At this point, slide a couple hose clamps over the garden hose, one at each end. If you forget to do this, you will absolutely hate yourself later.

Now one of the end fittings is installed. Slide it on, letting the inner tubing protrude through the hole drilled in the copper end-cap. The 1/2″ copper tubing at the other end of the fitting should go at least an inch inside the garden hose. Slide the hose-clamp over the tubing and tighten to seal. Now apply solder to seal around where the inner tubing protrudes through the end-cap.

I did not solder the the opposite end-fitting on yet, because the coiling-up operation might cause the inner tube to move relative to the outer hose. So, coiling it up is the next step. I wanted to get mine coiled up as a single orderly stack of turns, thinking that would help all the wort drain out. A random bunch of turns would trap a lot of precious beer in local low spots. I used a carboy as a form to coil it up on. Not easy to do a good job by yourself, get some help. I used a lot of nylon zip ties to hold everything together as I wound it up. Start with the already-soldered end.

When it’s all coiled up, you can install the second end-fitting. First, make sure the hose-clamp is already on there. This end-fitting is installed just the same way as before, but be careful when soldering not to burn the nearby turns of the hose. In fact, it’s probably better to unroll it just a bit while soldering.

And that’s about it.

You have some options on what you do with the wort-in and wort-out fittings. You can just put plastic tubing directly over the copper tubing, and clamp it. Or you could install hose-barbs. They suck though, too hard to take apart again.

I use flare-type fittings. I used a flaring tool to flare the ends of the tubing, and then installed flare-flare couplings. I have lots of hoses with flare-nut ends on them. And when the chiller is not in use, I leave it filled with weak iodophor solution, and sealed with flare caps.

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