by Steve Spielmann
In large buildings, chillers are often the single largest consumers of energy. As energy prices increase, chillers can be identified as a potential source of significant savings. Preventive maintenance, everything from keeping a daily log to the weekly, monthly, and annual operational reviews, will yield optimum efficiency of the chillers. Then there is tube cleaning.
The undoing of any chiller is the build-up of scale, biological growth, and corrosion inside chiller tubes. When this occurs, efficiencies are eroded and the chiller’s life cycle can be significantly reduced.
Throughout time, many methods have been employed to clean chiller tubes, some with greater success than others. There are five standard cleaning methods:
- Rod and brush
- Tube cleaning guns On-line systems
- Rotary tube cleaners.
This is, perhaps, the oldest and least pleasant method. It involves using a length of rod, usually metal of some sort, with a nylon or wire brush, larger than the tube’s inside diameter, attached to one end.
Typically, the tubes are first flushed with water from a hose, then the brush is manually pushed through the tubes. The tubes are then flushed again with water.
- Low cost.
- Simple operation ¯ requires little training or expertise.
- Time and labor intensive.
- Because the brush bristles are folded over, they tend to swab the tube more than brush it.
- Tubes cleaned at the beginning of the job tend to dry with debris still in them making the final water flush ineffective.
- Requires enough space to use a rod slightly longer than the tubes to be cleaned. Otherwise, rod sections must be coupled together, further increasing the time needed to complete the job.
- Nobody wants to do the job, so it either doesn’t get done or it gets done poorly.
Less prevalent now than in years past, chemical cleaning uses acid solutions that circulate through the tube bundles to break down or soften scale deposits in the tubes.
- Under the right conditions tubes can be cleaned to bare metal.
- Mineral scale can be softened enough to allow brush cleaning as a secondary operation.
- Expense ¯ chemicals can be costly and become an expense that is incurred every time tubes need cleaning.
- Time consuming ¯ chillers are off line for at least 24 hours.
- Chemicals are dangerous to work with, increasing employee health and safety risks and employer liability.
- With all the local, state, and federal restrictions on hazardous waste disposal, getting rid of used chemicals is difficult, costly, and carries some legal implications.
- Requires significant training and expertise.
Tube Cleaning Guns
This method uses compressed air and water or high pressure water alone to propel a cleaning projectile through the tubes to remove deposits. Projectiles can range from rubber bullets to brushes to plastic or various metal scrapers.
- Speed ¯ tubes are cleaned in a matter of seconds.
- Requires little training or expertise.
- Under the right conditions, tubes can be thoroughly cleaned with this method.
- This method has limited capabilities in terms of the types of deposits it can remove. Most projectiles are only suitable for light deposits such as mud and algae.
- Both end bells must be removed to allow recovery of projectiles increasing labor requirements.
- High water pressure units are costly and require considerable maintenance.
- Safety ¯ some guns aren’t vented to eliminate the possibility of “blow back,” which occurs when one attempts to shoot a projectile into a plugged tube. The tube becomes pressurized with air, and when the gun is removed from the tube, the projectile is launched back at the operator. This only occurs with air/water guns that don’t incorporate a pressure relief feature.
On Line Systems
There are two basic on-line tube cleaning systems in use today. One uses plastic brushes that are “trapped” in each tube using plastic baskets that have been attached to each end of the tube using an epoxy.
Periodically, the direction of the water flow through the tube bundle is reversed, causing the brush to travel to the other end of the tube. This back and forth brushing action is designed to keep the tubes clean.
With the other type of system, a quantity of foam balls are circulated through the system. In theory, a foam ball will travel through every tube in the bundle frequently enough to keep the tubes clean.
- Provided water treatment is right, these systems do work.
- The maintenance personnel need not even think about tube cleaning.
- Cost - these systems can be fairly expensive.
- They are not suitable where scaling commonly occurs such as areas where hard water is present or where water treatment is poor.
Rotary Tube Cleaners
These devices use either an electric or air motor to rotate a flexible shaft, which is encased in a plastic casing that transports water to the cleaning tool. Cleaning tools range from a variety of brushes to buffing tools, hones and scrapers. They can clean almost all types of deposits including hard scale. The operator simply feeds the flexible shaft through the tubes to brush and flush in one operation.
- Affordable - initial outlay is minimal and the cost of consumables is reasonable.
- Simple to use ¯ little training or expertise is required.
- Requires only one operator to reduce labor.
- Tubes are left thoroughly cleaned for better heat transfer.
- There are many models to choose from; ranging from simple, no-frills machines to more advanced units incorporating mechanical shaft feed, bi-directional rotation, and variable speed.
- Most effective method for cleaning the newer chillers that use internally enhanced tubes (more on this later).
- Only one end of the chiller needs to be opened for cleaning.
- Slower than tube cleaning guns.
- Untrained operators may tend to break flexible shafts
Internally Enhanced Tubes
The internally enhanced tube is a relatively new feature in the evolution of chillers. These tubes are manufactured with a “rifling” or spiral groove inside the tubes. They often have external enhancements as well. The idea is to provide more surface area for heat transfer. The internal enhancement also creates more turbulence in the water passing through the tube, allowing it to absorb more heat. The high points of the rifling are called the lands and the low points are the grooves.
Using internally enhanced tubes, chiller manufacturers get more cooling out of smaller chillers, thereby reducing costs and enhancing efficiency. This is great until the internally enhanced tube becomes fouled. These tubes aren’t easy to clean properly.
If you use the rod and brush method, you simply ride over the lands, leaving the grooves filled with deposits. If you spin the brush in the direction of the spiral, you can get some of the dirt in the grooves, but this is slow, tedious work.
Acid cleaning will work, but you must consider the downsides.
Projectiles from tube cleaning guns move through the tubes too quickly to effectively remove deposits.
On line systems don’t work.
Rotary tube cleaners have proven to be the best suited for cleaning internally enhanced tubes.
Chiller maintenance is important for optimizing efficiency and extending equipment life. It takes training and knowledge to perform chiller maintenance properly. Isn’t it interesting that the single biggest determinant of chiller efficiency is the cleanliness of heat transfer surfaces - primarily the tubes.
The technologies discussed here enable maintenance personnel to get their tubes clean quickly and with minimal effort. Keep those tubes clean! It will save you money.
Steve Spielmann is customer service/ technical manager for Goodway Technologies Corporation in Stamford, CT. He has been is the HVAC maintenance field for 27 years. He can be contacted at the following e-mail address: [email protected]