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Can Cooling Coils Be Cleaned? – Answered

 

As we learn more about indoor air quality problems, their prevention, and solution, we have identified several common problem areas. One of these is cooling coils. Immediately after being shipped from the factory, a cooling coil starts accumulating dust, and other contaminates. Some have suggested that part of the commissioning process for a new building should be removing any accumulated dust and debris from the cooling coil before it is placed into operation. This is because any level of contamination on the coil can dramatically affect its performance. Soil has insulating properties that affect the heat transfer properties of the coil and thus its operating characteristics. In addition, any buildup restricts airflow through the coil.

Dirty Coils Cause IAQ Problems.

Any change in heat transfer efficiency or airflow is a change to the designed operating characteristics of the system. If the change is too great, the system will not be able to maintain design efficiency and may become so out of specification that it is not able to hold up its part of maintaining a satisfactory comfort level. Any soil accumulated on or in the coil can also contribute directly to an IAQ problem. Dust represents an ideal food source for the growth of bacteria and fungi. These organisms can release gases as a byproduct of their growth that are unpleasant, irritating, and may cause many of the symptoms related to ‘sick building syndrome’. When growth is extensive, actual live organisms can be released into the air stream. These often trigger allergies in sensitive people and may even cause disease. As contamination builds up on fin surfaces, the flow of water removed from the air stream as condensation does not flow off the fin surfaces as efficiently. Water droplets remaining on the fin surfaces can blow into the air stream and fall out downstream in the duct system. It is this moisture that causes much of the growth that is seen inside of air ducts. There is a range of problems associated with dirty coils.

The question that is being asked more and more frequently is, ‘Can coils be satisfactorily cleaned?’ As we have come to better understand the importance of and indoor air quality, more and more contractors have began to analyze their coil cleaning techniques and evaluate their results. When a bright surface is restored to a coil face that was black and matted with lint, caked on dust and a residue of built up growth, it looks clean.

Looks Can Deceive

When the coil is removed and cut open exposing the full depth of the fins, another story emerges. Generally, only the first half inch or so is free of soil. Past the first row of tubes below each face, considerable contamination remains. This coating of grime may contain soil and be black or brown. When wet it is has a slimey; sticky feel to it. This is what is called a biofilm. When growing, bacteria secrete a sticky substance that helps attach them to the surface where they are growing. This material also tends to protect them from being affected by a chemical agent that would normally kill them. There have been documented cases where a biofilm containing live bacteria has been found lining pipes carrying an antiseptic in manufacturing plants. Some of this material flaked off into the fluid and when it was later used to treat skin surfaces, caused infections.

Generally it is felt that only mechanical scrubbing is effective in removing an established biofilm from a surface. No matter how creative we are in formulating coil cleaners so that they produce foam which expands and helps push soil out from between fins, we fall short of agressive direct mechanical action. Thus, much of any established biofilm remains undisturbed after cleaning.

Techniques That Will Help

Currently a number of procedures are being used in an effort to remove more of the soil and accompanying biofilm from the surfaces deep in the core of coils. These include:

  • Heat – The surfactants that are the heart of modern cleaning compounds use a chemical/physical action to ‘pry’ soil loose from surfaces. Heat enhances this action and speeds it up. As a rule, the hotter your cleaning mixture is, the more effective it will be.

 

  • Pressure – Water pressure imparts mechanical action. Although water pressure alone will never equal the mechanical action of direct scrubbing, it will enhance cleaning efficiency. When using high pressure, avoid pressures that will cause physical damage to delicate fins. Generally pressures over 1,000 will lead to damage. Very fragile materials may be damaged at even lower pressures. Carefully monitor fins for any bending, distortion or change in appearance when using high pressure.

 

  • Time – Modern cleaning formulations act through a series of chemical and physical actions. This interaction with the soil matrix takes time. Generally, five minutes or longer of contact time is required for full surfactant activity. Where a number of coils are being cleaned, all should be wetted with the cleaning solution then left to soak.

 

  • Rinsing – Many cleaners today are being marketed as ‘No Rinse’. This claim can be made if the cleaning solution does not contain ingredients that are so toxic that they must be diluted or they will cause severe air contamination when the unit is re-started. Thorough rinsing is still needed, however to carry away the soil that has been removed. Also, any ingredients that remain on the fin surfaces can act as dirt magnets and hasten re-contamination of the coil.

 

  • Removal – Coils are difficult to clean in place. Access is restricted. Drain pans can not handle the volume of water needed to do a through rinse. Heavy soil can block drain lines. Removing the coil to a cleanup area avoids these problems and can be used in conjunction with all of the above techniques.

 

Even with all of the above techniques, coils that are allowed to become very dirty will probably not be completely cleaned on the innermost surfaces. Thus, they will not operate like a new coil. Also, new soil will accumulate more rapidly. There are two options that should be considered: a) replace the coil with a new one; and b) clean the coil before the biofilm is established on the fin surfaces.

Since the cost of replacing a coil every time it becomes soiled is not acceptable to most owners, earlier cleaning is probably more acceptable to most. The problem is that it is difficult to determine how early is early enough. From our experience to date, we beleive that if we can see any evidence of soil on the face of the coil, it is too late. A resistant biofilm is probably already in place.

Emphasize Preventative Maintenance

Savvy contractors are signing owners onto comprehensive maintenance agreements when their systems are first purchased. They include with these both regular coil cleaning and filter replacement. Cleaning every six months is probably not too frequent and some contractors are advocating every 90 days. It is important to clean soil off the coil before it accumulates to the point where it can not be removed.

Harsh, aggressive chemicals are probably not the answer. Such agents utilize a chemical reaction with the surface metal of the fin to create the mechanical action that aids soil removal. Many fin stock surfaces contain microscopic grooves that help channel the water and aid in its rapid removal. Such chemical reactions remove these grooves and leave the surface of the fin with microscopic ‘pits’ that tend to hold water in droplets rather than allowing it to flow off.

Clearly, more research is needed before we can completely understand what most accept as a basic procedure but is much more complex in the impact it has on both system operation and IAQ. Coil cleaning is not a simple matter and deserves your serious attention.

Bob Baker is a member of ASHRAE and Chairman and CEO of BBJ Environmental Solutions, Inc., a manufacturing company specializing in providing clean air through environmentally responsible products.