In a chilled water system when heat exchange is less than design at the coil, chillers become less efficient because fewer BTUs are removed with each gallon of water as it passes through the cooling coil. When cooling coils operate efficiently, more BTUs are removed per gallon of water and pumps on variable speed drives systems ramp back. A system at a T of 8°F will remove the same heat load @ 250 gpm that a system with a T of 4°F @ 500 gpm. This has a dramatic effect on chiller and pump energy efficiency.

Particulates in the air become entrapped on the wet fin surfaces. Micro-organisms contained in the particles become attached to the surfaces and have a natural predisposition to form bio-films. Since the environment presents a plentiful supply of nutrients and water, the growth of the bio-film can be quite robust in spite of the relatively low temperatures.

The presence of the bio-film on the coil surface acts as insulation between the air and the cooling coil. In fouled coils, the air flowing through the coil is not reduced in temperature adequately enough to allow the system to cool the building efficiently. Due to the inadequate cooling of the air, the building is not reaching the required temperature and compressors run continuously in order to compensate for the poor temperature transfer. In a system where the air is cooling the building efficiently the compressors will cycle in and out. Poor heat transfer results in inefficient energy usage and unnecessarily increased energy costs.

The bio-fouling of these same surfaces leads to a definite shortening of the operational life of the coils by virtue of microbiologically induced corrosion (mica). There are two main causative factors contributing to this corrosion. One is the acidic exudates emanating from the bio-film itself and the other an electrochemical reaction.

MICROCROBIAL CORRISON

All those experienced in the maintenance and remediation of commercial HVAC systems are familiar with the dramatic loss of efficiency associated with bio-fouled heat exchange surfaces. Fouled coils can add significant resistance to the air handling system.

Fungal proliferation adds unnecessary resistance to the system, causing reduced service life and increased fan energy requirement to maintain supply duct and conditioned space values.

There are significant negative health effects on workers in buildings where the HVAC systems have become contaminated by micro-organisms. This condition is sometimes described as “sick building syndrome”. Biofouled coils, along with microbial contaminated dust filters and air ducts, are known to be the main factors in most cases.

Bio-films colonizing heat exchange fin surfaces have been found to be one of the major factors associated with health problems induced by microbial contaminated HVAC systems. Interestingly, these ailments are, in most cases, not associated with living micro-organisms. As bacteria and fungi die, the cells’ outer walls break down to form micro fine particulates. In the case of gram negative bacteria these particles are called endow-toxins. The inhalation of end toxins in significant quantities has been shown to cause a variety of symptoms including sore throat, sore eyes, headache, mucosal irritation and dry cough.

When fungi break down the particles are called beta glucans. The inhalation of significant quantities of these particles results in a decrease in the human immune response. In practical terms this can manifest as an increased propensity to pick up miscellaneous minor ailments.

As the bacteria within the bio-film regenerate, they shed bacterial endow-toxins and other potential toxic fragments. These fragments along with the micro-organisms themselves are introduced into the occupied space by way of the HVAC system

Exposure to the organisms can lead to various immunological responses, and allergic reactions. In some cases where one’s immune system has been compromised, exposure to these organisms can lead to pneumonia. This is of particular concern in hospitals and other health care facilities’ environments where poor indoor air quality is known to induce a wide range of nosocomial infections.

The cost of the loss of productivity of the staff in a building with a contaminated HVAC system is virtually impossible to quantify, but is almost certainly a far greater cost than the power inefficiencies discussed earlier. Quite recently quantitative tests1 have been developed and validated for the measurement of endo-toxins and beta glucans in the air. Since the generally safe levels of these particulates are known, it is now possible to demonstrate occupational levels by air sampling of the workplace environment.

Mold can grow on any surface in the building where moisture and nutrients are present. Ceiling tiles are particularly susceptible when they become damp. This generally occurs when the condensate drain line from a ceiling mounted unit becomes blocked, leading to an overflow of condensate water onto the ceiling tiles. This can be prevented by regular maintenance of the unit and a strategy to inhibit microbial growth on the coil and in the condensate drain pan.

Additionally, harsh acidic and alkaline based coil cleaning products can do more damage than good. As much as 30% of the fin stock can be removed after being etched away by the chemical reactions with the aluminum. These products are dangerous if not handled properly and cause long shut down intervals due to the typically noxious odors of the process. New coil cleaning technologies are emerging which are more effective and environmentally safe.

Companies are constantly forced to reduce staff, streamline operations and to seek efficiency opportunities in every aspect of operations. This creates demand on maintenance engineers to do more with less. To help improve maintenance efficiency and reduce turn-around time, companies now consider outsourcing to service companies more than ever before. Investment in coil maintenance programs can optimize operational efficiency, save energy, and prevent process interruption from indoor air quality issues.

Building managers should establish regular programs which incorporate indoor air quality management with standard mechanical maintenance. Coil cleaning should not be regarded as an arbitrary PM line item, rather as a vital mechanical maintenance procedure with significant and varied benefits.

A regular coil maintenance program will lower the level of contaminants within the building and contributes to the improved health and well-being of the building occupants. Effective maintenance of the HVAC system has additional benefits beyond indoor air quality. By inhibiting microbial contaminants the useful life of key air handling components is extended, and well-maintained mechanical plants consume far less energy than systems impaired by the presence of bio-film. Cost savings are achieved through improved operating efficiencies of the physical plant.

In a typical modern building the air conditioning system accounts for up to 50% of its total energy consumption.

Improvements in the operating efficiency of key components such as heat exchange coils will result in substantial energy cost savings.

ABOUT THE AUTHORS:

Grant Shallcross, ASCS, CVI is the Technical Services Manager for Aeris Technologies, Inc.