UVC Air Conditioning for Offshore Oil and Gas Industries
Many companies involved in the Oil, Gas and Mining Industries are active in improving the working and living conditions of their employees. One of the major factors in achieving this is to improve the Air Quality in Accommodation Rooms, Offices and Recreational Areas.
There are a number of suppliers offering solutions and systems to improve Air Quality, each standing by their own claims that their system is the best system and very few explaining why. The object of this article is to examine the various systems and explain the benefits and limitations of each.
Before we examine the solutions offered to improve Air Quality we must identify the problems that develop within a Heating, Ventilation and Cooling System (HVAC). The components which make up a typical HVAC are:
Air Handling Unit (AHU)
- Return Air and Fresh Air Mixing Chamber
- Pre Filter and Mixing or Bag Filter
- Cooling Coil Section and Drain Pan
- Droplet Eliminator
- Heating Coil Section
- Humidifier (not always fitted)
- A Fan Chamber
- An Attenuator or Silencer (not always fitted within the AHU)
- A secondary Filter (not always fitted)
- Supply Air Ducting (From The AHU to the conditioned rooms)
- Occupied Rooms
- Return Air Ducting (From the conditioned Rooms to the AHU)
The highlighted points in the list above are areas that can affect the Air Quality of the HVAC system. We will examine each point in the following text but before we do that we need to look at what we are trying to achieve and why.
Sick Building Syndrome
An article published in The Lancet in 2003 (The Lancet – Vol 362 – pp. 1785 – 1791, November 29 2003 – www.lancet.com ) proved that Employee absence due to sickness caused by microbiological contamination of ventilation and air conditioning systems can be drastically reduced by the integration of UV-C systems in to the HVAC System. The test which looked at two identical buildings with similar occupancy proved that the building treated with UVc resulted in:
40% fewer respiratory problems **
30% fewer mucous membrane problems **
50% fewer complaints about muscle pains **
The test also concluded that employees experienced higher concentration levels which resulted in higher productive output.
A viral epidemic in an Offshore Environment, Call Centre or any highly populated workplace can result in up to 30% absenteeism. No Human Resource Team could plan for this level of a reduction to their workforce.
Treatment to HVAC systems with UVc or Advanced Oxidation will reduce the potential of an epidemic by up to 85%
The capacity of the Cooling Coil in an AHU is calculated to cope with the load that is needed to maintain the system at the required level. The heat transfer capability is based on the size of the cooling coil and the surface area of the pipes and fins and the speed of the air passing through.
A “Bio-Film” comprising of Mould and Bacteria can develop on the cooling coil very soon after start up. The Bio-Film insulates the Fins and Pipes and restricts the Air Flow which results in lower heat transfer. This can often reduce the efficiency of the cooling plant by up to 20%.
Chemical Cleaning regimes are often in place but the Bio-Film will start to have an effect on the heat transfer within 10 days of a chemical clean down. The introduction of UVc or Advanced Oxidation will clean the coil constantly and therefore maintain the plant design 24/7. This will reduce the number of cleans required significantly and reduce the operating cost by 10 to 20%.
Treatment to the HVAC plant with UVc and Advanced Oxidation will:
- Significantly improve the Air Quality
- Improve the well being of the workforce
- Reduce the potential of a viral epidemic
- Improve and maintain the plant efficiency
How do we select the best method to achieve the summarised points above
- Return Air and Fresh Air Mixing Chamber: At this point the return air will be contaminated from the occupied areas. It may contain Viruses, Bacteria and other Pathogens. The fresh air may contain contaminants depending upon where the fresh air is taken from. It will certainly contain particulates.
- Pre Filter and Mixing or Bag Filter: Filtration is essential in any HVAC system to stop particulates. The normal system would have an F7 pre and bag filter. The bag filter is designed with concave chambers to increase the surface area so that the capacity of the filter to hold particulates is increased. The Filter density can be increased to HEPA which will filter very small particulates and some microscopic pathogens. Increasing filter density will increase the pressure drop within the system which in turn will increase the load on the Fan and therefore, increase the running cost of the system. Some HEPA Filters are impregnated with disinfectant but this can degrade very quickly in adverse conditions such as High Humidity.
Filters do not kill or eliminate living organisms. Moulds double every 4 hours and most bacteria and viruses double every 20 minutes. It does not take long for the pathogens to colonise the filter and work their way through to the discharge air side. It should be noted at this point that one Bacterium can develop into trillions in less than 24 hours.
- Cooling Coil Section and Drain Pan: This part of the HVAC system is the main cause of air pollution. The conditions on and within the cooling coil encourage the development of Microbial Colonies. Chemical cleaning is not an option. It cannot be scheduled to be completed regularly enough to have a consistent effect on air quality. Automatic Dosing Systems have been installed in some HVAC installations.
Health and Safety managers are aware that ingestion by the workforce of the chemicals used will cause mid to long term health problems. There can also be an issue in storing the reserve bottles of the chemicals which must be changed regularly. Ultra Violet Light emitters should be fitted across the cooling coil on the discharge side (air off). UVc works by an application of a continual dose of light at 254nM. Dosage is based on UVc power x time. Because the cooling coil is in a fixed position, a low amount of UVc will produce a large dosage over 24 hours. This will keep the cooling coil completely free of Bio-Film which in turn will significantly improve the general air quality.
Cooling Coil Pre UVc
Cooling Coil Post UVc
There are some negative issues with UVc coil cleaning: a) It is not always possible to find the space in the AHU to fit the UVc emitters. b) Often droplet / moisture eliminators are fitted immediately after the cooling coil. The eliminators need to use the cooling coil drain pan which makes it impossible to expose the cooling coil to the UVc. c) Because the UVc emitters are presented across the AHU there is not enough exposure time to treat the air which will contain airborne pathogens such as viruses, TB bacteria etc. Cooling coil disinfection can be achieved by strategic placement of Advanced Oxidation probes within the AHU. Advanced Oxidation generates Friendly Oxidisers, such as Hydrogen Peroxide, in very low but very effective quantities. At 0.02 parts per million, Advanced Oxidation Systems operate at 1/50th of the lowest safe limit for Humans. Advanced Oxidation is used throughout the world as a treatment in Domestic Air Conditioners. Over the past 5 years Advanced Oxidation has been developed to operate in High Volume HVAC systems.
- Supply Air Ducting (From The AHU to the conditioned rooms): As mentioned in statements 1 and 3 Air borne pathogens such as Viruses and Bacterium can pass through the filters and be unaffected by Cooling Coil Treatment. There are two methods we can deploy to ensure that the air supplied to the occupied rooms is as free as possible of harmful micro-organisms. a) A UVc Duct Module should be fitted into the discharge ducting. This can be either by a large plenum installed in between the AHU and the first division of the ducting or a number of smaller UVc Modules installed in each ducting leg. The most important point to understand when employing this method is that the UVc emitters must run in line with the duct. The Dosage of UVc applied must be calculated based on the size of the duct in section and the speed of the air as it passes through the duct. By offering the UVc emitters to run along the length of the duct we can ensure that the air is exposed to the UVc for as long a time as possible. b) When it is not possible to find the space for a UVc Duct Module or for a retro-fit, we can use Advanced Oxidation Probes to achieve the result. The number of Probes required will be based on the Air Flow and Volume. Advanced Oxidation can weaken over the distance it needs to travel, therefore, booster probes should be installed every 15 to 20 metres.
Typical UVc Duct Module
- Occupied Rooms: Mould and Bacteria often develops in occupied rooms such as sleeping quarters and bathrooms. This can be caused by sweat and exhaled air during occupation and vapour from showers. Utilisation of the Air Treatment as described above will eliminate this problem. In Modular Accommodation the deployment of a standalone UVc Air Disinfection Unit in each room will solve the problem.
- Return Air Ducting (From the conditioned Rooms to the AHU) Most Viruses and Infectious Pathogens will originate in the occupied rooms. If it is not possible to deploy either UVc modules or Advanced Oxidation probes in the supply air ducting, a reasonable alternative would be to fit UVc or Advanced Oxidation Probes into the return air duct. This method alone will not improve the general air quality without treatment to the cooling coil but it will prevent cross contamination by eliminating the Viruses before they can get into the supply air system.
|Location||Viruses||Mould + Bacteria||Constant Coil Cleaning||Maintain Plant Efficiency||Remove Particulates|
|Cooling Coil UVc||No||Yes||Yes||Yes||No|
|Discharge Air UVc||Yes||Yes||No||No||No|
|Return Air UVc||Yes||Yes||No||No||No|
|Advanced Oxidation in Fan Chamber and every 20m in the Discharge Ducting||Yes||Yes||Yes||Yes||No|
I am sure this document will cause a bit of an uproar with some equipment suppliers but hopefully it will take a lot of the confusion away from HVAC Design Engineers and Plant Operators.
Author: David Whittingham
Advanced Air Hygiene Ltd