Improving Ventilation Part 2: Redesign Considerations

In Part 1:  Why, How and What, of this series, I discussed why more ventilation is needed, how much is required and what type of filtration should be used.  In Part 2, I’ll discuss what needs to be considered to redesign an existing ventilation system to add more air flow and HEPA filtration.

Adding HEPA Filtration

In my article on Air Filtration Basics, I discussed that the two issues with adding HEPA filters to a ventilation system are:

1. increased static pressure; and
2. the area needed for the filter housing. 

The two issues are inversely related and there are trade-offs.  The faster the air flows through a HEPA filter, the higher the static pressure, but the filter area will be smaller.  Conversely, the slower the air flow, the lower the static pressure drop and the larger the filter area.   

To evaluate the trade-off between increased static pressure and filter area, start with determining the ventilation system’s current air flow rate, static pressure and where the fan is on its fan curve.  When static pressure is added to a ventilation system, the amount of air flow provided by the system will decrease according to the fan curve.  If adding HEPA filtration and the resulting increased static pressure will reduce the air flow from the ventilation system below the required indoor air change rate, then the fan performance will need to be increased.  Ultimately, the trade-off comes down to fan performance versus filter area with cost and area being the deciding factors.

There are two additional considerations when determining the static pressure increase related to adding HEPA filters.  First, because HEPA filters are expensive, it is practical to add pre-filters to extend the life of the HEPA filters.  The amount of static pressure added will depend on the MERV rating of the pre-filter and the air velocity.  Second, when the pre and HEPA filters collect dust and contaminants this is referred to as filter loading.   Depending on how frequently the filters are changed, filter loading can increase static pressure between 20%-50%.  If the requirement is to keep the indoor air change rate above a desired rate, you will need to include some amount of additional static pressure increase to account for filter loading. 

Alternative to HEPA Filters

The cost and area considerations for installing HEPA filtration may make it impractical.  An alternative is to install an Ultraviolet Germicidal Irradiation (UVGI) device to the duct work or air handlers.  UGVI devices can be expensive, but they have negligible pressure drop.  They are highly effective at killing viruses with a 99% kill rate for COVID-19.  However, they are less effective than HEPA filtration for removing bacteria from an air stream.  If your facility already has a good indoor air change rate and filtration, adding a UVGI device may be the quickest and least expensive way to add COVID-19 prevention to your ventilation system.

Adding Outdoor Air Flow

Although adding HEPA filtration may reduce the outdoor air change rate, it is always necessary to provide some amount of outdoor air to maintain positive pressure and for odor control.  Adding a small amount of additional outside air flow may be as simple as adjusting the dampers on the air handler, assuming that there is enough heating/cooling capacity to handle the additional outside air.  When adding larger amounts of outdoor air flow, a dedicated outdoor air system (DOAS) may be a good solution.  A DOAS unit delivers 100% outside air that is cooled or heated to the required indoor temperature.

The big drawback to adding more outdoor air flow is the cost of heating or cooling the air.  In my article, Indirect Evaporative Cooling Part 3:  Pre-Cooling Outside Air, I discussed how using Indirect Evaporative Cooling (IEC) can lower the cost of cooling outdoor air.  An IEC pre-cooler can be used with either a DOAS unit or an air handler with an outdoor air intake and reduce the amount of AC tonnage that is required.  Depending on the outdoor air conditions, the IEC pre-cooler will reduce the outdoor air temperature between 15^º and 20^ºF and deliver operating cost savings of 30% over direct expansion style AC.

Increasing Indoor Air Change Rate

Determining what ventilation system changes need to make in order to add more air flow to an existing ducted ventilation system requires application of The 3 Basic Fan Laws.  To provide an illustration of how to apply these laws, let’s go back to the simple example of a 10,000 sq. ft. restaurant dining area that was used as an example in Part 1 of this series.  The indoor air exchange rate that we want is 10 times per hour which would require an air flow rate of 1,667 CFM from the air handler.   Assume that the existing air handler provides 1,000 CFM with a belt drive blower wheel turning at 800 RPM using a 2-horsepower motor.  The First Fan Law says the new CFM increases at the ratio of the new blower speed over the original blower speed:

CFM_N = CFM_O x (RPM_N/RPM_O)

Solving for RPM_N where CFM_N = 1,667, CFM_O = 1000 and RPM_O = 800, we get 1,334 RPM.  This means that we need to change the sheave sizes so the blower wheel turns at 1,334 RPM.  To find out how much horsepower is required to turn the blower wheel at 1,334 RPMs and provide 1,667 CFM we apply the Third Fan Law which says that the new horsepower increase at the cube of the ratio of blower speeds:

HP_N = HP_O (RPM_N/RPM_O)³

Solving for HP_N where HP_O = 2, RPM_N = 1,334 and RPM_O = 800, we get 4.6 HP.  To produce 1,667 CFM from our blower turning at 1,334 RPM we will need a 5 HP motor.  Now we will need to consult the air handler manufacture to determine if the unit can be upgraded with new sheaves and motor.  Also, we will need to make sure that the electrical circuit can accommodate the amp draw of a 5 HP motor.

To make the illustration more complete, let’s add HEPA filtration to the redesign of the ventilation system.  The static pressure increase from the pre-filters, HEPA filters and filter loading is 0.6 in wg when the filters are new and clean.  From the fan curve for the existing blower, assume we find that with the increased static pressure from the filters will decrease the air flow rate to 750 CFM.  Applying the First and Third Fan Laws, we determine we now need the blower wheel to turn at 1778 RPM and the motor horsepower needs to be 11 HP.   Because the speed and motor size will most likely require a larger blower wheel and may be even an entirely new air handler, this is a situation where we should also look at a UVGI device as a lower cost solution instead of adding HEPA filtration.

Fluid Flow Analysis

Redesigning an existing ventilation system to add filtration and air flow can be difficult to do accurately.  Because the wrong answer can be costly, using fluid flow analysis (FFA) to model an existing ventilation system with all of its components is a good investment.  Eldridge’s proprietary FFA software goes a step beyond conventional FFA calculations.  Our FFA software allows us to input fan curves so that we can dynamically analyze the effect of changing variables such as air speed and filter size.   With this tool, we can find the optimal solution to redesigning an existing ventilation system to accommodate cost and area considerations.

Conclusion

The amount of money required to improve a ventilation system by adding additional air flow and filtration is a large investment for most businesses.   A problem this complex with such a large investment at stake requires a ventilation expert with the experience, tools and technical knowledge that will guaranty the right solution for the application.  At Eldridge, we stand behind our ventilation system designs with a guaranty that they will provide the required air flow.  Let Eldridge help you make the best investment to improve your ventilation system.