High Plume Dilution Fans | 3 Key Design Questions

There are three key design questions that we ask when designing every High Plume Dilution Fan application. They are the basic questions that determine every design choice down the line.  They will be asked at some point, so better to deal with them early in the design process.

 Q.  Is there redundancy required in the system?

Strobic Air High Plume Dilution Fans

A greater percentage of High Plume Dilution Fan Systems (Lab Fume Hood Exhaust Fan Systems) are now manifolded systems; there are more than one fan mounted on a plenum-mixing box. For these systems it is essential to figure out how much, if any, redundancy is required. We build redundancy into systems as a backup. Typically we look to make sure we are covered in the event of a fan failure. If one fan fails, will the system be able to cover the exhaust load?

Many times we just say make redundancy x1, meaning an additional backup fan is included. If one fan fails then it is backed up by 100%. This is the safest plan but it is also the most expensive. We don’t typically recommend it. 

  • We take a close look at every application. Is it a multimillion dollar process that cannot afford to go down or is a high school lab fume hood that’s not even used 24/7? It’s key to consider the application first. 
  • Then it’s worthwhile to ask the question whether the users could get by with a lower percentage of exhaust flow for a short period of time. In most lab applications this is the case. They run VAV systems and can afford to run the system at 75% of full capacity for the time it takes to fix a fan or replace a motor.  This is our typical recommendation.
  • Then it’s key to remember that good quality fans don’t fail very often.  If you are working with high quality equipment, like direct drive Tri-Stack Fans from Strobic Air, then you may not need the redundancy. The motors on these fans are rated for 22 years of operation and very rarely fail. Better to use the money in another part of the project and put in vibration sensing on the fans to provide an early failure warning.
  • Lastly the million dollar question; can the application afford the cost for the additional redundancy. If not, it’s better to consider less redundancy or none at all.

Q.   Is there energy recovery included in the design?

Strobic Air High Plume Dillution Fan Energy Recovery Unit

In many areas, like Massachusetts, energy recovery is now mandated for many applications that typically use High Plume Dilution Fans. If it’s not mandated then it should be considered. It should be considered early on in the project. Not at the end when no one wants to make any changes. 

Look at the application and determine a payback for the type of energy recovery that is being considered. It does not make sense to use energy recovery on applications that do not support a payback (like very small applications or applications with extremely dangerous exhaust). Most of the applications we see with High Plume Dilution Fans do support a payback and it’s usually less than 4 to 5 years. 

This is a key question to ask and explore right up front.

 Q.  Will sound attenuation be required for the application?

Nozzle Silencer from Strobic Air

Once again it is important to look at the type of application and the proximity to neighbors. If it’s a big industrial park and the building is miles from neighbors then we don’t look to incorporate sound attenuation. If the building is on a quiet campus and could impact other areas then it’s definitely considered.

There are several areas, like Cambridge, MA, where sound attenuation is fairly typical for these types of fans. Cambridge has one of the more strict thresholds for noise. They look to 50 dbA at the property line, which is the sound level of a typical home. That’s pretty quiet.

Keep in mind three things with respect to sound attenuation. 

  • Most sound is emitted from the fan upwards. If the building that the fan on is the tallest in the area then typically sound attenuation is not as important.
  • If the High Plume Dilution Fans are run on VFDs, like most are these days, then the sound at the reduced, operating RPM should be considered for the application.
  • Make sure that sound attenuation, if used, is also incorporated on bypass dampers also.  In many cases these can be the biggest sources of noise from the system.

Redundancy, heat recovery and sound attenuation all cost additional dollars. It is important to consider them early on and to justify the additional cost.



Project Snapshot: WCCC UMass Data Center | Custom Air Handling Units

Project:WCCC UMass Data Center
Engineer:Symmes Maini & McKee Associates
Contractor:KMD Mechanical Corp.
Equipment:Custom Air Handling Units
Size:(2) 83,000 CFM Units
DAC Sales Engineer: David Goodman

WCCC UMass Data Center

Project Highlights: 
Two Electronically Commutated Motor (ECM) fan array units.   Each array consists of (12) 7.5 HP ECMs, with backwards curved fans, that can deliver 83,000 CFM at 2.5” w.c.  The two arrays combined deliver a total of 166,000 CFM, which is enough air to cool the maximum design data center heat load of 1.2 MW (SA=75F to RA=100F).        

With typical annual energy costs per square foot 15 times (and in some cases over 40 WCCC UMass Data Center Rigging Picturetimes) that of typical office buildings, data centers are an important target for energy savings.  They operate continuously, which means that their electricity demand is always contributing to peak utility system demands.  The goal here was to create an energy efficient system that provided maximum flexibility to expand and variable capacity to meet load requirements. 

WCCC - UMass Data Center

We knew that better performance would be provided with a Custom-Designed Central Air Handling System.  A centralized system offers several advantages over the traditional multiple-distributed-unit systems.  The centralized system is well-suited for variable-volume operation to take advantage of the fact that the server racks are rarely fully loaded.  A centralized air handling system can improve efficiency by taking advantage of surplus and redundant capacity.  The maintenance-saving benefitsWCCC - UMass Data Center of a central system and  the reduced footprint are additional benefits.

The ECM Fan Array is a main design element of the system.  The ECM array provides both energy efficiency and redundancy.  Direct signals from the Building Management System controlWCCC - UMass Data Center all fans in the array.  Variable Frequency Drives were eliminated from the project,  saving first cost and maintenance cost.   The ECM motors also have a longer life cycle and take up less space than traditional fans.

Ashrae CCTC Article by Paul Hanbury on EMC Fan Array

EBM Papst – Brochure

Have a question about the ECM Fan Array, we invite you to   askRick?


Ask Rick: How much cross contamination occurs with an Energy Recovery Wheel?

Q.  How much cross contamination occurs with the use of an Energy Recovery Wheel?  Does exhaust air get mixed back into the supply air stream?

A. Cross contamination refers to the amount of air that is leaked from one side of the wheel to the other.  Leakage rates are significantly reduced by using higher quality wheels.  Higher quality wheels use better seals, minimizing leakage.

Fan Arrangement to Prevent Cross ContaminationCross contamination can also be reduced significantly by fan configuration.  If the Supply Fan is in a blow thru and the Exhaust Fan is in a draw thru configuration then any leakage will go from the supply to the exhaust side of the wheel.

Carryover is a different type of concern when using an Energy Recovery Wheel.  Carryover occurs when a contaminant is transferred from the exhaust side to the supply side of the airstream through the rotation of the wheel.  Research has shown that the potential for contaminants carryover varies as a function of the contaminant properties, the construction of the total energy wheel (quality of the wheel) and the desiccant utilized.

To minimize carryover, a Purge Section can be used.  The purge section uses outdoor air to clean or purge the wheel matrix before it rotates from the exhaust air stream to the supply air stream.

How much cross contamination and carryover occurs is completely dependent on the quality of Energy Recovery Wheel, the type of desiccant used and the Air Handling Unit design.

Ask Rick: Why aren’t direct drive fans used on more applications?

Direct Drive Fan - Ziehl Abegg

Q.  Why aren’t direct drive fans used on more applications?

A.  In Europe, 80% of commercial ventilation units sold today are equipped with direct drive fans.  In The US, the industrial and heavy commercial fields have already recognized the benefits of direct drive fans.  Reduction in maintenance has been the primary motivation.

In HVAC applications direct drive fans have not been used for the following reasons:

  • Habit –  It takes our industry a long time to change.  We are late adopters.  If it’s on the plans, it gets cut and pasted over and over.
  • Cost – Up until now direct drives had been more expensive than belt drive fans.
  • Space – The direct drive arrangement adds length to an AHU.  Sometimes that won’t work for a design.

At DAC Sales, we now use direct drive fans wherever we can.  They make up 90% of what we specify.  For Annexair and Strobic Air,  direct drive fans are all we use.


Pumped Glycol Energy Recovery | What’s so special about Konvekta?

Konvekta Pumped Glycol CoilTwo years ago Rick McGinley and I were contacted by Rudolf Zaengerle from KonvektaKonvekta had done research and had singled out DAC Sales as the firm to represent them in New England.

Rick and I were not convinced that Pumped Glycol Run Around loops were viable.  They had always been our least favorite means of air to air energy recovery.  Traditional Glycol Energy Recovery had efficiencies near 45% in design;  and 40% after pumping costs were included.  That’s on paper.  When they got installed and were not maintained we saw even worse results.  Needless to say, not our first choice in energy recovery.

We agreed to meet with Rudolf at the Boston Airport Hilton (we made him pay for lunch).  We let him know right up front that we were not big fans of Pumped Glycol Loops.  We needed to be sold.

Konvekta Pumpd Glycol Pumping PackageWhat’s so special about Konvekta?

We are sold.  Here is what we know is true.

  • Konvekta specializes in pumped glycol energy recovery systems.  That’s all they do.
  •  They are based in Switzerland and have been in business for over 60 years, doing only pumped glycol energy recovery systems.
  • Last year their sales were over $60 million.
  • Konvekta supplies the entire package.  They make their own coils, pump packages and controls.
  • The reason they are so successful is they are able to achieve 65-75% effectiveness with glycol loop systems and they guarantee the savings.   That’s right, they guarantee the savings.
  • The Konvekta system is the only pumped glycol energy recovery system that will meet the new minimum efficiencies in the revised ASHRAE code.

We have represented Konvekta for two years.  We have systems being installed at;  Dartmouth College, Mass State Dept. of Public Health, Hudson Valley Community College and VA Medical Center Jamaica Plain.  Contact Us for more information on Konvekta.

High Plume Dilution Fans | Online Selection For Lab Exhaust Fans

High Plume Dilution Fan Online Selection | Choose Tri-Stack.com

This is a quick overview of how to use the online choosetristack.com site from Strobic Air.  It takes minutes to get a login name and password.  It takes minutes to figure out the software.  Try it out at choosetristack.com

Feel free to follow up with questions and comments.  Sales@dac-hvac.com

Ask Rick: Pumped Glycol Systems | Where are Konvekta Systems manufactured?

Q.  I have read about Konvekta systems. They are a Swiss company. Do they manufacture any parts of their system in the US? 

A.  Most components are made in the USA; in particular, the Hydronic Module and its components (pumps, motors, valves, expansion tank, etc.) are made or are purchased from suppliers in the USA.
The coils are still manufactured in Switzerland.  They are a shorter lead item and are more cost effective to build in the main plant.

Ask Rick: When do you use a Desaturation Coil?

Q:  In what applications do you recommend using a desat coil?

Aerofin Desat CoilA:  In general, a desaturation coil (desat coil) will help in any application where the cooling coil is the last component in an air handling unit.  The desat coil is one of several options which can provide the necessary minimal reheat so supply air doesn’t leave the unit saturated.

We routinely use desat coils in laboratory or other critical applications in tandem with heat recovery options.  We design these applications with the supply fan in a blow-thru position upstream of the energy recovery device (wheel or heat pipe) to minimize the potential of leakage through the device.  In this arrangement, without the supply fan motor heat added downstream of the cooling coil, the desat coil provides an easy way to sensibly raise the leaving air temperature off the cooling coil (usually by 1.5° to 2.0°F).

Another option is to use a Wrap Around Heat Pipe. This setup provides both the reheat (5° to 15°F) and also the same amount of pre-cool upstream of the cooling coil.  The Wrap Around Heat Pipe option has a higher first cost than the desat coil but provides a significantly lower life cycle cost.

Read more on the Aerofin Desaturation Coil


Project Snapshot: Lab Exhaust Energy Recovery Systems | Mass General Hospital CNY149

Project Name: Mass General Hospital CNY149
Architects: Chan Krieger NBBJ
Engineer: Thompson Consultants Inc.
Contractor: JC Cannistraro Inc.
Manufacturer: Air Enterprises, Thermotech, Strobic Air
Size: 320,000 CFM Lab Energy Recovery Unit
DAC Contact: Rick McGinley


Project Overview:

MGH - Aerial PhotoMass General Hospital CNY149 is an extremely large energy recovery system.   The system includes Air Enterprises Custom Air Handling Units,  Thermotech Energy Recovery Wheels and Strobic Air High Plume Dilution Fans (Tri-Stack Fans).  The system was designed to replace individual fan sets that served separate hoods in all lab spaces.  Massive duct sections were provided to allow for combining all exhausts. It also served general lab exhaust. 

Project Challenge:

Energy Recovery Device Selection:   Selecting the right energy recovery device for the application was a challenge.  Energy Recovery Wheels were of course the desired choice.  They are much more efficient and transfer both sensible and latent energy.  They save both heating and cooling energy costs.  The Energy Recovery Wheel Savings Analysis showed an expected annual savings of $767,253 ($2.4 per CFM). 

Because some of the exhaust air was from fume hoods the building owner was concerned about cross contamination.  In order to verify that energy recovery wheels could be used,  the hospital requested a carryover assessment study. The study assumed the worst case spill for the facility with a variety of chemicals.  The theoretical carry over amounts were compared with NIOSH Threshold Limit Values (TLV).  In all cases carryover amounts were significantly below the NIOSH TLV.  Wheels were also tested after installation with SF6 gas. 


There are 4 energy recovery wheels used on this project.  Each handles 80,000 CFM and   is over 18 feet in diameter.  At the time of install they were the largest wheels used in North America.  The specification included the following:  “The structural frame and casing shall be designed and manufactured so as to allow a maximum rotor deflection of 1/32 inch, as measured at the outer radius, during maximum rated airflow condition.”  The installed wheels meet the required deflection specification.





For more information on this application; 

Related Blog Posts:
Ask Rick: Energy Recovery Wheels on Laboratory Fume Hood Exhaust?
Energy Recovery Wheels | Understanding Cross Contamination / Leakage

Related Materials:  
In the Air, Volume1