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Convert Fume Hoods from CAV to VAV (In Progress)

Project History

  • 2/1/2017

    There were nine projects approved by the Revolving Loan Fund selection committee, see attached image.

  • 4/19/2014

    My information is a bit dated, but I had calculated $5.25/cfm for constant air versus $3.23/cfm for variable flow hoods.

Description

The University should continue their current approach of upgrading chemical fume hood exhaust systems.  The iCAP called for converting all existing constant air volume chemical fume hoods to variable air volume chemical fume hoods with energy recovery.  The University’s current approach of reducing total building air change rates to be in-line with current recommendations from various regulatory agencies and standards publishers, and conducting thorough life cycle analyses of each potential exhaust system allows adoption of the best solution for each laboratory building. 

Introduction

In May 2010, the University of Illinois (U of I) published iCAP A Climate Action Plan for the University of Illinois at Urbana-Champaign in an effort to fulfill its obligation to the 2008 American College & University President’s Climate Commitment.  The goal of the iCAP is to make U of I carbon neutral by 2050.

Several strategies were presented in the iCAP to reduce greenhouse gas (GHG) emissions.  One strategy presented for GHG reduction is to convert all existing constant air volume (CAV) chemical fume hoods (CFHs) to variable air volume (VAV) with energy recovery systems.

Background

A CFH is laboratory exhaust equipment specially designed to contain and exhaust hazardous chemicals being used inside them.  CFHs operate by taking in conditioned air from the lab room through an opening at its face and exhausting it through ducting.  U of I standards require that CFHs operate with an average face velocity of 100 linear feet per minute (lfpm) through an 18 inch-high opening.  At this face velocity and opening height, a six foot CFH exhausts at a flow rate of 900 cubic feet per minute (cfm). 

In VAV systems, the CFH exhaust flow, general laboratory exhaust flow, and supply air flow for a laboratory are interconnected.  Closing the sash on a CFH with a VAV system can result in a reduced CFH exhaust flow and supply flow, although VAV systems are not always setup in this manner.  Using the Trane TRACE energy model, the iCAP estimates that a VAV CFH costs $2,100 per year to operate.  It is important to note that this estimate is based on a model that assumes that CFH users will shut their sashes when not in use and may uses parameters or make assumptions that may not be representative of the actual conditions on this campus.

Energy recovery systems use a media to transfer sensible (heat) and/or latent (humidity) energy from the exhaust air stream to the supply air stream.  Energy recovery systems are typically used to recover energy from the total exhaust stream (CFH plus general exhaust).  Using the Trane TRACE energy model, the iCAP estimates that a CAV CFH with energy recovery costs $3,200 per year to operate.  However, in this model, only the CFH exhaust flow was considered. 

Using the Trane TRACE energy model, the iCAP also estimated the operation cost of a VAV CFH with energy recovery to be $1,500 per year to operate.  Again, in this model only the CFH exhaust flow was considered.      

Analysis

Converting all CAV CFHs to VAV with energy recovery is not feasible.  As was shown below through building-specific engineering analyses and publication of new U of I Facility Standards, the combination of VAV with energy recovery is not always the best solution due to research-associated hazards, lack of existing control and sensing equipment, and CAV CFHs that do not have the components (e.g., airfoils) necessary to function properly with a VAV system. 

This approach is also too narrow when developing energy conservation efforts in laboratories.  While CFHs exhaust a significant amount of conditioned air from a laboratory building, the total air change rate should be analyzed, not just that of the CFHs.  In many cases, air change rates in laboratories were inflated during the initial design as a safety factor.  Air change rates in the 12-20 air changes per hour (ACH) range are typical for most of the laboratory buildings on campus.  Various regulatory agencies, professional societies, and standards and codes publishers have determined that lower air change rates, ranging from 4-12 ACH, are acceptable to maintain a safe environment in normally operating laboratories.  The new U of I Facility Standards recommends an occupied air change rate of 6 ACH and an unoccupied rate of 4 ACH.  U of I feels that these rates meet all applicable regulations and standards, provides a safe environment under normal laboratory conditions, and promotes energy conservation.  Engineering analyses may show that there are variables, such as high CFH densities (or heating load), which may ultimately drive the air change rate above the recommendations of the standards. 

U of I VAV Studies

In two studies conducted by the Division of Safety and Compliance (S&C) on the U of I campus, no energy savings were observed.  The two studies are summarized below:   

A study of the VAV system at the Illinois Sustainable Technology Center in April 2009 showed that the VAV system was designed to reduce CFH exhaust flows when the sashes were shut, but the reduction in CFH exhaust flow corresponded to an increase in laboratory general exhaust.  The result was a constant air change rate regardless of the CFH sash height resulting in no reduction in total exhaust from the building and no reduction in energy consumption.   

The Division of Safety and Compliance received a grant from the Student Sustainability Committee (SSC) to conduct a Shut the Sash Pilot Study at the Beckman Institute from October 2009 to February 2010.  The study was successful in changing CFH-user behaviors by encouraging the closing of CFH sashes when not in use, however, no reduction in energy was observed during the study period compared to energy usage during the same period the prior year.  Background data for the study were limited and other building specific factors that may have contributed to the observed energy data trends were also limited.  Further studies need to be conducted to determine why the anticipated reduction in energy consumption did not occur.

To-Date Engineering Analysis

Facilities & Services (F&S) Engineering Services (ES) explored adding a VAV system to the CFHs in Chemical and Life Science A Building, which had an existing energy recovery system.  After an extensive engineering review, it was determined that the project was not feasible at that time because of unrealistic payback due to incomplete scoping. Architectural changes and mechanical infrastructure upgrades were unavailable and could not be included in the estimate.   

F&S Engineering Services also explored adding an energy recovery system to the exhaust system at Edward R. Madigan Laboratory, which has a VAV system.  After an extensive engineering review, it was determined that the project was not feasible at that time because of unsatisfactory payback due to the extensive mechanical infrastructure upgrades required.  Supply and exhaust fans would have to be replaced with fans that could handle the additional static pressure drop of the new heat recovery coils.  Ducts and air handling unit housings would have to be modified to accommodate the new heat recovery coils.  Floor space would have to be provided for the required new heat recovery pumps.

An Energy Service Company (ESCO) project is ongoing at Veterinary Medicine Basic Science Building.  An Energy Audit Report was prepared by the ESCO contractor to analyze various options for saving energy.  The report estimated that installing a VAV system with a total enthalpy wheel energy recovery system would cost $8,940,960, provide an annual energy savings of $98,507, and result in a payback of 91 years.    Also, the report estimated that installing a VAV system with a run-around coil energy recovery system would cost $5,082,000, provide an annual energy savings of $140,394, and result in a payback of 36 years.

Recent Projects

Three buildings that contain CFHs have undergone significant upgrades to their air handling systems, including CFH system modification, within the past few years.  In Roger Adams Laboratory (south), exhaust ducting from approximately 200 CFHs and general exhaust ducting are being manifolded into a single stream and exhausted through a total enthalpy wheel energy recovery system.  The cost for this project is $33,000 per CFH with a ten-year payback.  VAV was not considered feasible for this project because of unsatisfactory payback due to the extensive mechanical infrastructure upgrades required.  To do this, the existing dual duct HVAC system would have to be replaced with a VAV system.  All new ductwork, air handling units, terminal units with reheat coils, hot water piping, and controls would be required.  Also, extensive modifications or complete replacement of CFHs would be necessary.

In Morrill Hall, exhaust ducting from approximately 70 CFHs and general exhaust ducting were manifolded into a single stream and exhausted through a heat pipe energy recovery system.  The cost for this project was $33,000 per CFH with a ten-year payback.  VAV was not considered feasible for this project because the researchers work with bio-hazards that require constant volume exhaust.

In the Medical Sciences Building, exhaust ducting from approximately 17 CFHs and general exhaust ducting were manifolded into a single stream and exhausted through a heat pipe energy recovery system.  The cost for this project was $40,000 per CFH with a ten year payback.  VAV was not considered feasible for this project because the researchers work with bio-hazards that require constant volume exhaust.

U of I Facility Standards

The U of I Facility Standards established the use of constant air volume with a total enthalpy wheel energy recovery system for all laboratory systems as the baseline system.  Variances from this baseline for alternative energy recovery systems and variable air volume systems can be granted if a thorough engineering study can show that installation of a total enthalpy wheel is not feasible (e.g., significant infrastructure modifications would be required, etc.) or the life cycle analysis makes the alternative a better solution (e.g., higher return on investment, reduced maintenance, etc.). 

VAV systems were not included in the baseline due to past experience on campus.  VAV CFHs required voluntary actions by their users in order to conserve energy.  Attempts in the past to change user behaviors to shut CFH sashes were unsuccessful.  Controls and sensors required ideal conditions to work which often are not present in campus buildings and they required continuous maintenance and calibration.

VAV systems are also excluded from use based on the types of hazards associated with some buildings and CFHs.  The U of I Facility Standards state:  “Energy conservation by reduction of exhaust airflow and associated outdoor air makeup airflow shall not be applied to laboratories where regulated carcinogens, highly toxic chemicals, nanomaterials of unknown toxicity, or other compounds of unknown (and suspect) toxicity are used.”

Recommendations       

The current process that has been utilized over the past few years of completing a thorough engineering evaluation of each building and associated mechanical system should be maintained.  This approach allows U of I to examine all factors in determining the best solution rather than applying a one-size-fits-all approach as presented in the iCAP.  All energy reduction solutions should be considered using a life cycle analysis.  Exhaust flow reduction should be broadened beyond CFHs to include the entire building exhaust. 

Project Team

  • Project Leader:

    Maureen Banks

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