Heat Recovery: A Necessity
By:  Drew Godfrey, P.E., LEED AP

Heat recovery is not something that is done because an engineer simply doesn't want to waste energy any longer.  LEED and High Performance Buildings, as required by the new ASHRAE Standard 189.1, and other federal and state requirements (Mississippi Senate Bill 3007 requires a 30% decrease in energy consumption beyond ASHRAE Standard 90.1) are making it a necessity to use heat recovery.  A simple method commonly used is the total energy wheel recovering heat from exhaust air streams.  However, much attention should be given to the Cromer cycle which reduces the latent load and helping a cooling coil remove much more moisture by use of desiccants and to recovering heat from compressors and heat of compression generated by cooling equipment.  This article will attempt to discuss each of these briefly in an effort to illustrate their validity.

Exhaust Air Heat Recovery:   Recovering heat from exhaust air streams are being used so frequently now that they are considered the norm rather then the exception.  Whether the technology of choice is a Silica Gel Desiccant, an Alumina Substrate Desiccant or a 3 Angstrom Desiccant, the use of Energy Recovery Ventilators (ERV) has gained popularity over the last 15 years.  Prior to this, energy recovery from exhaust air streams were limited to the use of run-around loop heat recovery which used water coils in each air stream with interconnecting piping, pumps and glycol solution.  These type systems were often eliminated or discarded due to economics.  It should be said that these type systems should be given due consideration in exhaust systems that are considered hazardous and/or where fear of cross contamination is a concern.

Engineers have used and continue to use ERVs in today's systems due to their obligations to the owner and their desire not to waste energy more so than any code or their attention to ASHRAE Standard 90.1.  However, today's projects necessitate the use of these systems because equipment energy efficiency improvements alone cannot achieve the degree of energy efficiency required by owners, standards and/or codes.

Exhaust airstream heat recovery is applied simply by routing the exhaust air duct systems parallel and in close proximity to the outside air stream in order that warm air, in winter, or cool air, in summer, can be used to pre-treat or pre-condition the incoming outside air.  Typically, under design ambient conditions, the amount of pre-conditioning that can be achieved is approximately 2,700 Btuh (based on 95/76 ambient conditions and 80/67 conditions from the ERV) of OA, in the cooling season and 3,700 Btuh (based on 21F ambient and 70F exhaust air temp.) in the heating season.  This can be significant coil load reductions in any system but especially in situations where the major cooling systems are unitary applications where very high percentage of OA is required such as school applications.  Under these scenarios, it is recommended that the ERV is coupled to the conventional systems.  Also, due to the fact that these systems are usually selected based on the worst case scenario with respect to design conditions, part load operation must be addressed as this part load operation can and will result in high space humidity conditions.  One way to accomplish this might be to dehumidify the OA via a cooling coil in the ERV and to specify unitary systems that can vary zone level airflow and capacity.

The Cromer Cycle:  The Cromer Cycle is a not so well know desiccant system that has an extremely high capability of reducing system latent load.  In this system, the desiccant wheel is placed in series with the return airstream, after the OA is mixed, and the cooling coil at the system level, as opposed to preconditioning location.  This desiccant system works as a regenerative desiccant wheel where the wheel transfers moisture from a high moisture content airstream (post cooling coil position) to the regenerative position (just after the mixing box).  This scenario increases the moisture removal capability of the cooling coil and thus decreases the leaving air dew point.  Lower dew points to 50F can be realized with standard 55F LAT and actually allowing increasing the chilled water temperatures from the standard 42F (leaving the chiller) to as high as 46F.  This rise in temperature will result in higher efficiency at the chilled water generation level.  This efficiency can also be seen if a direct expansion (D/X) cooling coil is used because a warmer suction temperature can be used similar to the warmer chilled water temperature as discussed.  The ability to achieve these lower dew points while allowing the chilled water system to work easier, can be an incredible energy reduction design application.  If you combine all of the energy reducing characteristics of the system described above with the fact that a drier space will inevitably result in space setpoint increase, we can see that the proverbial "domino effect" might be realized.  Also, we could conclude that a drier space results is a healthier space for occupants due to the reduced risk of mold growth and is a more comfortable space due to the dryness.  Reference article in August 2009 edition of the ASHRAE Journal.

Chiller Energy Recovery:  One type of energy recovery that is not given much thought is that of recovering energy from motor heat and/or heat of compression from chillers.  As engineers, we all know that a large amount of heat is rejected to the atmosphere either by the air-cooled condenser coil, in air-cooled equipment, or by cooling towers, in water-cooled equipment, all while a demand for heat probably exists for domestic water heating requirements or, most likely, for system reheat requirements created by increased minimum airflow rates that are created by increased OA requirements.  The use of de-superheaters in the refrigerant systems of air-cooled chilled water systems should be resurrected in order to achieve the recovery of heat available.  However, for the purpose of this article, the same heat recovery used in a water-cooled system could be used in an air-cooled application.  The system intended for discussion is a sidestream chiller application.  A water-cooled chiller can be applied in a sidestream situation where the water-cooled chiller is used to pre-cool the chilled water while the heat generated from this chiller is removed via the condenser bundle.  Using the refrigerants available today can result in water temperatures up to 140F.  Due to heat of compression, the heat available to be recovered from the condenser can be as high as 30% greater than the Btuh of cooling capacity.  In this scenario, the chiller is controlled by leaving condenser water temperature and therefore the chiller controller must be able to accommodate this change from leaving chilled water temperature, when in the cooling mode, to leaving condenser water temperature, when in the heating mode.  Selection and sizing of these heat recovery chillers should be done by comparing the heating and cooling loads of the building.  Theoretically, there is a point, at some ambient condition, where the heat load and the cooling load are identical.  This is the point at which the chiller should be sized/selected.  This evaluation usually requires at least 12 months of data, for an existing facility, or can be estimated using a load profile of an energy analysis program.

Conclusion:  Energy recovery systems once ignored due to the lack of economic feasibility should be considered in every application.  No amount of energy recovery is too small.  Energy recovery must be used more and more in today's HVAC systems if the industry is to realize the degree of low energy usage desired.  More than the desire of low energy usage, mandates by federal and state codes are increasing as well as that of organizations such as the United States Green Building Council (USGBC) and ASHRAE Standard 90.1.  However, the oblivious consideration of saving energy without regard to the economic impact is counterintuitive to conventional design considerations and to those of us who believe in the definition of "Is it worth it?",  but there are design applications that have merit in both energy to be recovered and in payback on invested capitol.  Our job as engineers is becoming more challenging and will require more time to evaluate these systems,  However, our engineering community both locally and nationally has the brightest minds ever, in my opinion.  This is a very exciting time in the HVAC industry and I believe that we are up to the challenge.

Lessons Learned

When selecting fans (exhaust fans, fans for AHU's) make sure that the BHP is selected with drive losses. Drive losses occur and will drastically effect the size of the motor horspower selected for the fan. Coordinate this with your fan representative and check catalogue data.
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ASRE: The Early Years

The American Society of Refrigerating Engineers (ASRE) was founded in 1904 as a national institution.

However, by 1906 when its memebership totaled 146, it already had members from Canada, England, India, the Argentine Republic, Australia and New Zealson.

--Source: ASHRAE Journal
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