Industrial Utility Efficiency

New York City Transit Authority Reduces Energy Use with Compressed Air Heat Recovery System

When the New York City Transit Authority (NYCT) set out to comply with local regulations calling for reductions in energy usage, it leveraged new air compressors for use in transit bus maintenance and repair – and took things to another level by recovering air compressor waste heat to provide hot potable water for the bus depot. The air compressor and heat recovery system, installed in spring 2017, is on its way to helping NYCT achieve the best energy savings possible.


Local Laws Require 18% Reduction in Energy Use in Large Buildings by 2025

In 2009, New York City began to implement a series of local laws that requires building owners to reduce energy usage 18% by 2025. The local laws (84-88) focus on building upgrades in lighting, HVAC, Energy Star Appliances, and other updates to buildings. The laws cover commercial, residential, municipal and city government buildings larger than 25,000 square feet.

In addition to greatly tightening building codes for the city, the laws require the establishment of baselines for building energy usage. The city of New York decided to take a leadership role in this effort and made no exception for buildings it operates.

One building that qualifies under the laws is the Mass Transit Authority Building on 4805 Grand Ave. Maspeth, NY.  At the bus depot, NYCT wanted to replace air compressors used for maintenance and repair. When tackling the project, NYCT wanted a system that would give it the best impact for energy efficiency and energy recovery so they could apply the energy reduction to the new legal requirements. It also wanted to take advantage of financial opportunities available through the new laws when procuring equipment.

NYCT retained the services of the New York Power Authority (NYPA), who then hired Wendel, an engineering construction management firm, to specify the job and oversee construction of the project. Wendel hired SRW Engineering for the detailed design. Iacono Inc., Bohemia, NY, was selected to supply the air compressors and an energy recovery system.


Air Compressors Designed for Energy Recovery

The specified system includes three Atlas Copco GA26VSD+ air compressors. The load dictated that one air compressor would likely run continually, while the second unit would be required some of the time, and the third air compressor would be used as needed and serve as a backup.

The air compressors are oil-flooded and each is equipped with a Variable Frequency Drives (VFD) to control/match capacity requirements. Wendel suggested the energy recovered from the air compressors be used to heat potable water to reduce the use of the existing oil fired boiler. As such, the machines have an added energy recovery heat exchanger that allows for heat transfer from the lubricating oil to an energy recovery fluid. Any heat not recovered is sent to an air-cooled heat exchanger on the air compressor. The heated water is used for general cleaning, restrooms, and showers. The boiler was left in place should it be needed for hot water if the demand exceeds the capabilities of the heat recovery unit.

Iacono, Inc. partnered with HydroThrift Corporation to design a heat recovery unit capable of heating the potable water from 110 oF to 120 oF, at a given flow of 20 gallons per minute (gpm) based on significant analyses and discussions. The conditions needed to represent the best compromise between the actual normal air compressor running mix and realistic performance for heat recovery.

The high temperature of 120 oF was chosen for personnel safety and continual heat transfer at the upper limit of the potable water temperature, plus the upper limit of the coolant temperature from the compressor, and allowing the approach for the heat exchanger.    Water temperatures below normal would take all of the heat available from the air compressors. At the upper limit, the heat recovery system would control the temperature and send any unneeded heat back to the air compressors.

Once the conditions were set, the amount of heat to be transferred to the water could be calculated.  The calculated heat is as follows: 20 gpm X (120 oF – 110 oF) X 500 =100,000 Btu/hr. This represents the nominal heat that would be deferred from the oil-fired boiler at the higher temperature limits. The actual amount of heat could be more if the entering temperature of the water is below 110 oF and/or the number of air compressors and load are higher.

Hydrothrift Heat Recovery System

The HydroThrift Heat Recovery System at the Mass Transit Authority Building in Maspeth, NY.


Heat Recovery System Designed for Optimal Results

Once HydroThrift knew how much heat was to be transferred to the potable water, it was able to determine how this would be accomplished with the air compressors. Using air compressor data sheets, the team knew it could recover as much as 31.1 horsepower from each air compressor at full load. The system was designed so that all of the heat could be recovered from all of the air compressors as long as the outlet potable water temperature was at or below the design maximum of 120 oF. 

An Alfa Laval double-wall stainless steel heat exchanger was selected for the heat recovery system. The coolant flow was engineered for the best flow of 30% industrial inhibited propylene glycol and water mix with the proper temperature differential for efficient heat transfer from the air compressors. The water side was selected from the driving set of conditions at 110 oF in and 120 oF out at 20 gpm of potable water.

A double-wall heat exchanger was selected to protect the potable water from cross contamination from the PG/water coolant mix. The flow of 20 gpm of potable water was provided by the end user. The closed-loop glycol and water side of the heat exchanger pump was selected by determining the total pressure loss in the closed circuit, including the field piping, compressor heat exchangers, the heat recovery heat exchanger pressure loss, and the loss through all of the piping on the packaged skid.

The pump and control skid also included a motorized three-way valve that could regulate the flow through the heat recovery heat exchanger and bypass around it. The valve was driven by the maximum required outlet for hot potable water and set to 120 oF. As the hot potable achieves the 120 oF outlet temperature, the bypass valve dumps the excess heat back to the air compressors to be dissipated into the ambient air from the air compressor’s air-cooled heat exchanger. 


System Up and Running

As part of the installation process, HydroThrift traveled to the site and performed a post start-up inspection. There, the team adjusted the balancing valve to mirror the flow through the heat recovery heat exchanger.

The air compressor and heat recovery system began operating continually beginning in spring 2017. Since then, NYCT has capitalized on high-efficiency air compressors that not only provide the required amount of compressed air to match the need, but also deliver the added benefit of heat recovery to the bus depot’s potable hot water system.

Given the need for NYCT to establish a baseline and begin to reduce energy at the Mass Transit Authority Building, the upgrades to the compressed air system in combination with the heat recovery system will go a long way to allowing the building to achieve the required reductions in energy usage and a reduced carbon footprint.

For more information, please contact Bruce Williams, Regional Sales Manager, HydroThrift Corporation, tel: 330-236-2023, or visit

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