The owners of 1845 Walnut Street in Philadelphia were presented with a challenge not uncommon to many building owners. They had a 32-yr-old building with a cooling plant the same age. The chillers were still in good condition. However, the bigger concern was the CFC refrigerant they used. As further motivation to re-examine the system, the local steam utility had a rebate program that would help with the initial cost of new equipment.

The building, owned by Frankel Enterprises, is the only office tower located on the prestigious Rittenhouse Square in the Center City section of Philadelphia. It is a 352,000-sq-ft building with 25 floors. The first five floors are comprised of a parking garage with mechanical space in the center of each floor. Everything above the fifth floor is professional office space.

The building has been on the local district steam system, owned by Trigen-Philadelphia Energy Corporation, since its original construction. Through heat exchangers, steam heats hot water for heating and for generating domestic hot water. The cooling system had consisted of two 600-ton electric centrifugal chillers located on the fifth floor of the parking garage with the cooling towers located on the roof above the 25th floor.

In January 1999, Frankel Enterprises asked two prominent chiller manufacturers to work with Trigen to propose the replacement of the chillers and cooling towers. However, the company required that at least one of the chillers be steam-turbine driven or use steam-absorption technology.

Frankel Enterprises had four primary reasons for going in this direction. First, Pennsylvania had recently started electric deregulation, and no one could really predict where electric prices were going. Consequently, they didn't want to rely solely on electric chillers to cool the building for the next 25 to 30 years.

Secondly, the owners were looking for the lowest possible first cost. In exchange for a long-term agreement, Trigen offered an attractive rebate program that could pay for a portion of the new steam cooling equipment.

Thirdly, Trigen also had an attractive rate for cooling steam consumption that was competitive with the alternative utility.

Finally, Trigen also offered to provide a 150-kW backpressure turbine generator as part of the rebate package.

Hybrid Generator Reduces Electricity Use

From the various options proposed, Frankel Enterprises decided to purchase a hybrid cooling plant from York International (York, PA) and entered into a 20-yr agreement with Trigen. The new cooling plant would consist of a 600-ton single-stage absorption chiller, a 600-ton electric centrifugal chiller (with new cooling towers), and a backpressure turbine generator.

The equipment was designed to work in the following manner: Steam street pressure was 170 psig, and the building operating pressure was 15 psig. The steam would enter the fourth floor mechanical room at the street pressure of 170 and immediately go through the backpressure turbine generator. The generator would reduce the steam pressure from 170 psig to 15 psig and at the same time would generate 150 kW of electricity to the building motor control center. In the summertime, steam would travel to the 600-ton absorption chiller, that operates at 15 psig. The condensate from the absorption chiller would go to a heat exchanger to preheat the domestic hot water for the building.

In the winter, the steam would still go through the backpressure turbine generator but would be diverted from the absorption chiller to the heat exchangers for the hot water heating system. The condensate from the hot water heat exchangers would also go to the heat exchanger for preheating the domestic hot water.

The new design allows the building to use one unit of steam three times prior to sending it to drain. In the summer, electricity, chilled water, and domestic hot water are generated. In the winter, electricity, heating hot water, and domestic hot water are generated.

Spreading the Weight

Some interesting issues arose in the construction phase. The most significant was the weight of the chillers. The mechanical room had two 600-ton electric centrifugal chillers. The new design replaced one electric machine with a significantly heavier single-stage absorption machine.

To add to the problem, no structural drawings were available from the original construction of the building. The engineer and the owner devised a design that incorporated additional steel supports within the mechanical space to spread the weight of the absorption chiller evenly across the floor joists.


The summer of 2000 was the first cooling season for the new chillers. The absorption chiller was not operational until the end of June 2000. Consequently, the 600-ton electric chiller was used much more during that summer than it will be in future years. However, even with the electric chiller carrying the load for the first part of the summer, the effect the steam absorption chiller had on the electric bills was dramatic. Electric consumption decreased by approximately 200,000 kWh, and peak demand was reduced by 400 kW.

By using absorption cooling, Frankel Enterprises lowered the peak electric demand of the building by 400 kW and reduced electric consumption as well. The backpressure turbine generator further lowered that peak electric demand by 150 kW.

Additionally, the condensate from the steam system preheats the domestic hot water system on a year-round basis, adding to the overall efficiency. The owner has begun to see the benefits of using a hybrid cooling plant. "It's clear that we saved money on our electrical costs," says Ed White, building manager for Frankel Enterprises.

"We're hoping the savings will be even bigger this coming year. We should be able to achieve this because we used the absorption chiller more this past summer, and hopefully we won't see the spikes in the fuel market like last winter." ES