Right-Sizing Heat Exchanger System Spells Comfort & Simplicity
The first priority for facility managers today is reducing operating costs, particularly for energy and maintenance. But when a solution is poorly designed, it often causes more problems than it fixes.
Allentown, PA, USA
To reduce energy and maintenance costs
A turnkey design and installation of a larger heat exchanger and connecting piping
Improving steam system efficiency and reduced energy and maintenance costs
That was the situation at a Level 1 trauma center in Allentown, Pennsylvania, after it tried to reduce energy costs in 2007 by replacing two boilers in its HVAC reheat system with a steam heat exchanger. The system never functioned properly and, three years later, an engineering audit team from Spirax Sarco found that the local contractor who did the installation had undersized the heat exchanger and associated piping.
Parallel Systems, Rising Costs
During the winter, the heat exchanger could not provide enough hot water to heat the area. The maintenance staff was often called in late at night to fire the hot water boilers, which are powered by natural gas, and shut down the heat exchanger to provide the needed heat. When outside temperatures warmed again, workers would have to shut down the boilers and then spend a great deal of time adjusting the water valves to compensate for pressure drops and cavitations in the system caused by the undersized 3-inch piping.
Consequently, the hospital was operating two parallel systems, neither of which worked optimally, increasing costs for both energy and maintenance.
Audit Findings Pinpoint Design Flaws
The boiler system with two sets of TACO pumps circulated water heated to 180 degrees and pressurized to 30 psig through the air handling units, fan coils and reheat coils. The system assured that one boiler was always online with the second on standby to provide 100 percent redundancy.
When the original heat exchanger and 3-inch piping were installed, the heat transfer capacity (3 MMBtu/hr) was only half of the design load for one boiler (6.28 MMBtu/hr) and the design flow was only 300 gpm, about half of the combined design flow of the pumps (586 gpm). The pumps, capable of delivering between 36 psig and 41 psig, were unable to provide the 70 psig needed to compensate for frictional losses through system components and from the undersized piping.
To reduce the pressure drops through the heat exchanger and associated piping to enable sufficient hot water flow to be heated and circulated through the loops, the Spirax Sarco engineering team recommended a larger heat exchanger and connecting piping be installed.
The new components were sized to handle the water flow of 586 gpm and to transfer 6.28 MMBtu/hr of heat, matching the combined hydraulic capacity of the pumps and the design heat load of one boiler.
If at some point a higher discharge pressure is needed, the team recommended the pumps be fitted with 8.4-inch impellers.
A Turnkey Solution
To make the process of re-engineering the system easier for the hospital’s small maintenance staff, Spirax Sarco provided turnkey design and installation, including project management.
This comprehensive solution included the larger heat exchanger and 8-inch piping, a new steam main PRV, moisture
separators and proper drip traps. It also allowed maintenance personnel to operate either the boilers or heat exchanger
without valve adjustments.
More Efficiencies Ahead
The success of this installation in improving system efficiency and reducing energy and maintenance costs has led to two new turnkey assignments at this hospital, including identifying the boiler room piping and creating a P&ID of all systems with proper identification. Spirax Sarco is also in the process of testing all steam traps in the hospital and replacing the failed traps.