Big Dollar Steam Savings Achieved at Duke's Laundry Facility
- By Bob Friedman, Jerry Black
- August 1st, 1999
Many campuses use steam as a primary energy source for equipment operation. Therefore, it should come as no surprise that steam line problems are common. Duke University’s Medical Center Laundry, operated by steam, is a prime example. The 25,915-sq.-ft. facility is located adjacent to Duke’s East Campus. The laundry receives all linens from the hospital, amounting to 5.5 million pounds per year. The facility has 58 employees and is in operation eight hours per day, 6.5 days per week.
The steam-using equipment includes washers, dryers, ironers and presses. Steam is supplied to the laundry from the Duke University Heating Plant through a six-in., high-pressure (125 psi) line. Condensate is returned to the central plant.
In recent years, laundry steam consumption has averaged about 20 million pounds per year. Unaccountably, in fiscal year 1996/97, consumption soared to more than 64 million pounds. Duke Facilities Management Department (FMD) was asked to investigate possible steam equipment problems at the laundry.
FMD employee Gary Teater was sent to look at the laundry steam system. His examination revealed three general areas of problems, which led to a renovation project in September and October 1997. Total project cost was $107,213. Steam consumption immediately fell. Consumption for fiscal year 1997/98 was less than 10.5 million pounds. This was more than 54 million pounds less than was used the previous year, representing a one-year cost savings of $405,185.
Here are the problems Teater found and how they were resolved.
Flash Tank Problem Solved
Of greatest concern to Teater was the flash tank located in the mechanical room above the condensate return pump. Under normal conditions, steam gives up its latent heat and condenses in a coil or other device. As the steam condenses, liquid condensate is formed at the same pressure and temperature as the steam vapor. The condensate is then discharged through a trap to an area of lower pressure. Depending on temperature and pressure conditions, some of the condensate returns to a vapor state, known as flash steam.
It is important to remove the flash steam from the condensate. Otherwise, pressure will build and the removal of condensate from the system will be impeded. The purpose of the flash tank is to remove the flash steam from the condensate. The condensate is then pumped back to the central plant to be reheated and converted to steam again. The flash steam is either vented to atmosphere or piped to a low-pressure main to be used elsewhere.
In the case of the laundry, the flash tank has a two-inch Spence back pressure regulator valve maintaining a positive pressure of 10 psi at all times. Back pressure regulators are necessary when flash steam is recycled to a low-pressure application, which was not done at Duke’s facility. Instead, the excessive pressure caused several problems.
1. Pressurized condensate caused temperature-controlled equipment to run in a stall mode. This occurred when the pressure of the condensate was greater than that of the steam. The condensate actually backed up into the heating equipment itself and reduced the available area for heat transfer. In addition, it caused water hammer, erratic temperature control and corrosion of metal surfaces due to the presence of carbonic acid that forms when air is present in a steam system.
2. The existing preheater also failed due to pressurization of the flash tank. Steam was supplied at five psi inlet pressure, but the 10 psi back pressure kept the device flooded with condensate and unable to heat.
3. The Kent condensate meter had flash steam going through it instead of condensate. This caused meter failure and inaccurate readings. The pressure from the flash tank was sufficient to turn the condensate meter even when the condensate receiver pumps were not discharging any condensate. This was a factor in the abnormally high steam consumption readings.
4. The new Cemline heater installed in the mechanical room at the loading dock experienced severe water hammer. This occurred when condensate was not properly removed from the steam system. The liquid condensate was pushed through steam lines by the force of the steam and caused noise and vibration where the steam lines changed direction or diameter. The force of the water under these circumstances can even cause damage or leaks, as was the case with the heater, which developed a leaking head gasket.
In order to correct the problems associated with the pressurized flash tank, the system was changed from a closed loop system to a vented system. The back pressure regulator, flash tank, preheater and electric condensate pumps and receiver were removed. The equipment was replaced with a Spirax Sarco 3X2 PRF ASME Code pressure-powered pump and vent condenser package. This new equipment solved several problems. Because it was vented to atmosphere, the condensate was no longer pressurized. Condensate was free to flow by gravity to the new receiver. Also, any flash steam produced was cycled through the vent condenser where the heat energy was captured and used for water preheating.
Condensate System Repiped
The second problem area identified by Teater was the condensate system on the main floor. The laundry was designed with steam and condensate lines overhead. This worked fine for the steam supply lines. But most of the steam-fed equipment on the floor was trapped at floor level with condensate lines running up to the ceiling height for return to the mechanical room. This was not good steam system design. Some steam traps were capable of lifting most of the condensate when they cycled, but this was inadequate for proper system operation.
Every morning at start-up, the condensate risers from each piece of equipment were full of cold condensate left over from the previous day’s operation. As each piece of equipment heated up and tried to cycle the condensate, the trap was forced to work against the pressure of the accumulated condensate. This caused flooding, water hammer and flashing. Also, when condensate was left in pipes and equipment, it caused carbonic corrosion and led to premature deterioration of traps, check valves, pipe and heater exchanger tube bundles. When condensate was properly removed, better heat transfer occurred and premature equipment failure was minimized.
To resolve this problem, Teater recommended repiping the condensate system to eliminate vertical risers, avoiding the lift problem. To accomplish this, it was necessary to cut a pit in the main operating floor and install a three-in. PPC pump package. Trenches were also cut in the floor to enable equipment condensate lines to drain the pump by gravity flow. From the pit, all condensate was then pumped back into a main header near the mechanical room. It was further recommended by Teater that the condensate drain pit be centrally located in a space accessible from all directions in order to facilitate the addition of new equipment.
The installation of this pump and the rerouting of condensate lines caused an immediate improvement in the operation of the laundry equipment, especially the ironers. Before, cold spots in the ironers left wrinkles on sheets and other flat linens. These cold spots were caused by the presence of condensate in sections of the heating coil that was displacing the steam. The efficient removal of condensate allowed the ironer coils to heat fully and do a uniform ironing job. Teater also recommended the replacement of existing buck traps on the ironers with float-thermostatic traps. These have an internal air vent, which helps remove air from lines on start-up. This allows a quicker and more efficient heat transfer.
Steam Metering System Installed
The third area of concern was that of metering steam usage. In the past, steam consumption was measured by condensate flow. This did not capture all steam use because there are several pieces of equipment in the laundry that do not recover condensate. The tunnel washer, for instance, heats water by the direct injection of steam and does not recover condensate. To measure total consumption, Teater recommended installation of a steam flow meter on the incoming high-pressure line itself. In addition, a cycle counter was installed on the new pressure-powered condensate pump. This provided a backup reading of steam consumption to use for comparison.
Jerry Black is director of the facilities management department at Duke University in Durham, N.C. Bob Friedman has responsibility for energy management in the same department.