Minnesota is one of the coldest places of the continental United States. The complex centralized utility plant at Minnesota State University, Mankato (MNSU), was designed to address at high energy demands in a cold state. Located in Mankato, in the Minnesota River Valley of south-central is Minnesota, MNSU currently supports more than 15,000 students. There are 16 academic buildings, some of which are interconnected, and three dormitory complexes on the campus. The Utility Plant generates steam and chilled water that are distributed through a network of pipes and tunnels to the buildings on campus.
Central Plant History
Over the last two decades, a variety of components were added to the central plant: four boilers, one steam turbine-driven generator unit, three chillers, two emergency power diesel generators, three standby diesel generators, and several pumps and compressors.
The decision to install a central chilling system is based on factors associated with maintenance, efficiency, and lifespan. Each buildings equipped with an air-conditioning unit also require an individual cooling tower to reject the heat to the environment, and the water in the towers must be chemically tested on a daily basis. The establishment of a central unit shifted the inspection and maintenance work from many individual units to just a handful at the main plant. Furthermore, by 1992 many of the building units were approaching the end of their lifespan. As any good thermal design engineer will tell you, the best time to upgrade is when you have to purchase new equipment, so this was an economically rational time to make the changeover.costing $145,000 was added, and during the final stage, in 1999, a 1000-ton Trane chiller was installed.
Steam Heating of Campus
Whether the outside temperature is -10°F or 95°F, heating air or water is always required somewhere on the MNSU campus. Steam is used to heat both the campus buildings and water. The steam system used currently consists of four boilers that are located in the facilities plant at the northwest corner of campus. These four boilers provide all of the heating required on campus year round. Each of the four boilers is a "D" type water tube boiler. Each water tube boiler, which includes two drums connected by steam generating tubes, produces steam at 150 psig. Two of the boilers are rated for 35,000 lb/hr, the third is rated for 75,000 lb/hr, and the fourth boiler was originally rated for up to 90,000 lb/hr.
If the campus were heated using furnace typically used in a residential home, more than 600 units would be needed to satisfy the heating requirements. And if the campus were cooled with a typical window air conditioner more than 1000 air conditioning units would be needed to maintain the required temperature level.
Water Cooling of Campus
In addition to the four boilers, the utility plant also houses three chillers that provide air conditioning indirectly for most of the campus buildings. In this system air-handling units use water to air heat exchangers to cool the air down to the specified temperature. The chilled water can then be distributed to other locations.direction of the refrigerant as it enters the compressor. These adjustable inlet guide vanes pre-swirl the refrigerant to increase or decrease the capacity of the impeller to take in refrigerant, thus increasing or decreasing the refrigerant flow rate.
Used chilled water returns to the plant in one main pipe. Once it reaches the chiller room, the water distributed among the three different chillers. Each of the chillers can produce chilled water at 42°F. Once the chilled water is used within in the air handlers in each building, the water returns to the chilling facility at roughly 54°F. The water chilled again and sent back out on this continuous loop.
The ideal water temperature for this cycle is 45°F going out to the buildings and 55°F coming back. These temperatures are low enough to enable the formation of condensation on the outside of the pumps. Therefore, all of the pumps that handle chilled water have to be insulated to prevent condensate dripping onto the floor from their outer casings and causing damage and/or unsafe conditions.decides whether or not to turn on the third unit. The campus population shrinks significantly at 4 pm when most classes are over. Without the third unit, building temperatures may be slightly higher, but there is a corresponding savings in electricity and operating costs.
In 1995, the university decided to look into constructing a cogeneration unit that would provide an additional 434 kW to campus using excess heat from the boilers. An external engineering firm researched the benefits of installing a cogeneration unit by filed a report that described the amount to be spent on the unit, installation, additional fuel, and the estimated cost savings. After the university approved the plan, the engineering firm submitted a bid data sheet with the plant specifications for the unit. The university accepted a bid that featured a Coppus model RLHB24 single stage steam turbine and a Reliance Frame E5010S generator with an expected cost for both of $453,000 (in 1995 dollars).
The cogeneration cycle began with steam from the existing boilers at 150 psig and 366°F. This steam was fed into the Coppus steam turbine where energy was extracted and converted to mechanical power in the form of a rotating shaft. The shaft was directly coupled to an electrical generator, which produced the supplemental electricity for the university. In the original design, the steam exited the turbine at 50 psig and 297°F and distributed to heat campus buildings.
Ambient temperatures on campus can vary drastically, from 100°F+ during the summer to lower than -20°F in the winter. Building occupancy also changes based on the academic schedule. Because of this variability, steam requirements change throughout the year. During the summer, there is excess steam capacity because of the low demand for heat, while during winter more steam is produced to meet the high demand. Large amounts of hot water and heat are also needed to compensate for the dropping temperature. To handle these changes in demand, the cogeneration system design included a control valve to regulate the steam flow through the turbine.
However, there was an oversight in the design of the cogeneration system. The way that the system was designed using a back pressure turbine design, this means that the turbine was used to reduce the pressure of the steam while getting power from the steam going through the turbine. This design caused the originally 150 psi steam to be reduced to 50 psi before being sent to the buildings for heating. The flaw in this design was that the piping network outside of the central facility was not considered in the initial design. What was found was that due to the much lower steam pressure and current sizing of the pipe network, there was not enough steam flow. The solution to this problem was to install substantially larger pipes and valves throughout campus, but the cost of this solution was far more than what would be saved using the cogeneration system. Because of this, the system is no longer in use and has been dismantled.
Three additional generators, diesel Caterpillar stand-by generators, were installed in 2005 to provide stand-by power for the university's full electrical load. The installation of these generators qualified the university for a reduced utility rate on electricity. A curtailment permits the electric company to notify the university that the university electrical load will be removed from the electric company's local grid
The installed system, which includes three 3516B LOW BSFC 2250 kW Caterpillar generators, is monitored and controlled for the university by an external company that is responsible for turning the system on and off, and for monitoring temperatures, pressures, and various other specifications while the generator is operating. The campus facilities staff rarely needs to monitor this system.
The stand-by generator building that houses the generators was designed to hold a total of four generators. However, the three current generators provide enough power to meet the current energy needs of the campus. The initial construction allows for the installation of additional unit to meet the demands of the expanding campus.
When the engines are not running, they are closed, effectively sealing the building. However, starting the engines activates motors that open the louvers to allow intake air into the building. The exhaust is channeled to the roof of the building. The engines are tested regularly, even during the winter months. If it is snowing outside, snow is pulled in with the air creating a virtual blizzard inside the building!
Each generator has its own 275 gallon day tank storing diesel fuel.
Personnel in the Plant
The equipment is monitored from the utility plant office. The Johnson Controls Metasys system allows facilities staff to view data generated from various facilities on the campus in the main office to check for equipment malfunctioning. Before this installation in 1989, all steam line readings were collected on location. Former Chief Engineer Berger explained that he spent half of his workday walking around the campus taking the readings. By 1989, however, the majority of the readings were conducted by computers. Today most of the readings can be accessed remotely on a computer. If something appears to be outside the usual range utility personnel are sent to investigate the problem. Despite the new construction and addition of multiple boilers over the years, fewer people are required to operate the utility plant.
|Information current as of:
|Click here to see a line drawing of the MNSU plant.|
|Water tube boilers are the most common type of boilers because of their fast steam generation process and efficiency.|
|Want to know more about adjustable inlet guide vanes? Click here.|
|Line drawing showing the refrigeration system at MNSU.|
|A ton of refrigeration is the amount of thermal energy required to melt a short ton (2,000lb) of ice.|
|Want to learn about a greener, less expensive cogeneration plant? Read this ASME article.|
|Want to learn more about genset maintenance? Cummins discusses this here.|
|Louvers are a series of adjustable slats that are used for admitting light and air into a structure while shutting out rain and noise.|
|Video Interview with Dudley Berger|
|Video interview with Steve Ardolf|
|Interview with Paul Corcoran|
|Interview with Jeff Rendler|