Minnesota State University, Mankato

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.

Facilities Plant at MNSU
Facilities plant at MNSU [129].


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.

Boiler room at MNSU
Boiler room at MNSU [129].
Until 1992, the MNSU campus was air-conditioned by individual building units. Each unit operated independently based on temperature set points for a particular building or air-conditioning zone. However, in 1992, the university decided to install the first stage of a central chilling system. This system consolidated the cooling functions in one location by replacing individual units with several larger chillers.

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.

McQuay chiller at MNSU
McQuay chiller at MNSU [129].
The central chilling system was completed by 1999 in three stages. Each stage included the addition of a chiller. The first, installed in 1994, was a 1000-ton McQuay chiller. In 1998, during the second stage, a 1200-ton Trane chiller 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.

Boiler at MNSU
Boiler at MNSU [129].
If you think the paperwork you have to fill out as a student is cumbersome, consider this: The permit acquired by MNSU allows for steam production of up to a total of 100 million Btu/hr (MBtuH). If the fourth boiler were to run at full capacity the campus would exceed its permit limit. However, running the boiler at only 80,000 lb/hr satisfies both the campus demand and the permit requirements. Therefore, the fourth boiler was re-rated. De-rating the boiler by physically modifying the spud burners (a device used to draw fuel into furnace) was an easier solution than applying for a new permit from the state.

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.

Centrifugal Chiller Compressor at MNSU
Centrifugal chiller compressor at MNSU [129].
While all of the chillers on the MNSU campus use centrifugal compressors, the two Trane chillers operate quite differently from the McQuay chiller. The biggest differences between the two brands are the type of refrigerants used and the refrigerant compression process employed. The Trane chillers use constant speed compressors and the McQuay chiller uses a variable speed compressor. The McQuay chiller controls the speed of its compressor to change the load. Trane chillers, on the other hand, employ guide vanes to control the 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.

Piping network at MNSU
Piping network at MNSU [129].
The chilled water system has one main supply line and one main return line which connect to each of the campus buildings. To operate effectively, the supply/return lines must have the same pressure for all of the buildings in the system. The chilled water that exits each chiller is routed through the main pipe to two 200 hp pumps. These pumps provide the pressure needed to supply the chilled water to the campus buildings. As the chilled water reaches the buildings it is sent through air handlers to cool the air supply. Once used, the chilled water is sent back to the utility plant through return pipes.

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.

Trane chiller at MNSU
Trane chiller at MNSU [129].
The total capacity of the central chilling system is 3200 tons. However, the chillers don’t usually operate all at the same time. One chiller is started and run until it is at 85% capacity. At that point a second chiller is turned on. In most instances, the university meets its cooling needs using only two chillers; but in the middle of summer when the temperature and humidity are very high, all three units are needed. In some cases, the third chiller is not needed until around 3 pm, when the facilities staff 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.

Cogeneration

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.

Electrical Generation

KatoLight Generator at MNSU
KatoLight generator at MNSU [129].
The university currently purchases bulk electricity from a local utility company. However, the utility plant is responsible for emergency and stand-by power generation. Emergency generators must have the capacity to generate power within 10 seconds of a power failure. During an emergency, this power is produced by two model D600FRX4T1 Katolight diesel generators. Running at full capacity, a Katolight diesel produces 600 kW at 60 Hz, which is used to power emergency lights, exit signs, and fire alarms.

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

Caterpillar Generator at MNSU
Caterpillar generator at MNSU [129].
In exchange for this load control, the utility charges to the university decreased to a constant 0.045 $/kWh as of 2011. The stand-by generators are used a total of approximately two weeks per year. The utility company applies a large fine if the campus load is not taken off the grid when the electric company makes a request. To meet this requirement, the generators must be well maintained so they will work properly when needed.

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.

Caterpillar Generator at MNSU
Caterpillar generator at MNSU [129].
This building was designed with several unique features. Because the three diesel engines generate a tremendous amount of noise when operating all at the same time, everyone in the building must wear special ear protection. However, someone standing just outside the building would have a difficult time hearing the engines, because the building was designed with highly effective noise suppression. Running these three engines also requires a large volumetric airflow into the building, so one wall of the building was equipped with louvers.

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.

Fuel holding tank at MNSU
Fuel holding tank at MNSU [129].
The day tank containing diesel for each generator is located to the right of the unit inside the building. These tanks are connected to the 8500 gallon main tank outside the building. Each day tank has enough fuel to power the unit for approximately two hours; when the fuel level in the tank gets low, fuel from the main tank is pumped into the day tank. No. 2 diesel yields higher energy per gallon. However, it would crystallize in the cold Minnesota winters, it is blended with No. 1 diesel fuel, which is more refined than No. 2 diesel.

Emissions

Exhaust stack at MNSU
Exhaust stack at MNSU [129].
The utility plant staff and the state government closely monitor the amount of emissions produced by the boilers. Each year MNSU is allotted a certain quantity of expelled emissions. At the end of the year, the university must pay a certain amount per ton of emissions expelled. The university is fined if it exceeds its allotment. Imagine if you were charged for the exhaust emissions from your car! When the boilers were operated using No. 6 fuel oil, which contains high levels of sulfur, the plant operators had to monitor the emissions to ensure that the sulfur discharge limits were not exceeded.

Personnel in the Plant

Dudley Berger
Dudley Berger
The Facilities staff is responsible for keeping the buildings comfortably warm or cool and campus operational. Therefore, to make sure all of the equipment runs smoothly, personnel monitor the utility plant 24 hours a day. Plant personnel are required to have specialized licenses to operate the boilers and chillers installed at the plant. From 1998 to 2007, Chief Engineer Dudley Berger managed the utility plant operations. To become a licensed Chief Engineer, he was required to have five years of operational experience and pass the State of Minnesota Department of Labor and Industry exam. Operators under the Chief Engineer are required to have three years of experience to acquire a licensed. Surprisingly in a state that lies over 1200 miles from the nearest ocean, a large number of these employees are former Navy personnel. Many, like Dudley, gained knowledge and experience by working on Navy ships.

Steve Ardolf
Steve Ardolf
When Dudley left, Steve Ardolf became the new chief of the plant. Ardolf joined the Navy in 1981 and became a boiler technician. When he left the Navy he moved to Springfield, MN, where he worked for over four years as a boiler operator and a maintenance technician for the city. After working as a plant maintenance technician, a stationary engineer, and a utility machine builder at a security hospital in St. Peter, MN, for 16 years, he moved to Mankato, where he is working happily in the role of. Chief Engineer.

Paul Corcoran
Paul Corcoran
Paul Corcoran, who started working for the university as a mechanical engineering student and went on to become a project manager at the plant for several years, was promoted to Physical Plant Director. Paul is in charge of grounds, engineering, plumbing and steam fitting, and building maintenance. Physical plant operators take care of all aspects of the plant except custodial work.

Jeff Rendler
Jeff Rendler
All of the equipment must be maintained throughout the year but most maintenance is carried out during the summer months. In late May, all the boilers are emptied, cleaned, and inspected. Jeff Rendler, a Boiler Inspector with the Minnesota Department of Labor and Industry, inspects each boiler on campus once a year. The chillers are also cleaned once a year. Shutting down the chilled water supply to campus in preparation for winter takes four days and includes running antifreeze through many of the pipes to prevent them from freezing and cracking. The plant facilities staff test the emergency generators once a week at 1/3 the load, and, Katolight tests them once a year at full load. The stand-by generators are also turned on once a month to ensure they are working properly.

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:
July-2013
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