Centralized Heating and Cooling Plants

Centralized heating and cooling plants, also known as district energy systems, provide heating and cooling to residential, commercial, and industrial complexes. A district energy system produces steam, chilled water and hot water at a central plant and distribute the thermal energy through underground pipes to destination sites. After it is used at the sites, the water is returned to the central plant to be re-heated or re-chilled and then recirculated through the network. Centralized heating and cooling plants are an efficient and reliable option for a network of buildings because they eliminate the need to install and maintain multiple individual facilities and reduce capital and maintenance costs.

McQuay chiller at MNSU. [129]


Energy Use

District Energy systems have been in operation over 100 years and currently supply the heating and cooling needs for more than 4.3 billion sq.ft. of building space in the U.S. [47]. District heating and cooling facilities are used in such sites as the Mayo Clinic and Harvard Medical School. Approximately 78% of all U.S. heating systems distribute steam rather than hot water [48]. Approximately 34% of the energy used in commercial buildings is for heating, cooling, and ventilation, and cooling alone accounts for more than half of that energy [49].

The central source or central production plant in a district energy system can include any type of heat inducing system, including a boiler, incinerator, geothermal source, solar energy, or cogeneration facility. Many district energy systems are transitioning to the use of local sources of fuel: the city of St. Paul, MN, uses waste wood [50]; the University of California, Los Angeles, uses biogas from landfill [51]; and the University of Missouri-Columbia currently provides steam and electricity to the entire campus using fuel derived from shredded automobile tires (tire-derived fuel, or TDF) [52]. The use of TDF, derived from over 350,000 tires annually, has replaced 20% of the energy that otherwise would be generated by burning coal, by burning discarded tires as fuel. The use of local resources supports the local economy and often qualifies the xx for tax credits [53].

Steam Heating

In a central heating facility, a boiler generates steam or hot water. The two types of boilers used for gas and oil firing are fire tube and water tube boilers.

Boiler Steam Drum
Boiler steam drum at MNSU. [129]
Because of the material complexity of pulverized coal, coal fired boilers are not well suited to use in a central heating facility and are more commonly used in power plants. In a central heating and cooling facility, convection created in the combustion process in the boiler heats the water, which is then released from the steam drum at the top of the boiler. To increase the efficiency of the boiler, many facilities incorporate an economizer, which uses the flue gas to heat up the water entering the boiler. The heated steam is then sent through the piping to designated locations served. The heated steam has many uses once it reaches the main facility. It can be used for direct heating, service water heating, or absorption cooling, or it can be directed through a pressure reduction station for use in lower pressure steam heating.

Before MNSU decided to construct a central heating and cooling plant in 1958, there were 19 boilers on campus, one for each building, each of which had to be tested regularly and maintained. The centralized utility plant was adopted when hot water cycles in high-pressure boilers were introduced and became practical. Installed in 1959, the first two boilers in the central system were used until 1962 when they were replaced by two 35,000 lb/hr boilers. A third boiler was installed in 1968 and a fourth in 1972, which was replaced in 2004. After this last replacement, four boilers instead of 19 could be used to heat the entire MNSU campus!

Super boiler
MNSU boiler room [129].
Steam is produced by the boilers at 150 psig and 366°F and distributed to the campus buildings. Steam flow rates vary from 7000 lb/hr up to 60,000 lb/hr depending on the need. Once the steam reaches each building, it travels through pressure regulating valves (PRVs), which control the steam flow to the building and maintain the building steam pressure below 15 psig. Maintaining that steam pressure value is critical because steam pressures with values above 15 psig create what is considered "high pressure steam" and require additional safety considerations. Within the building, the steam is used for a variety of purposes. Some of it is sent through heat exchangers that heat a separate supply of water for distribution to the perimeter of the building to heat traditional units like radiators. Other steam is circulated through a hot water heater to supply hot water to laboratories and rest rooms. The remaining steam is sent to air handlers throughout the building for space heating.

Once it has been used, the steam must be returned to the utility plant to be "re-boiled." In this process, the steam travels through a grid of concentric tubing going into and out of each condensate tank. The bottom of the inside tube is lined with pencil-sized holes. As the steam travels down the inner tube, it loses energy and condenses into a liquid. The condensate then drips through the holes and down the outside pipe to the condensate collection tank. Similar processes occur within the heat exchangers as energy is transferred. As condensate forms, steam traps separate the two phases by allowing only the condensate—and not the steam—to pass. The condensate then passes into the holding tank while the steam is held back until it has cooled enough to condense, and then it is processed in the same manner.

Condensate Tank at MNSU
Condensate Tank at MNSU [129].
Once water reaches a certain level in the main condensate tank, a pump engages and sends it back to the utility plant. Each building has a unique layout of heat exchangers and air handlers plus a holding tank. As the condensate returns to the utility plant, it collects in a main condensate holding tank. This tank, which can hold up to 5000 gallons, supplies water to the boilers. The city water supply is connected to the tank to resupply water lost through the heating and cooling operations for the buildings. As the liquid water, which loses pressure through the system, leaves the tank, it is pumped back up to a pressure of 150 psig and is sent through the boilers to restart the process.

Steam vs. Liquid Water Systems

Steam systems provide energy for uses other than heating. Unaided by external energy sources, such as pumps, low density steam can be used for water systems in tall buildings, where pumps must create high pressure, to lift the hot water vertically. The steam components can be varied or modified by closing the steam supply without the difficulties associated with draining and refilling water systems.

There are advantages to using a water system, which loses very little water and requires little to no makeup water. In a steam system, on the other hand, 12 to 25% of the condensate can be lost. The thermal energy stored in the supply and return water is not lost through traps, condensate losses, nor pressure reducing valves (PRVs). A closed loop requires minimal water treatment and no deaeration as compared to a steam system, which requires steam equipment including a deaerator, steam drum, and economizer. Additional costs are incurred to replace piping corroded by oxygen and bicarbonate in the feedwater.

Central Cooling

The cooling process involves four major systems: production, distribution, building bridge, and building cooling. The production system consists of the chillers, which provide the chilled water. The distribution and building systems contain the chilled water piping in the building, chilled water pumps, cooling coils and heat exchangers.

Chiller
Condenser on a Trane centrifugal chiller at MNSU [129].
In the production system, chillers are modeled upon the basic vapor compression cycle, which consists of four basic components: a compressor, condenser, evaporator, and expansion device. The compressor raises the pressure of the refrigerant vapor. The temperature of the refrigerant rises as the pressure rises, to a level slightly above that of the cooling medium within the condenser. The condenser heat exchanger rejects heat from the refrigerant to a cooling medium, which is usually water or air. Water cooled condensers use cooling towers to remove the heat from the water in the system. The water is run through the chiller, which extracts heat from the refrigeration liquid. The expansion valve expands the liquid refrigerant, decreasing the pressure inside the chiller, and sends it to the evaporator. At a decreased pressure level, the refrigerant will boil at a much lower temperature. It is then run through the evaporator where the refrigerant absorbs heat from the water that is being chilled. In a chilled water system, transferring the heat in water to the refrigerant inside the chiller cools the water. Some of the more advanced systems utilize multi-stage compressor, economizers, and multiple expansion valves in order to increase the overall efficiency of the system.

In most cases, the chilled water is used for air conditioning purposes. Warm or chilled water is run through devices called air handlers, which are usually located near the area that is to be heated or cooled. Air handlers are sheet metal box structures that contain a fan and a cooling coil. The cooling coil is copper tubing bent into a serpentine shape with aluminum fins bonded to the copper to increase heat transfer. The air handler includes a heating coil to heat air. The fan, or blower, draws in a mix of outdoor and return air through air filters that remove impurities. The air is heated or cooled as it passes over the hot or cool coils and then distributed throughout the facility in air ducts. The chilled water piping is heavily insulated using insulation covered by a vapor barrier. Moisture in the air can condense on the outside of uninsulated pipes that carry low temperature fluids and drip onto the equipment below, potentially causing erosion and rust.

Chilled water systems are found mainly in large buildings. Separate central chillers and air handlers along with a network of pipes and pumps connect the chilled water system. Of all buildings larger than 100,000 sq. ft., 39% contain a chilled water system [54]. Chilled water systems normally draw approximately 6-12% of the total plant energy consumption per year. In conventional chilled water systems, the chilled water is distributed at a constant flow rate, regardless of actual cooling demand. Since the peak demand for most air conditioning systems is around mid-day, much energy is wasted by not having the capacity for variable flow.

Supplying the buildings with air conditioning works in a similar manner to heating buildings with steam. The chillers operate on the same basic vapor compression refrigeration cycle used by home refrigerators and air conditioners.

Cooling Tower at MNSU
Cooling Tower at MNSU [129].
In the evaporator, energy is removed from the chilled water loop by transferring it to the refrigerant. The energy is then rejected from the refrigerant to a separate water loop in the condenser which is heated using the energy removed from the chilled water. The water then exits the condenser and is sent to three dual cell cooling towers outside the utility plant, where it pours into a basin that overflows onto the cooling tower wet deck. Air is brought into the tower through the wet deck and exits the top of the unit via a 30 hp fan. Energy is rejected by evaporation and the remaining water is collected in a basin at the bottom of the tower, where it is topped up to make up water lost through evaporation. The water is then pumped back to the plant.

The chilled water system has one main supply line and one main return line which connect to each of the campus buildings. To operate correctly, the supply/return lines must have the same pressure for all of the buildings. The chilled water exiting each chiller is routed through the main pipe to two 200 hp pumps that provide the pressure needed to supply chilled water to the campus buildings. As the chilled water reaches the buildings it is sent through air handlers to cool the air supply. After the chilled water has been used, it returns via return pipes to the utility plant.

Sent back to the plant in one main pipe, used chilled water, once it reaches the chiller room, is divided among the three different chillers. Each of the chillers can produce chilled water at 42°F. Once the chilled water is used in the air handlers within each building, the water returns at roughly 54°F to the chilling facility, where it is chilled once again and sent back out on a continuous loop.

The ideal temperature for this cycle is 45°F going out to the buildings and 55°F coming back. But because these temperatures are low enough to allow condensation to form on the outside of the pumps, all of the pumps handling the chilled water must be insulated to prevent condensate from dripping from their outer casings onto the floor, which creates safety hazards.

Chilled Water System Problems

Insufficient Flow

Like an electrical current, water follows the path of least resistance. The greater the resistance to flow, the less distance the water will travel. Total resistance to flow in any distribution network is a result of factors relating to the length of piping, pipe diameter, material, pipe roughness, valves, and fittings. Balancing valves are installed on each branch of piping to add resistance to flow in order to guarantee that each branch receives the volume of water it was designed to handle.

Piping
MNSU chiller piping. [129]

Personnel change the balancing valve settings to increase flow to individual units, such as air handlers, in order to increase cooling capacity. Increasing the volume of water to a single air handler decreases the amount of water received by another.

A second cause of insufficient flow of chilled water is clogged strainers. Composed of wire mesh, strainers are installed at each air handler to protect the valve and coil from rust and mud that collects in the piping system. Over time, the solids removed from the chilled water system by the strainers clog the openings in the wire mesh, impeding the flow of the water and reducing the ability of the air handler to transfer heat.

Dirty Coils

Water coils in air handlers have an average life span of 30 years. Chilled water coils that become covered with airborne dirt do not operate efficiently, cannot deliver the required capacity to cool a space, and have a shorter life span.

Condensate Leaks

Like most air conditioning systems, chilled water systems lower the temperature of air below the dew point in order to remove moisture from the air. Moisture condenses on the cold coil surfaces, drips down the fins, and collects in a drain pan. The drain can become clogged with debris, causing condensate to overflow the drain.

As in a steam system, air handlers can develop leaks as a result of carry-over, which occurs when droplets of moisture condense on the coil and are blown off the coil into the air stream at velocities as high as 500 fpm which can cause damage to the water pipe.

If a water pipe is damaged or the temperature of the piping is below the dew point, moisture will condense on the exposed pipe, saturating the insulation. Continuous dripping from the wet insulation can cause damage to the equipment.

Winter Freeze Ups

Devastating problems can occur when chilled water in the air handler freezes the chilled water coil. If water is trapped inside the tubes of the coil and freezes, it expands the tubes causing them to split and burst. Because the tubes have limited expansion capacity, they develop blister-like protrusions that pop open. This type of freezing can also occur when coils are exposed to air temperatures less than 32°F, when outdoor air dampers become stuck in open position, or when the heating system fails, causing the temperature to drop below 32°F.

Locations

Physical sites that include Central Heating and Cooling Plants:
Minnesota State University, Mankato
The College of New Jersey

Components

Components on Engaged that are included in Central Heating and Cooling Plants:
Boiler
Chiller
Cooling Tower

Want to learn more about the University of Missouri's Utility Plant? Click Here!
How destructive and intense is a steam leak? Click here to watch a video on this.
This line drawing shows a chiller with economizer.
How can cooling towers save energy? Read this article from ASHRAE.
Line drawing showing MNSU's refrigeration system.
What are types of chilled water HVAC systems and chilled water system problems? Brinco Mechanical Services describes them.
Want to learn about geothermal central systems? Read this ASHRAE journal.