Appendix D

D.7.4 Central Plant Systems

Central plant systems comprise the mechanical equipment that heat and cool water (boilers and chillers) to provide heating or cooling to a single or group of buildings. Distribution systems circulate the heated or cooled water through heat exchangers to condition air or meet process loads. Begin considering the central plant design and obtaining input from other design team members about the building loads, space conditioning requirements, and process loads early in the design process. Participation in the architectural design activities to minimize building loads, then design the central plant and distribution systems to meet the loads while using the least amount of energy.

Advantages of central plants versus dedicated mechanical systems include but are not limited to:

Allow for diversity of equipment capacity for multiple buildings.
Provide future flexibility for increased loads on the system (remember to allow space and accessibility for future expansion).
Lower maintenance costs because all major equipment is in one location.
Increase system efficiency because multiple chillers, boilers, and cooling towers can be staged so that they operate near their maximum efficiency and provide useful redundancy. Waterside economizers can also be easily incorporated.
Reduce amount of space dedicated to mechanical equipment in a building.
Reduce the noise and vibrations associated with operating combustion- and refrigeration-based equipment by removing this equipment from the building.
Increase the potential for combined heat and power (CHP) systems.

D.7.4.1 Water-side Economizers

The low wet-bulb temperatures in Fort Carson are especially suitable for water-side economizer systems. Select chiller systems designed to operate at as low a condenser (tower) water temperature as possible, down to about 50°F. When the CWS temperature is this low, a water-side economizer system can offset the entire chilled water load. Water-side economizer systems are particularly applicable for meeting large cooling water loads such as that which may be required by some laboratory activities. Provide a chiller bypass when using a water-side economizer.

D.7.4.2 Chillers

Refrigerant Options

Anticipate the actual operating conditions of the chiller and select accordingly. Most chillers operate between 40 percent and 70 percent of capacity a majority of the time and rarely operate at full load. Select chillers with a high-integrated part-load value (IPLV) rating so that they operate efficiently under full and part load conditions.

Consider selecting multiple chillers of different capacities to provide flexibility in meeting varying loads in addition to selecting chillers with high IPLV ratings. It is better to operate chillers near full capacity and start up additional chillers as needed than it is to operate large chillers at part load most of the time.

The energy use of central plant and distribution systems can vary by a factor of two or more based on the system design and operation. For example, an air-cooled chiller operating in Fort Carson will have an energy use of 1 kW/ton or more; whereas, the same sized water-cooled chiller with a cooling tower can have an energy use of less than 0.5 kW/ton.

Achieve improved compressor part-load kW/ton ratings by installing a variable-speed-drive (VSD) on the compressor. The VSD allows the compressor to run at lower speed under part-load conditions.

Operating chillers to provide higher chiller water supply (CHWS) temperatures increases the efficiency and provides greater cooling capacity (tons) for a given chiller size and constant condenser water supply (CWS) temperature. Keep this in mind when selecting coils for the chilled water system. Select coils using a 50°F or higher CHWS temperature. The larger face area of these coils reduces the air velocity and pressure drop across them. Also, designing for warm chilled water temperatures increases the number of hours of potential “free” cooling using a waterside economizer.

D.7.4.3 Boilers

Boiler Options

Select boilers based on the lowest life cycle cost. In most cases, it is best to purchase high-efficiency condensing boilers. But, if the heating load is small (so the boiler would not be operating very many hours per year), the added cost of these efficient boilers may not be justified. Discuss the various capital costs and full-load and part-load efficiencies with the project team as well as with the boiler manufacturers to determine the right boiler for a particular load.

Consider selecting multiple boilers of different capacities to provide flexibility in meeting varying loads. It is better to operate boilers near full capacity and start up additional boilers as needed than it is to operate large boilers at part load most of the time. Systems relying on multiple hot water boilers are more flexible and result in better peak and part-load performance.

Avoid selecting steam boilers for heating Fort Carson buildings. Steam systems are not recommended because of their typical high maintenance and poor efficiency. Should a steam boiler system be included in the design, consult an experienced boiler manufacturer regarding the boiler, heating surfaces, valves, combustion, condensate, condensate return, flashing, automatic temperature control, steam traps, pressure reduction, and steam metering.

D.7.4.4 Combined Heat and Power

the Efficiency of CHP is approximately 85%, compared to about 30% for a typical central power plant. This diagram asumes operation of 1-MW or larger gas tubines or fuel cells.

Combined heat and power systems generate both electricity and heat. Consider using CHP systems where the heat can be used for space heating, powering an absorption cooling system, or providing heat for a particular research activity. Size the CHP system so that all the waste heat is used most of the time. One appropriate application of CHP systems is to provide standby (emergency) power instead of installing an emergency generator for a building with a process heat load.

The low cost and high thermal-to-electrical efficiency (23 percent to 27 percent) of micro-turbines are making CHP systems viable in sizes of 30 kW and larger. CHP systems are also developing the reputation of being low-maintenance systems.

CHP systems reduce peak electrical demands of buildings. If CHP systems are considered for all new Fort Carson buildings, then installing these systems may help delay construction of new high-voltage feeds to the Fort Carson campus. This can be an important factor where available electrical power is limited. Remember to derate the capacity of all combustion devices (e.g., boilers and turbines) at Fort Carson for altitude.

D.7.4.5 Heating Systems

The primary heating energy categories are:

Outside air preconditioning
Space reheat
Overcoming envelope heat loss
Heating domestic hot water and process water.

Precondition ventilation air for freeze protection by using exhaust air heat recovery systems, natural gas-fired furnaces, or hot water coils. Natural-gas-fired furnaces located in the preconditioning unit can be direct-fired (combustion products go into the airstream) or indirect-fired (combustion products go out a flue). Direct-fired furnaces are more efficient, but only if used in systems needing 100% outdoor air. Use modulation controls with good turndown for all gas-fired units.

Minimize space reheat requirements by supplying air to the space at a temperature appropriate to the loads in that space. Minimize laboratory space conditioning system energy consumption by supplying air as warm as possible to the space when it is warm outside and as cool as possible when it is cold outside.

Work closely with the architectural design team members during the early design phases to ensure inclusion of a good thermal envelope for the building. Reducing building loads minimizes the need to provide heating system equipment to overcome heat losses through the envelope (see Appendix D, Section 6).

There are many variations in hot water system requirements at Fort Carson, from very light domestic hot water (DHW) loads to process-level loads. Typically, the DHW loads are quite small in most Fort Carson buildings. In all buildings, minimize the DHW demand as much as possible by specifying low-flow sink and shower fixtures.

Design the DHW system to meet the anticipated loads (DHW systems are often oversized in commercial buildings). Consider point-of-use hot water systems in buildings with light DHW loads (also known as instantaneous hot water heaters). These systems avoid the central hot water storage tank and system circulator pumps found with centralized systems. They save energy by eliminating thermal losses through the storage tank and eliminating the pump loads.
Central gas-fired hot water systems are typically a more efficient solution as the DHW loads increase and to meet process loads. For these systems, be sure to schedule circulator pump operation based on use patterns.

D.7.4.6 Distribution Systems

Properly engineered distribution system design, good specifications, and accurate installation result in system that efficiently deliver heated and cooled air or water from the point where it is generated to the point where it is used. In addition to the chillers, boilers, air handling units, and other components discussed so far in this section, good distribution system design includes effective insulation, condensation control, and minimized air leakage. Specify all motors controlled by variable speed drives as “inverter duty.”

The two most common and efficient types of water distribution systems within a building are primary/secondary and variable flow primary pumping systems. Primary/secondary systems provide energy-saving opportunities through variable flow (only pumps the water actually needed to meet the required loads) and elevated return-water temperature. The cost of variable speed drives has decreased significantly in recent years, resulting in an extremely cost-effective approach to reducing wasted pumping energy. Two-way valves cost less to buy and install than three-way valves.

Issues such as the minimum, maximum, and acceptable change of flow rate through the boilers or chillers, and the installation of a bypass to satisfy the minimum flow through the chiller will affect the design of variable flow primary pumping systems. These systems cost less to purchase and operate than the primary/secondary systems because there are fewer pumps in the system.

Provide controls that automatically reset supply water temperatures by representative temperature changes responding to changes in building loads or by outside air temperatures. It is best if fluid temperatures for heating equipment devices are as low as practical and as high as a practical for cooling equipment, while meeting loads and minimizing flow quantities.

D.7.4.6.1 Water Distribution Systems

The following are guidelines for water distribution design:

Design systems for the maximum temperature differential to improve equipment efficiency and reduce pumping energy
Vary the flow quantity with the load, using two-way control valves and variable speed pumps
Design for the lowest practical pressure drop
Provide operating and idle control modes
Identify the critical pressure path and size the pipe runs for minimum practical pressure drop when locating equipment
Specify high-efficiency pumps with high-efficiency (NEMA Premium) motors