Using the AEDG in large hospitals
Concepts to employ
Commissioning large hospitals has led to confirmation of many successful energy conservation concepts. The following are conceptually sound, simple, and proven to produce better environments, lower energy use, and sustainable results. The following is low-hanging fruit—applications of equipment and systematic design that do not add cost and may even reduce cost. Savings obtained by applying the following can be used to fund other energy-reduction features as recommended by the AEDG, which may add cost.
Low-cost HVAC concepts that work, and not likely to be subjected to value engineering cuts by category, include:
Primary equipment sizing/pumping
- Selecting equipment with better part-load performance. This is a combination of equipment sizing and better turndown, considering that most equipment operates well below full load the majority of the time.
- Variable flows and variable speed drives (for everything). Variable flow system design is the first step; applying drives is the next step.
- Variable primary, not primary-secondary pumping for cooling systems. Less pump, and better flow control options.
- Manifolded/paralleled variable speed drive chilled water and condenser water pumps. Trueredundancyand better flow control options.
- Hot water in lieu of steam boilers—no centralized steam or condensate systems, no blow down, and lower losses. Steam for sterilization and humidification by unitary equipment that operates only as needed for the load, and only when needed.
- Low-temperature heating hot water—less heat loss and better heat coil modulation. This will require larger coils and/or low-temperature supplemental under-floor heat, and will rule out supplemental higher temperature radiant or fin tube heat.
- Use outside air free cooling, whenever free cooling is available. Feasible for most, if not all, space conditioning but may not provide the process cooling needed in specialized rooms (MRI, data center, etc.). This is already required by ASHRAE or IECC codes in many climates (see Figure 1).
- If air side cooling is not possible, install hydronic free cooling. Hydronic free cooling may be the answer if air side free cooling cannot be used for process loads. If hydronic free cooling is required, design with higher chilled water temperatures to permit longer periods of free cooling use, and design the plant with independent mechanical and free cooling condenser water systems. Switching back and forth with colder condenser water reduces the number of hours when free cooling is used, based on operator objection to starting chillers with cold condenser water temperatures. And finally, size free cooling heat exchangers for maximum flow needs (not just tonnage or delta T).
- Do not use, or minimize supplementary direct expansion (DX) cooling, computer room air conditioning units, etc. However, supplemental DX cooling for surgery sub-cooling is recommended. It is more energy-intensive, but will permit running the much larger plant with warmer chilled water, and may permit a later start of chillers as outside air temperatures rise.
- For high-heat equipment cooling, provide supply air near or under the floor, and take exhaust /return air out at the ceiling.
- Open cooling towers: lower horsepower and with variable speed drive fans.
- The larger the cooling towers, the better (not true for the other equipment).
- Use of indoor condenser water sumps—no tower heaters, drain down, or equalization lines.
Air handling units (AHUs)
- Use of dual (opposing) outside air entrances with air blenders (in cold climates) to eliminate stratification and minimize the use of preheat coils (see Figure 2).
- Lower air velocities through filters, coils, ducts, and fittings. Filter and coil velocities should not exceed 400 fpm. Damper velocities should not exceed 1500 fpm. Supply air should not exceed 1800 fpm, and return or exhaust air velocities should not exceed 1300 fpm.
Instrumentation and controls
Instrumentation and controls are the most important aspects of controlling and refining operations. Most equipment will operate at part load the majority of the year, and minimizing operation of equipment is the most important part of the process.
- Economizer cycles with proper setup of minimum outside air and supply/return fan tracking. Consideration for indoor and outdoor humidity levels is required. Enthalpy high limit control is not recommended due to the inaccuracy of measuring humidity and calculating enthalpy.
- AHU discharge air temperature control of all AHU functions, in sequence, and with high and low limits as needed to properly control component performance.
- Separate (from mixed air) control of exhaust air dampers. Hospitals reject a substantial amount of air via fixed exhaust air streams. Ejecting even more from the major AHU is not needed or desired, except during full airside economizer cooling periods.
- Supply air temperature and static pressure reset.
- Higher AHU supply air temperatures, with less reheat. Zoning by exposure will permit greater uniformity of loads, but it is difficult to do in a hospital and not likely to be implemented without significant extra cost.
- Larger amounts of warmer discharge air to provide the needed cooling—less air side cooling, and less reheat. This may require larger interior zone airflows to provide equivalent interior cooling with warmer, but more, airflow.
- Adding stages of cooling is easy. Dropping stages of cooling is far more difficult.
- Measure and totalize cooling equipment power. Based on experimental combinations of equipment operation, select operating routines that yield lower total energy consumption for any given load.
- Occupied/unoccupied control of spaces that are not continually occupied by patients, or continually used by administrative staff. If pressure relationships for infection control are required, maintain the pressure relationships, but with lower amounts of airflow when unoccupied.
- The most significant example is operating rooms (and other high-airflow rooms), where there are very high occupied airflow requirements but much lower requirements when unoccupied. Note this is airflow setback only when the space is not in use, not temperature or humidity setbacks. The resistance to this simple and effective energy-conservation practice is based on perception, not actual infection control risks.
- Control by individual zones, which is easy to do with variable air volume boxes. For example, efforts by the operating staff of a newer and larger hospital in Colorado reduced night and weekend airflows by more than 20,000 cfm compared to the previous same period, saving fan, heating, and cooling energy for many hours each day, and significantly improving air distribution system diversity.
- By totaling energy used by plant cooling equipment and experimenting with combinations of pumps, chillers, and cooling towers as well as water temperature resets, operators can determine actual energy performance variations, and can operate equipment at the lowest total energy use. When using variable speed drive equipment, more equipment at part load performance may be surprisingly more energy efficient.
- Utility meter monitoring via the building automation system (BAS). Utility meters continually measure usage and demand, but that information is not available to the operators other than via the monthly utility bills, or by special request for other than monthly histories. Continual monitoring, in conjunction with time of day, outside air temperatures, and so on, permits the operators to gain greater understanding of when and how peaks are set. Also, of even greater interest is observing how usage does not drop when conventional wisdom says it should.