4 FM quick reads on HVAC
1. How To Minimize HVAC Energy Used For Cooling
Today's tip from Building Operating Management comes from Daniel H. Nall of Flack + Kurtz. One major opportunity to reduce HVAC energy use is to improve the efficiency of the cooling sources.
There is a basic conflict between optimizing the efficiency of the distribution systems and optimizing the efficiency of the heating and cooling source equipment.
When it comes to cooling sources, the frequent need for dehumidification introduces a complicating element. The usual temperature setpoints of a chilled water system are often set by the fact that the dew point temperature of the ideal comfort condition (75 degrees F temperature, 50 percent relative humidity) is 55 degrees F. Maintaining that dew point in the space would typically require a supply air dew point temperature of 53 degrees F. Producing that temperature at a cooling coil typically would require an entering chilled water temperature at the coil of 45 to 46 degrees F, and a supply-chilled water temperature from the chiller of 44 degrees F. For an optimized all-air HVAC system, reduction of air transport energy is usually more effective than reduction of chilled water production energy, so selection of a coil with a 6 degrees F approach temperature should be accompanied by 44 degrees F chilled water and 51 degrees F temperature off the coil, reducing the fan volume and energy by 10 percent.
Minimization of cooling energy can also be achieved by reduction of the temperature at which the refrigeration device rejects the heat that it has removed from the conditioned space. Utilization of evaporative heat rejection with cooling coils or evaporative condensers is one way of achieving this end. Rejection of heat-to-surface water features such as ponds or rivers is another method. With either of these evaporative heat rejection methods, resetting the condenser water set point in response to non-design exterior conditions results in additional savings.
Another strategy for enhanced energy efficiency is rejection of refrigeration heat to the ground itself, although this method usually requires accompanying heat extraction from the ground. The ground-coupled heat pump is a popular strategy for enhanced energy efficiency, although failure to maintain a balance of heat rejected into the ground during cooling cycle and heat extracted from the ground during the heating cycle can result in long-term temperature drift in the soil surrounding the ground heat exchanger. Many systems have failed or delivered poor energy performance, over the years, due to this imbalance. Well-designed and balanced systems can deliver significant energy savings compared with air source heat pumps or cooling systems with evaporative heat rejection.
5 Steps to Chiller Efficiency
When developing a PM plan for chilling equipment, maintenance and engineering managers should consider five essential areas.
- Maintain a Daily Operating Log: This process allows the operator to assemble a history of operating conditions, which can be reviewed and analyzed to determine trends and provide advanced warning of potential problems.
- Keep Tubes Clean: One large potential hindrance to desired chiller performance is heat-transfer efficiency. Chiller performance and efficiency relate directly to its ability to transfer heat, which begins with clean evaporator and condenser tubes
- Ensure a Leak-Free Unit: Manufacturers recommend quarterly tests of compressors for leaks. Low-pressure chillers using either CFC-11, which has been phased out, or HCFC-123 have sections of their refrigeration systems that operate at subatmospheric pressure. Although these chillers are the most common in today's facilities, it is difficult to create a perfectly sealed machine, and leaks allow air and moisture, commonly referred to as non-condensables, to enter the unit.
- Sustain Proper Water Treatment: Most chillers use water for heat transfer, so the water must be properly treated to prevent scale, corrosion and biological growth. A one-time chemical treatment is required for closed-water systems, which are typical of chilled-water systems connected to the chiller evaporator.
- Analyze Oil and Refrigerant: Annual chemical analysis of oil and refrigerant can aid in detect chiller-contamination problems before they become serious. Testing consists of spectrometric chemical analysis to determine contaminants, including moisture, acids and metals, which hamper performance and efficiency. A qualified chemical laboratory specializing in HVAC equipment must perform the analysis. Most manufacturers provide annual oil and refrigerant analysis services.
Benefits of an HVAC Upgrade
Facility managers know that major HVAC upgrades can be some of the most difficult projects of their professional careers. There exists tremendous potential for workplace disruption, unanticipated equipment challenges and the wide-ranging opinions from building occupants over the definition of comfort — a factor that should never be underestimated. But a healthy implementation plan, open communication between tenants and service providers, and realistic expectations can prevent HVAC equipment installation from being a bane.
The starting point is proper planning. Every savvy facility manager instinctively knows this. But the mantra can never be repeated often enough, according to Jim Cooke, national facilities operations manager for Toyota Motor Sales U.S. operations. "Plan, plan and plan prior to starting any physical work," he says.
He also recommends developing back-up scenarios for any disruptions to the schedule. Cooke's list of "what-ifs" include unfavorable weather, equipment delivery delays, structural considerations demanded by new HVAC units on existing roofs, crane availability and access, and more.
Rick Martorano, director of space and facility operations for Arizona State University, echoes the sentiment. In his case, he had to plan far in advance to accommodate the needs and requirements of lab users on ASU's campus.
"Make sure you have their buy-in, and leave room in the schedule for unforeseen issues since they're unavoidable on projects of this magnitude."
Pay Attention To Part Load Efficiency Of Heating And Cooling Distribution System
Today's tip from Building Operating Management comes from Daniel H. Nall of Flack + Kurtz. The creation of energy-efficient HVAC systems can be difficult. Many different and often conflicting factors must be optimized to achieve the best system. The prevailing climate and the function of the conditioned space are the main determinants of the most effective system. The challenge is to recognize the opportunities inherent in the climate and application so as to select the best heating and cooling sources and distribution system. One key step in the process of creating an energy-efficient HVAC system is to improve the efficiency of the heating and cooling distribution system for the building. This improvement should be thought of not only as improvement of peak-load efficiency but also of part-load efficiency, because most HVAC systems spend the preponderance of their operating life at part load.
Good part load efficiency for distribution system components often involves the use of variable speed drives along with components that allow those drives to operate at lower frequencies as often as possible. For fans and pumps, facilitating variable flow operation is a must.
For variable flow systems to be effective, capacity reduction should be accompanied by flow reduction. Two-way control valves should almost always be specified for hydronic distribution systems. Some systems require a minimum flow rate, so the use of a controlled minimum flow bypass may be required.
The bypass is preferred to the option of utilizing a limited number of three-way valves, because the three-way valves will increase flow through the system when the actual required flow is above the minimum, resulting in increased pumping energy.
Appropriate selection of the prime movers is also important for energy efficient distribution systems. Pump and fan curves can be compared to find the best selection for each application. In general, larger diameter, slow rotation speed selections are more efficient, up to a point, although the designer should avoid selections for which a slight miscalculation of the system pressure drop might result in an undesirable operating point.