Creating Efficient HVAC Systems

Transcription

1 Creating Efficient HVAC Systems Heating and Cooling Fundamentals for Commercial Buildings Heating, ventilating, and air conditioning (HVAC) systems account for nearly half of the energy used in a typical commercial building. There are a number of ways to better manage your energy budget by focusing on this important area. This whitepaper describes the types of HVAC equipment, key terminology including efficiency ratings, and the impact of the building envelope on heating and cooling energy use.

2 Equipment The HVAC industry has its own language to identify different equipment types. Unitary refers to equipment that contains all of the components necessary to heat, cool, dehumidify, filter and move air in one or more factory-made assemblies. Unitary equipment is available in packaged or split system designs. The most common type of commercial system is the packaged system design, which provides both heating and cooling to about 70 percent of the commercial building floor space built over the last 30 years. Packaged units are typically sized from five to 30 tons in cooling capacity. They are generally mounted on the rooftop but can also be installed at ground level. The evaporator and condenser are kept together in one package, delivering conditioned air directly into a room or ductwork. A split system is a combination of an indoor air handling unit and an outdoor condensing unit. The indoor air handling unit contains a supply air fan and an evaporator (or cooling) coil. The outdoor condensing unit consists of a compressor and a condenser coil. Split systems are typically found in smaller commercial buildings. Heating The heating side of an HVAC system can include electric resistance, natural gas or heat pumps. In the case of electric resistance, the heating elements used are similar in concept to those used in a toaster only quite a bit larger. Most manufacturers use heating elements in the 10 to 15 kw range, and then combine multiple elements to achieve the desired heating capacity. The heating outside design temperature (coldest expected temperature) varies by climate zone, and is used in conjunction with heat loss calculations to size the heating portion of the HVAC system. Natural gas heating is also used in unitary HVAC systems. However, there are some differences compared to a conventional gas furnace for a home due to the common placement of these commercial HVAC units on building rooftops. The piping to deliver the natural gas to the rooftop or other locations should be considered in the system installation costs. A heat pump works much like a conventional air conditioner, except it can also run in reverse during the winter to provide heat. Heat pumps are more efficient because they do not create heat, but merely move or pump it from one place to another. These work best in moderate climates. Cooling Cooling is produced through the clever use of refrigerants, and HVAC professionals use different terms such as mechanical compression, vapor compression or direct expansion to describe the basic cycle. A conventional mechanical vapor compression system consists of four major mechanical elements: a compressor, a condenser, a metering device and an evaporator. Essentially, indoor heat is absorbed by the liquid refrigerant in the evaporator coil. This causes refrigerant boiling (evaporation) and transforms the refrigerant from a cold liquid to a warm gas. The vapor continues on to the compressor, where it is mechanically raised in temperature and pressure. It then circulates to the condenser coil, where heat is rejected and the vapor condenses to a liquid. The liquid refrigerant is metered back into the evaporator via a thermal expansion (TX) valve, which regulates the flow of liquid refrigerant using spring pressure in the valve body. The amount of liquid refrigerant released must be restricted to keep the evaporator pressure low enough to support evaporation. The refrigerant enters the evaporator again as a liquid-rich vapor/liquid mixture and the evaporation portion of the cycle is repeated. Compressor designs may be reciprocating, screw, scroll or centrifugal types. Reciprocating types are like the pistons in a car engine, displacing an increment of vapor with each stroke. This type is costly, noisy, bulky and limited in total cooling capacity. Screw compressors also displace increments of vapor but do so with counter-rotating screw shafts that mesh together. This results in a higher flow rate with small exterior dimensions. Scroll compressors function similarly to screw compressors except only one helical 2

3 coil rotates; the other is stationary. Scroll compressors are cheap and efficient, but limited in cooling capacity. Centrifugal compressors are like jet engines. Vapor is continuously drawn into the center of a rotating impeller with radial blades, and then thrown out toward the periphery of the impeller by centrifugal forces. The velocity of the gas is transformed into pressure when it is forced to decelerate within the impeller structure. They can achieve very high cooling capacities more efficiently. Cooling load, usually expressed in refrigerant tons (RT) or tons, is a big driver in HVAC equipment selection. One ton of cooling is the amount of heat energy (12,000 Btu) required to melt 2,000 pounds (1 ton) of ice in one hour. Commercial facilities and manufacturing plants with larger cooling loads often use chillers, which produce chilled water for air conditioning or process cooling applications. Chillers typically use mechanical vapor compression (centrifugal, screw, scroll or reciprocating) to achieve the refrigerant phase change that cools. The waste heat is then discharged to the outside air, either through an air-cooled condenser or cooling tower. Chillers using reciprocating compressors are generally found in the 15 to 200 ton range, while centrifugal chillers range from 200 to 1,500 tons. Chillers with screw and scroll compressors serve the mid-size capacities. Local climate, fuel cost, fuel availability, building type and capacity requirements are the most important factors in the HVAC system selection process. When deciding which type of HVAC system best fits your situation, operating, installation and maintenance costs should also be considered. Building Envelope Your building envelope doors, windows, walls and roof protects your indoor environment from the elements. The condition of your building envelope strongly impacts HVAC system energy consumption and occupant comfort. As a general rule, roughly 35 percent of a typical building s energy loss is through the roof. Twenty-five percent is lost through the walls, 20 percent through windows, and the balance is lost via fenestration through spaces like cracks and doors. A detailed assessment of the building envelope is required to make a heat loss calculation. Similar-sized buildings with variations in the number of windows or the degree of insulation will have heat loss calculations that are quite different. One of the most cost-effective steps is to weatherize windows and doors by filling cracks and placing weather-stripping around gaps where air can escape. Also, properly insulated walls and roofs can substantially reduce heating and cooling costs. Tightening up your building envelope is a great first step in an HVAC upgrade, since it can provide a quick payback toward lowering your energy costs. You may even be able to reduce the size and cost of your new equipment! Local building codes can provide specific recommendations for insulation levels and best practices. You can also reference the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standards, the International Energy Conservation Code (IECC), or the Building Owners and Managers Association (BOMA) standards for more information. Efficiency Ratings Determining the efficiency of your existing HVAC equipment is the first step in deciding whether to upgrade to a more efficient unit. Since the efficiency ratings for HVAC equipment can be confusing, a brief description of applicable HVAC terms follows: A British thermal unit (Btu) is a commonly used unit of measure for energy use in heating and cooling equipment. A Btu is the amount of heat required to raise the temperature of one pound of water by one degree Fahrenheit. One kilowatthour (kwh) of electricity contains 3,412 Btu. Energy efficiency ratio (EER) is a measure of how efficiently a cooling system operates when the outdoor temperature is at a specified level (95 F). A higher EER means the system is more efficient and will use less energy for the same cooling output. Typically, EER is used in rating commercial cooling systems that have a cooling capacity greater than five tons, with ASHRAE specifying values from 9 to 12 EER depending on size. Seasonal energy efficiency ratio (SEER) is a measure of efficiency over an entire cooling season. Residential units are rated in SEER, with a higher SEER indicating higher efficiency. Simply put, SEER is the total amount of cooling the air conditioner provides over the entire cooling season divided by the total number of watt-hours it consumes. Manufacturers must now meet a minimum of 13 SEER, though efficiencies over 20 SEER are available. 3

4 Coefficient of performance (COP) is typically found on chiller systems, gas cooling equipment and heat pumps. COP measures how efficiently a system operates at a single outdoor temperature typically 47 F for heating or 95 F for cooling. A higher COP means higher efficiency. The COP is equal to the Btu output divided by the Btu equivalent of electricity to produce that energy. COP ratings can vary from less than one to greater than five. Heating Seasonal Performance Factor (HSPF) is a ratio used to rate heat pumps, determined by dividing the seasonal heating output in Btu by the seasonal power consumption in watts. Like SEER ratings for air conditioners, HSPF is a seasonal measure. HSPF ratings of seven to nine are common for air source heat pumps, while geothermal (water source) heat pumps typically have HSPF ratings greater than 10. Annual fuel utilization efficiency (AFUE) measures the efficiency of heating equipment on an annual basis. AFUE is calculated by dividing the total heat delivered to a conditioned space for a heating season by the total fuel used (in Btu). Higher AFUE ratings indicate higher efficiency. Ratings of 80 to 96 AFUE are typical for furnaces. Upgrading Your Equipment If you have an HVAC system that is more than 15 years old, you may want to consider an upgrade. Newer systems are 20 to 40 percent more efficient than older technologies, which can result in a good return on your investment. When deciding to replace, however, you should also consider the maintenance costs and condition of your current system. Full-load Value (FLV) is the most useful efficiency rating for electric HVAC equipment. FLV is measured in kilowatts per ton capacity (kw/ton) and a lower FLV is preferred. COP and EER can easily be converted to FLV using these conversion factors. FLV = 3.516/COP FLV = 12/EER Knowing FLV, the capacity in tons, and cooling equivalent full load hours, you can make a rough estimate of energy consumption: Cooling energy consumption (kwh) = Capacity (tons) x FLV (kw/ton) x Cooling Load Hours The best available chiller efficiency is in the range of 0.5 kw/ton The best available packaged rooftop units operate at 0.8 kw/ton The best available packaged terminal air conditioners (PTACs) operate at 1.0 kw/ton When purchasing a new system, size equipment to the current load requirements rather than attempting to match the old design. Oversizing can result in higher installation and operating costs, shortened equipment life and poor humidity control. Oversizing is frequently caused by reliance on old nameplate data or rule-of-thumb formulas for general estimating. When sizing your new system, take into account insulation or lighting system upgrades that may have reduced space conditioning needs. Newer systems also provide a higher level of performance than older models. Consult with an HVAC engineer or trusted equipment supplier about properly sizing your system. HVAC Accessories An economizer is an HVAC accessory that incorporates bypass vents in the air handler to allow cool (30 F to 55 F) outside air to be brought directly indoors without being cooled by the evaporator. It is often called a free cooling air system since no cooling energy is used. Economizers are subject to requirements spelled out in ASHRAE 90.1, a standard that provides minimum requirements for energy efficient designs for all buildings except low-rise buildings. This standard states that economizers are required for systems larger than 5 tons in cool/dry climates (West, Southwest) and for systems larger than 11 tons in cool/moist climates (Midwest), but not in warm/moist climates (Southeast). Heat recovery ventilators (HRVs) are a heat exchange system between incoming make-up air and exiting exhaust air. In the summer, the temperature of outside air is reduced by cooler air from indoors that is being exhausted. In the winter, it works in reverse. HRVs can recover about 60 to 70 percent of heat in warm exiting air as cold outside air passes through the heat exchanger on its way into the building. 4

5 Energy/enthalpy/desiccant wheels are similar to HRVs, except that the heat exchanger is coated with a moisture-absorbing desiccant material. Desiccant wheels can reduce about 70 to 80 percent of the thermal energy in incoming humid summer air, because they remove moisture as well as heat. HRVs and desiccant wheels are best used in climates with extreme temperatures. Thermal energy storage systems use HVAC equipment to make ice or chilled water during the night. When air is passed over this cold media during the day, you can reduce or eliminate the HVAC contribution to peak demand. Nighttime operation of the HVAC system may also qualify for lower time-of-use rates. Conclusion HVAC systems are complex, but understanding the language of HVAC and principles of operation will help you achieve optimum indoor air quality while minimizing energy use. A well-executed preventive-maintenance program, combined with building upkeep and equipment upgrades, can provide substantial energy-cost savings. 5