HVAC BASICS AND HVAC SYSTEM EFFICIENCY IMPROVEMENT SECTION O INTRODUCTION HVAC systems or Heating, Ventilating and Air-Conditioning systems control the environment for people and equipment in our facilities. HVAC use in an office building might be as high as 30 50% in some climates. HVAC systems and chillers are also significant energy consumers in many manufacturing facilities. Section O -2 1
FUNCTIONS OF HVAC SYSTEMS The purpose of a Heating, Ventilating and Air Conditioning (HVAC) system is to provide and maintain a comfortable and safe environment within a building for the occupants or for the process being conducted. Section O -3 Many HVAC systems were not designed with energy efficiency as one of the design factors ENVIRONMENTAL CONTROL FACTORS An HVAC system functions to provide an environment in which these three factors are maintained within desired ranges. Typical design conditions are: 23 degrees C temperature (dry bulb) 40-60% relative humidity ASHRAE 62 1999, 2001 and 62.1 2004, 2007 Ventilation Standard 10 LPS outside air per person, or CO2 less than 1000 PPM Section O -4 2
COMFORT ZONE ON PSYCHROMETRIC CHART Section O -5 TEMPERATURE CONTROL STRATEGIES Vary the temperature of the supply air to the space while keeping the air flow rate constant. This is the constant volume, variable temperature approach. Vary the air flow rate while keeping the supply air temperature constant. This is the variable volume, constant temperature approach. VAV variable air volume system. Vary the supply air temperature and the flow rate, as in a variable volume reheat system. Section O -6 3
RELATIVE HUMIDITY CONTROL Humidification - The air is too dry and water vapor must be added for comfort. Dehumidification - The air is too wet and water vapor must be removed for comfort. AC systems typically over-cool the air to remove water vapor, and then may have to heat the air back up - this is called reheat, and requires additional energy. Either way, energy required is around 2300 kj per kg of water. Section O -7 SENSIBLE AND LATENT HEAT Sensible heat - The heat associated with a temperature change of a substance at a constant moisture level. Section O -8 Latent heat - The heat associated with the phase change of a substance. Enthalpy - Total heat content of a substance, including both sensible heat plus latent t heat. 4
PRIMARY EQUIPMENT Chillers Direct expansion (DX) systems Boilers Section O -9 Furnaces SECONDARY SYSTEMS Single duct, single zone system Single duct, terminal reheat system Multizone system Section O -10 Dual duct system Single duct, variable air volume system This is the most common system going in to large commercial buildings Fan coil system 5
HVAC SYSTEMS The summary below illustrates the types of systems frequently encountered in heating and air-conditioning systems. 11 12 6
13 VAV HVAC SYSTEMS Most common new system going in to large commercial buildings. High efficiency because of use of variable speed drive on the supply air fan. For example, cutting air flow from full rate to 80% of full rate, cuts the fan power almost in half! Fan is controlled most often to keep a constant static pressure in the supply duct. Temperature is controlled by a local l terminal box in the zone. Section O -14 7
POWER AND ENERGY IN AIR CONDITIONING One kw of A/C = 3600 kj/h A kw is a measure of A/C power, and is used when sizing systems, or when determining electrical demand. Section O -15 One kwh of A/C = 3600 kj A kwh is a measure of A/C energy, and is used when sizing storage tanks for thermal energy storage (TES) systems, or when determining electrical energy consumption. VAPOR COMPRESSION CYCLE OUTSIDE AIR EXPANSION VALVE OUTSIDE AIR CONDENSER COMPRESSOR Section O -16 INSIDE AIR EVAPORATOR INSIDE AIR 8
DIAGRAM OF A TYPICAL CHILLER 29 o C Condenser Water 35 o C Condenser Section O -17 Expansion Valve High Pressure Side Low Pressure Side Compressor Motor Evaporator 6 o C Chilled Water 12 o C HRU DESUPERHEATER Adding a small plate and frame heat exchanger into this system allows heat recovery from the 85 degree C hot refrigerant gas coming out of the compressor. Cold water (10-20 degrees) goes through the other side of the HRU, and is heated to 55 to 65 degrees C. Section O -18 The amount of heat recovery is about 800 kj/h per kw of capacity of the AC unit or air cooled chiller. 9
HVAC SYSTEM PERFORMANCE MEASURES Energy Efficiency Ratio (EER) EER = kj of cooling output Wh of electric input = kj/h of cooling output W of electric power input Section O -19 Coefficient of Performance (COP) COP = Energy or heat output (total) Energy or heat input (external only) = EER / 3.6 kj/wh SOME MAGIC NUMBERS FOR EER AND COP EER = kj/h of cooling output W of electric power input COP = EER /3.6 kj/wh Section O -20 kwin = 3.6 = 1.0 kwcool EER COP 10
EXAMPLES 1. A 30 kw cooling roof top A/C unit has an EER of 7.2. What is its COP? 2. A 30 kw cooling roof top A/C unit has an EER of 7.2. What is its kw input load at full capacity? Section O -21 3. A 30 kw cooling roof top A/C unit has an EER of 8.5. 85 What is its COP? 4. A 30 kw cooling roof top A/C unit has an EER of 8.5. What is its kw input load at full capacity? HEATING SEASONAL PERFORMANCE FACTOR For heating with an electric heat pump, the test is conducted at a different temperature than for the SEER, and this performance measure is called the HSPF the Heating Seasonal Performance Factor. For heating, there is also the heating COP, measured at different temperatures than the cooling COP. Section O -22 11
SYSTEM IMPROVEMENT OPTIONS Make building envelope improvements to reduce HVAC load insulation, high performance windows and roofs Replace old HVAC units and chillers with more efficient models Section O -23 Possibly downsize units and chiller Energy Star (US) and ASHRAE say chillers are oversized 60% in US. Consider multiple chillers Consider a chiller with a variable speed drive Consider installing a small chiller or separate HVAC system for 24/7 loads Section O -24 Use VSDs on pumps, cooling towers Replace constant volume systems with VAV - variable air volume systems 12
USE HEATPIPE FOR DEHUMIDIFICATION Section O -25 Consider adding a gas engine driven chiller with heat recovery for hot water Retrofit to DDC controls Use cooling towers where possible Section O -26 13
USE VARIABLE REFRIGERANT VOLUME SYSTEMS The term variable refrigerant flow (VRF) refers to the ability of the system to control the amount of refrigerant flowing to each of the evaporators, enabling the use of many evaporators of different capacities and configurations, individualized comfort control, simultaneous heating and cooling in different zones, and heat recovery from one zone to another. Current era technology using electronically commutated motors, inverter driven scroll compressors, multiple compressors, versatile configurations, and complex refrigerant and oil circuitry, returns, and controls, can permit as many as 60 are more indoor units to operate off one outdoor unit. Section O -27 ENERGY EFFICIENCY The energy efficiency of VRF systems derives from several factors: The VRF system essentially eliminates duct losses, which are often estimated at 10% to 20% of total air flow. But, there must be a separate ventilation system. The VRF systems typically include 2 3 compressors, one of which is variable speed in each condensing unit, enabling wide capacity modulation, and high part-load efficiency. HVAC systems typically operate between 40 80% of full load most of the time. Section O -28 14
ABSORPTION CHILLERS Absorption chillers can produce large quantities of chilled water using very little electric power and energy. Their prime energy source is heat from hot water or steam. Section O -29 Absorption chillers have no CFCs. Most absorption cycles use either ammonia and water or lithium bromide and water. Absorption chillers are not very efficient. Single stage -- COPs about 0.6-0.8 Two-stage -- COPs about 1.0-1.2 GAS DRIVEN CHILLERS Gas engine driven chillers offer significant electric demand savings, and good part-load performance. Most applications are in areas with high demand rates and low or moderate gas rates. In many cases, heat recovery and beneficial use of the heat is necessary for this approach to be cost-effective. Dual drive chillers are available (electric and gas). PowerCold Corp, Alturdyne Corp, Jenbacher Section O -30 15
CEM REVIEW PROBLEMS 1. In a vapor compression cycle air conditioner, the refrigerant is always in the vapor state. A) True B) False Section O -31 2. A roof top air conditioner has an EER of 9.2. What is its COP? 3. Reheat may still be needed in an HVAC system even if the outside temperature is very high. A) True B) False 4. A roof top air conditioner has an EER of 13.5. What is its kwin per kwcool rating? 5. How many kwh electric input is used to provide 120 million kwh of air conditioning i with a system having a COP of 3.0? Section O -32 5. How many kwh electric input is used to provide 100 million kj of heat removal (air conditioning) with a system having a COP of 5.0? 16
HVAC SYSTEMS APPENDIX Section O - 33 LATEST RESULTS ON CHILLED WATER PUMPING Study just out from the Air-Conditioning and Refrigeration Technology Institute (2004) on Variable Primary Flow Chilled Water Systems: Potential Benefits and Application Issues. To measure energy use and savings of VPFCWS, ARTI conducted an extensive study that compared these systems energy use with that of other common systems, including: Constant flow/primary-only CWS Section O -34 17
ARTI CWS STUDY (CONTINUED) Constant primary flow/variable secondary flow CWS Primary/secondary CWS Primary/secondary CWS with a check valve installed in the decoupler According to the ARTI study, the VPFCWS: reduced energy use 3-5% reduced first cost by 4-8% Reduced life cycle cost by 3-5% compared to conventional constant primary flow/variable secondary flow CWS. Section O -35 HVAC TESTING AND BALANCING The systems that distribute air and water for space conditioning throughout a facility may need to be balanced and cleaned as part of the retrocommissioning i i effort. In a process known as testing, adjusting, and balancing (TAB), HVAC system components are adjusted so that air and water flows match load requirements. The process begins with testing to evaluate the performance of the equipment in its current state. Adjustments to flow rates of air or water are then made for the purpose of balancing the system and matching the loads throughout a building. Indications that TAB is needed include frequent complaints from occupants about hot or cold spots in a building, the renovation of spaces for different uses and occupancy, and the need for frequent adjustments of HVAC components to maintain comfort. Section O -36 18
HVAC TESTING Typical HVAC system investigations include: Air system flow rates, including supply, return, exhaust, and outside airflow (flows go through main ducts, branches, and supply diffusers that lead to specific spaces in a building) Water system flow rates for chillers, condensers, boilers, and primary and secondary heating and cooling coils Temperatures of heating and cooling delivery systems (air side and water side) Positions i and functioning i of flow-control l devices for air and water delivery systems Control settings and operation Fan and pump speeds and pressures Section O -37 HVAC TESTING The savings associated with TAB come from the reductions in the energy used by the heating and cooling system and can range up to 10 percent of costs. The heat exchange equipment that cools and heats the air that ultimately reaches building spaces should also be inspected and cleaned if necessary. This consists of heating and cooling coils installed in air handlers, fan coil terminal units, or baseboard radiators. These units are typically supplied with chilled water and hot water. The heating and cooling coils can also be part of a packaged unit such as a rooftop air-conditioning unit. Section O -38 19
HVAC MAINTENANCE All surfaces and filters should be clean dirty surfaces reduce heat transfer and increase pressure loss, which serves to increase energy use. For air-side heating and cooling coils, whether in an air handler or in a rooftop unit, the methods for cleaning include compressed air, dust rags or brushes, and power washes. Any of these techniques will reduce deposit buildup. In addition, check baseboard heating systems for dust buildup, and clean them if necessary. The water side of heating and cooling systems is generally inaccessible for mechanical cleaning. Chemical treatments are often the best solution for cleaning these surfaces. Ongoing water treatment and filtering of the water side are recommended to reduce dirt, biological, and mineral-scale buildup. Filters for both air-side and water-side systems should be cleaned and replaced as necessary. Section O -39 HVAC MAINTENANCE In addition, make sure that terminal fan coil units and baseboards are not blocked or covered with books, boxes, or file cabinets. Besides creating a fire hazard (in the case of radiators), blocking the units prevents proper air circulation and renders heating and cooling inefficient. In general, the cleaner the heat transfer surfaces, the greater the savings. In addition, cleaning coils and filters may reduce the pressure drop across the coil and reduce fan or pump energy consumption. Savings can range up to 10 percent. Section O -40 20
CHILLER PLANT SAVINGS CHILLER PLANT SAVINGS Use controls to properly sequence chillers. Monitor the capacity of all chillers in the plant and turn chillers on or off so that each one is loaded enough to keep it in its most efficient zone Monitor outdoor conditions and reset the chilled-water temperature accordingly. This strategy can help match chiller output to the actual load. Monitor outdoor conditions and reset the condenser-water temperature accordingly. Higher condenser-water temperatures decrease cooling tower fan power but increase chiller power. The optimum operating temperature occurs at the point where these two opposing trends combine to produce the lowest total power use. Section O -41 Section O -42 21
CHILLER PLANT SAVINGS Take full advantage of available cooling towers. Most chilled-water plants have excess capacity, with one or more cooling towers not operating during low-load periods. To make the most of existing cooling towers, simply run condenser water over as many towers as possible, at the lowest possible fan speed. Inspect tubes annually and clean as needed, or use automatic tube-cleaning equipment. As a chiller runs, water may leave behind scale, algae, or slime on the inside of the condenser tubes Prevent scale formation in cooling towers. Scaling, corrosion, and biological growth all impede tower efficiency and increase maintenance costs from the resultant condenser fouling and loss of heat transfer. Section O -43 CHILLER WATER SAVINGS Replace standard valves with low-friction units to reduce flow resistance for the chilled water. This measure reduces pump energy use and returns less heat to the chiller. Insulate chilled-water pipes. Insulation helps ensure that the chilled water only absorbs heat from the spaces where it is intended to do so. Replace standard-efficiency or oversized pumps with highly efficient units sized for the newly reduced loads. Most induction motors that drive pumps reach peak efficiency when about 75 percent loaded, and are less efficient when fully loaded. Thus, wherever possible, size pumps so that much of their operating time is spent at or close to their most efficient part-load factor. If a pump is oversized, then it likely operates at an inefficient loading factor. Section O -44 22
CHILLER WATER SAVINGS Control chilled-water pumps with variable-speed drives (VSDs). VSDs can ensure that pumps are performing at maximum efficiency at part-load conditions. As with fan systems, the power required to operate a pump motor is proportional to the cube of its speed. For example, in a pump system with a VSD, a load reduction that results in a 10 percent reduction in motor speed reduces energy consumption by 27 percent: 1 (0.9)3 = 0.27. However, it is necessary to ensure that flow rates through chillers are maintained at safe levels. Upgrade the chiller compressor. For a centrifugal compressor, install a VSD. This will allow the chiller to run at lower speeds under part-load conditions. For reciprocating and screw chillers, replace with one that uses new magnetic bearing technology. These achieve better efficiency. Section O -45 CHILLER WATER SAVINGS For chillers without a VSD, use low-voltage soft starters. The motor windings of constant-speed compressors experience great stress when the chiller is first started due to the high inrush of current. Replace an old or oversized standard-efficiency chiller with a properly sized high-efficiency water-cooled unit. When replacing an existing chiller, select one that will be most efficient under the conditions it is likely to experience. Install water-side economizers to allow cooling towers to produce chilled water when weather conditions permit. Section O -46 23
EFFICIENT PACKAGED A/C UNITS HVAC CONTROLS The EMS and controls within a building play a crucial role in providing a comfortable building environment. Over time, temperature sensors or thermostats may drift out of tune. Wall thermostats are frequently adjusted by occupants, throwing off controls and causing unintended energy consumption within a building. Poorly calibrated sensors can increase heating and cooling loads and lead to occupant discomfort. Occupants are likely to take matters into their own hands if they consistently experience heating or cooling problems! Section O -47 Section O -48 24
TUNING UP THE CONTROLS Calibrate the indoor and outdoor building sensors. Calibration of room thermostats, duct thermostats, humidistats, and pressure and temperature sensors should be in accordance with the original design specifications. Calibrating these controls may require specialized skills or equipment and may call for outside expertise. Inspect damper and valve controls to make sure they are functioning properly. Check pneumatically controlled dampers for leaks in the compressed-air hoses. Also examine dampers to ensure that t they open and close properly. Stiff dampers can cause improper modulation of the amount of outside air being used in the supply airstream. In some cases, dampers may actually be wired in a single position or disconnected, violating minimum outside air requirements. Section O -49 TUNING UP THE CONTROLS Review building operating schedules. HVAC controls must be adjusted to heat and cool the building properly during occupied hours. Occupancy schedules can change frequently over the life of a building, and control schedules should be adjusted accordingly. Operating schedules should also be adjusted to reflect daylight saving time. When the building is unoccupied, set the temperature back to save some heating or cooling energy, but keep in mind that some minimum heating and cooling may be required when the building is unoccupied. Review the utility rate schedule. Utilities typically charge on-peak and off-peak times within a rate, which can dramatically affect the amount of electric bills. If possible, equipment should run during the less expensive off-peak hours. Savings from these control tune-up measures can range up to 30 percent of annual heating and cooling costs. Section O -50 25
COOL SAVINGS Chilled-water and condenser-water reset. In facilities with a central chiller system, the operating efficiency can be increased through a practice known as chilled-water reset modifying the chilled-water temperature and/or condenser-water temperature in order to reduce chiller energy consumption. Section O -51 Chiller tube cleaning and water treatment. Cleaning chiller tubes and improving water treatment can also improve performance of a chiller system by providing cleaner surfaces for heat transfer on both the refrigerant and water sides of the chiller tubes COOL SAVINGS Reciprocating compressor unloading. For smaller chiller systems that use reciprocating compressors with multiple pistons, part-load performance can be improved by making sure the system properly unloads pistons as the load decreases. If not, the system may cycle unnecessarily during low cooling loads. Increased cycling can lead to compressor and/or electrical failures. Unloading is typically controlled by a pressure sensor that is set for a specific evaporator pressure. This sensor, and the controls dependent upon it, can fall out of calibration or fail. Chiller tube cleaning and water treatment Cleaning Chiller tube cleaning and water treatment. Cleaning chiller tubes and improving water treatment can also improve performance of a chiller system by providing cleaner surfaces for heat transfer on both the refrigerant and water sides of the chiller tubes. Section O -52 26
END OF SECTION O Section O - 53 27