Economizer Applications in Dual-Duct Air-Handling Units
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ESL-HH-02-05-44 Economizer Applications in Dual-Duct Air-Handling Units Ik-Seong Joo Mingsheng Liu, Ph.D., P.E. Graduate Student Associate Professor Energy Systems Laboratory Energy Systems Laboratory University of Nebraska-Lincoln University of Nebraska-Lincoln ABSTRACT This paper provides analytical tools and Single-Fan, Dual-Duct (SFDD) System engineering methods to evaluate the feasibility of the economizer for dual-duct air-handling units. The The SFDD system (see Figure 1) includes an results show that the economizer decreases cooling economizer, supply and return air fans, a pre-heating energy consumption without heating energy penalties coil, and cooling and heating coils. The terminal for dual-fan, dual-duct air-handling units. The boxes modulate the cold airflow, the hot airflow, or economizer has significant heating energy penalties both to maintain the room air temperature. for single-fan, dual-duct air-handling units. The penalties are higher than the cooling energy savings Exhaust Air Louver when the cold airflow is less than the hot airflow. Exhaust Air Detailed engineering analyses are required to Damper Return Air Fan evaluate the feasibility of the economizer for single- fan, dual-duct systems. Return Air Damper Heating Coil INTRODUCTION The economizer is widely acknowledged as one Cooling Coil Outdoor Air of the popular energy conservation measures. It Supply Air Fan Damper Pre-heat Coil eliminates or reduces mechanical cooling by using Filters free cooling. For a single-duct system, the economizer reduces mechanical cooling with no Terminal Box heating penalties. For a dual-duct system, however, Cold Air Hot Air heating energy penalties may exist. Liu et al. [1997] Suppy Duct Suppy Duct pointed out that the heating energy penalty is often TTZone 1 Zone 2 higher than the cooling energy savings for single-fan, Figure 1. Schematic diagram of the SFDD system dual-duct (SFDD) air-handling units, and they developed an advanced economizer algorithm to The economizer modulates exhaust air, return air solve the economizer operational problems. and outside air dampers accordingly to maintain the However, there are no general guidelines or mixed air temperature at its setpoint. In practice, the recommendations for the economizer design for dual- mixed air temperature setpoint is slightly lower than duct systems. This paper presents economizer the cold air temperature setpoint to avoid the chilled models, performs the energy performance analyses, water valve hunting or frequently opening and and develops the design recommendations. closing. The action of the exhaust and outside air dampers opposes the return air damper. When the outside air damper is fully open, the return air SYSTEM MODELS damper is closed. When the outside air damper is in The dual-duct system supplies both hot and cold the minimum open position, the return air damper is air to each zone, where a terminal box modulates the totally open. total airflow rate and/or the mixing ratio of hot and cold air to maintain the room temperature. The dual The economizer can be activated using either duct-systems are defined as single-fan, dual-duct outside air temperature (temperature economizer) or (SFDD) systems, where a single fan is used to push outside air enthalpy (enthalpy economizer). The air through both hot and cold ducts, and dual-fan, temperature economizer is activated when the outside dual-duct (DFDD) systems, where two fans are used air temperature is within a predefined range. The to push air through cold and hot ducts, respectively. Proceedings of the Thirteenth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, May 20-22, 2002 ESL-HH-02-05-44 enthalpy economizer is activated when the outside air air duct. When the cold airflow rate is smaller than enthalpy is smaller than the return air enthalpy. Both the outside air intake rate, however, a portion of the the temperature and enthalpy economizers use the outside air is supplied to the hot air duct. Figure 4 same control sequence after activation. Figure 2 and and Equation (3) present the economizer schedules. Equation (1) present the economizer schedules. Exhaust Air Louver Exhaust Air Damper b Return Air Fan 1 Return Air Tr Tc,d Damper VSD Heating Coil TTroa VSD Filters Cooling Coil Controller min Outdoor Air Pre-heat Coil Toa Damper PP Te,min Tc,d Te,max Figure 2. Economizer schedules for the SFDD system Terminal Box Cold Air Hot Air Suppy Duct Suppy Duct $ TTO TTZone 1 Zone 2 ' min oa e,max 1 TTT? Figure 3. Schematic diagram of the DFDD system ' c, d oa e,max %TTr c, d (1) b TTT? ' e,min, b oa c, d ' TTr oa o &' min TToa e,min, b 1 o(Tr Tc,d) When the outside air temperature (T ) is higher o oa Tr Toa than the cold air temperature setpoint (Tc, d ), the economizer uses 100% outside air. When the outside min air temperature is lower than the cold air temperature Toa setpoint, the economizer maintains the mixed air Te,min Tc,d Te,max temperature at the cold air temperature setpoint. When the economizer is off, the system receives the Figure 4. Economizer schedules for the DFDD minimum outside intake. system The minimum economizer temperature,Te,min, b , $ TTO ' min oa e,max varies depending on the minimum outside air intake TTT? ' o c, d oa e,max ratio, min(/)moa ,min md . % o()TT r c, d (3) o TTT? ' e,min, o oa c, d ' TTr oa 1 ' TT ()TT (2) & min TToa e,min, o e,min, b r c, d r min When the outside air temperature is between the The heating and cooling energy consumption maximum economizer temperature (Te,max ) and the depends on economizer cycles, entering air conditions, setpoints of leaving air conditions, and cold air temperature setpoint (Tc, d ), the outside air is cold and hot airflow rates. When these parameters directly supplied into the cold air duct. The cold are given, the heating and cooling energy airflow rate equals the outside airflow rate, and all consumptions can be calculated using energy balance hot air is from the return air. principles [Joo and Liu 2002, under review]. When the outside air temperature is between the cold air temperature setpoint and the minimum Dual-Fan, Dual-Duct (DFDD) System economizer temperature (Te,min, o ), the outside The DFDD system (see Figure 3) has two supply airflow is modulated to maintain the cold deck mixed fans. Outside air is directly introduced into the cold air temperature at the cold deck setpoint. The hot Proceedings of the Thirteenth Symposium on Improving Building Systems in Hot and Humid Climates, Houston, TX, May 20-22, 2002 ESL-HH-02-05-44 deck air is from the return air unless the cold airflow protected if the return and outside air is well mixed ratio ( o ) is smaller than the minimum outside air during the economizer cycle. Therefore, the intake ratio. minimum economizer temperature is selected to be 20°F (-6.7°C). When the outside air temperature is higher than the maximum economizer temperature or lower than The maximum economizer temperature depends the minimum economizer temperature, the primarily on the outside air moisture contents. For a economizer is disabled. When the cold airflow ratio dry climate, such as New Mexico, the maximum is smaller than the minimum outside air intake ratio, economizer temperature can be as high as the return outside air is allowed into the hot deck in any air temperature. For a humid climate, such as schedule. Galveston, TX, the maximum economizer temperature should be limited to 62°F (16.7°C) or is an outside air intake ratio (a ratio of the lower. To consider general conditions, the maximum o economizer temperature is selected to be 65°F outside airflow to the total airflow), which may differ (18.3°C). from the outside air intake ratio of the SFDD system ( b ) because of the dual-fan system’s characteristic The minimum outside air intake depends on the that the outside air is directly supplied to the cold building functions. It often varies from 10% (office deck. buildings) to 30% (hospital buildings). At a low outside air temperature, an office building may The minimum economizer temperature of the require 30% outside air intake due to a reduced total DFDD system, T min,, oe , varies depending on min airflow rate. Therefore, the simulation is conducted and . using minimum outside air intakes of 10%, 20% and o 30%, respectively. TT o TT )( (4) The partial building load can be expressed using e min,, ro , rdc min the cold airflow ratio and the supply air temperature. When the cold airflow ratio is 1, the building is in The energy consumptions can be calculated full cooling. When the cold airflow ratio is 0, the using general energy balance principles. Joo and Liu building is in full heating. provided the detailed models [Joo and Liu 2002, under review]. 0 5 10 15 20 25 30 oC oF 110 oC hot deck temperature 40 ANALYSIS 100 cold deck temperature room temprature C) Economizer performance depends on the o 35 F or 90 following parameters: minimum and maximum o 30 economizer temperatures, minimum outside air 80 intake ratios, cold airflow ratios, room conditions, 25 and deck setpoints. The parameter ranges are 70 selected carefully so that simulation results can be 20 Set-up temperature ( temperature Set-up directly used and serve as a guideline for engineers. 60 15 50 10 The temperature economizer is used in this study 30 40 50 60 70 80 90 o due to the following reasons: (1) it is more popular Outside air temperature (oF or oC) F than the enthalpy economizer, and (2) both Figure 5.