Objectives and Requirements For the Creative and Performing Arts High School (CAPA), the main objective of the mechanical design is to provide an energy efficient system that is easily maintainable and capable of providing the building occupants with a comfortable environment. Many of the aspects of the design and final design decisions were based upon the tenant s, Pittsburgh Public School, familiarity with the equipment, such as using a pneumatic control system as apposed to a direct digital control (DDC) system. Site Factors that Influenced The design of the CAPA building mechanical system was greatly influenced by site conditions. One of the largest influences was caused by the existing building s owner. The owner had sold the lot and the use to the school under certain conditions, one of which gave the owner full access to the roof above the new school building. As part of the request, the owner desired to place a garden and patio space on the roof of the CAPA building. This requirement forced the designers to move the penthouse to the far East side of the roof, closest to the adjacent FISERV building. The placement of the penthouse along with the limited plenum space, caused by the need to match floor levels with the existing Bitz Building, made it impossible to run the necessary ductwork to spaces along the western façade of the building. To meet the ventilation needs of these spaces, unit ventilators were used to bring in outdoor air through penetrations in the exterior wall. Another area of the design that was influenced by existing site conditions was the placement of the cooling towers. Originally, the cooling towers were going to be placed on the roof of the CAPA building, but because the original cooling tower placement was to be in close proximity to office spaces for the upper management of the FISERV building, the mechanical system designer was forced to place the cooling towers on the roof of the existing Bitz Building. The FISERV management did not want the cooling towers, or the condensation plumes formed by the cooling towers, to obstruct the view out of their windows. The cooling towers themselves were also the result of a site factor that influenced the design of the mechanical system. The site for the CAPA building previously served as the location of a gas station. The initial design idea for the building included the incorporation of using ground water to reject heat from the chillers. This design was ultimately turned down because the underground fuel tanks used by the gas station had leaked fuel into the soil in the past and despite approval from the EPA, the school district was not comfortable with the use of the groundwater in the school s system and the mechanical designer was forced to use cooling towers to reject the heat of the cooling system. Existing Conditions Evaluation 1
Conditions conditions for heating and cooling were obtained via Bin Maker Plus Software. For Pittsburgh, PA, the design conditions for heating and cooling according to ASHRAE Handbook of Fundamentals 2002 (0.4%) are as follows: Conditions Location = Pittsburgh, PA Elevation = 1224 ft above sea level Dry Bulb Wet Bulb Relative Humidity Indoor Dry Bulb Indoor Wet Bulb Temp. (F) Cooling 89 72 75 50 Heating 2 50% 75 50 Heating and Cooling Loads The design heating and cooling loads for the CAPA High School building according to available design documents are as follows: Heating: Cooling: 5902.8 MBH 432.8 Tons Heating and cooling loads for the administration spaces are not included in these values because they were not available in the design documents provided by the engineer. Cost Factors that Influenced the The sources for electric power generation and natural gas were limited to Duquense Light Company and The Peoples Natural Gas Company (Dominion), respectively, and since neither company offers rebates, costs associated to utilities did not influence the design of the system, and there were no additional cost factors that influenced the design. Existing Conditions Evaluation 2
Energy Sources and Rates Electrical power for the CAPA building is provided by Duquense Light Company and natural gas is provided by The Peoples Natural Gas Company (Dominion), the only providers available. The CAPA High School is part of Pittsburgh Public Schools (PPS) which is classified into the rate brackets of Duquense Light Company and The Peoples Natural Gas Company as Rate GL General Service Large and Rate CS-L Commercial Service Large, respectively. Tables with the utility rates from the utility providers follow: Utility Rate Tables Existing Conditions Evaluation 3
Conditions conditions for heating and cooling were obtained via Bin Maker Plus Software. For Pittsburgh, PA, the design conditions for heating and cooling according to ASHRAE Handbook of Fundamentals 2002 (0.4%) are as follows: Conditions Location = Pittsburgh, PA Elevation = 1224 ft above sea level Dry Bulb Wet Bulb Relative Humidity Indoor Dry Bulb Indoor Wet Bulb Temp. (F) Cooling 89 72 75 50 Heating 2 50% 75 50 Heating and Cooling Loads The design heating and cooling loads for the CAPA High School building according to available design documents are as follows: Heating: Cooling: 5902.8 MBH 432.8 Tons Heating and cooling loads for the administration spaces are not included in these values because they were not available in the design documents provided by the engineer. System Operation Chilled Water System (See Chilled Water System Flow Diagram schematic) The primary chilled water pump (PCHWP-1) is started by the DDC control system when a need for chilled water is determined via space temperature and an outside air optimized time schedule. When flow through the pump is verified by a flow sensor, the chiller starts up and remains online for a minimum amount of time. The lead secondary chilled water pump is started upon the operation of the primary chilled water pump and pumps chilled water to the system. The secondary chilled water pump s VFD speed is controlled by a differential pressure control and maintains a fixed pressure differential. If the loop pressure falls 2 psi below the setpoint for a 5 minute time period, the lag pump is energized and is ramped up to match the speed of the lead pump to maintain loop pressure setpoint. Existing Conditions Evaluation 4
The cooling tower fan is controlled to maintain an adjustable condenser water supply (CWS) temperature setpoint. When the CWS temperature rises above the setpoint, the cooling tower fan is turned on, and when the CWS temperature drops below the setpoint the fan is turned off. A cooling tower bypass valve (see schematic) is used to maintain the CWS temperature setpoint when the CWS temp drops below the setpoint. A free-cooling system is utilized when the outside air temperature is below 55 F and cooling is required in the building. The free-cooling system utilizes a plate and frame heat exchanger to provide chilled water to the system. When the plate and frame heat exchanger is being used, the chillers are disabled and the heat exchanger is isolated from the chillers. When more cooling is required, the heat exchanger is isolated from the system and the chillers are once again utilized. Hot Water System When the outside air temperature drops below 60 F, the DDC system starts the hot water system. The DDC system selects the lead pump based upon operation hours and when flow is detected in the system by flow sensors, the boilers begin operation. The DDC system modulates the boilers in sequence to maintain the hot water temperature to the system based on hours of operation. A 3-way diverging valve is used to control the hot water supply temperature, which is reset based upon the outdoor air reset schedule. The hot water return temperature is controlled to maintain a minimum temperature of at least 140 F. Variable Air Volume (VAV) Air Handling Units (AHU -1,-2,-4,-5.-6,-7,-8) Each air handling unit (AHU) is comprised of a supply fan with VFD, return fan with VFD, hot water coil, chilled water coil, filters, and economizer controls (see schematic of typical VAV AHU). For AHUs 4, 5, & 6, the supply fan operates continuously during warm-up and occupied modes, and is modulate as needed to maintain a constant duct static pressure sensed by a 2/3 duct pressure sensor. For AHUs 1, 2, 7, & 8, the supply fan operates continuously during warm-up and occupied modes and is modulated to maintain space temperature. The return fans, for all AHUs, track the supply fan and maintain a CFM differential as specified by the System Air Balance schedule. During the occupied mode, the outside and return air dampers are modulated in sequence with the chilled water valve to maintain a constant supply temperature of 55 F. When the outside air temperature is above 70 F, the outside air damper is returned to minimum position. During the un-occupied mode, the chilled water valve is closed, the hot water valve is opened, the outside air damper is closed, the return air damper is opened, and the supply fan is cycled at a night setpoint sensed by a space sensor. During the occupied mode of AHU-1, -2, -7, and -8, the heating coil control valve is modulated to maintain a discharge air temperature that is reset by the return air temperature. During Existing Conditions Evaluation 5
the unoccupied mode of all AHUs, the heating coil control valve is cycled with the supply fan to maintain an unoccupied setpoint. During morning warm-up, the outside air dampers are closed and the heating valve is fully opened for a set period of time before the building is set to occupied mode or until space temperature is satisfied. A CO 2 sensor is located in the return air duct and will override the outside air damper position when CO 2 setpoint is exceeded. Outside air damper will return to normal position when CO 2 level returns to normal. VAV Air Handling Unit (AHU-10) AHU-10 is used to serve the administration spaces year round and is served by its own air cooled condensing unit (see schematic for AHU-10). The AHU is comprised of a supply fan with VFD, return fan with VFD, a hot water coil, a DX coil, filters, and economizer control, and operates in the same manner as AHU-4, -5, and -6 as described in above section. Constant Air Volume (CAV) Air Handling Unit (AHU-3) AHU-3 is comprised of a supply fan, return fan, hot water reheat coil, chilled water coil, filters, and economizer controls (see schematic of AHU-3). The supply fan operates continuously during warm-up and occupied modes and is cycled as required to maintain the unoccupied space temperature setpoint during unoccupied modes. During the occupied mode, the outside and return air dampers are modulated in sequence with the chilled water valve. When the outside air temperature is above 70 F, the outside air damper is returned to minimum position. During the unoccupied mode, the supply fan cycles at a night setpoint as sensed by the space DDC sensor. During the occupied mode, the heating coil control valve is modulated to maintain a discharge air temperature that is reset by the return air temperature. During the unoccupied mode, the heating coil control valve is cycled with the supply fan to maintain an unoccupied setpoint. During morning warm-up, the outside air dampers are closed and the heating valve is fully opened for a set period of time before the building is set to occupied mode or until space temperature is satisfied. A CO 2 sensor is located in the return air duct and will override the outside air damper position when CO 2 setpoint is exceeded. Outside air damper will return to normal position when CO 2 level returns to normal. Existing Conditions Evaluation 6
CAV Air Handling Unit (AHU-9) AHU-9 is comprised of a supply fan, hot water reheat coil, filters, and economizer controls (see schematic of AHU-3). For this system, the supply fan operates continuously during the warm-up and occupied modes. During the occupied mode, a discharge air sensor modulates the outside and return air dampers in sequence with the hot water valve, and whenever the outside air temperature is above 70 F, the outside air damper returns to the minimum position. The heating coil control valve modulates to maintain a discharge air temperature that is reset by the return air temperature. During the unoccupied mode, the supply fan cycles at a night setpoint sensed by the space DDC sensor, and the heating coil control valve cycles with the supply fan to maintain an unoccupied setpoint. During morning warm-up, the outside air dampers are closed and the heating valve is fully opened for a set period of time before the building is set to occupied mode or until space temperature is satisfied. Heat Recovery Unit (HRU-1) & Horizontal Unit Ventilators The 3 rd, 4 th, 5 th, and 6 th floors of the existing Bitz Building are serviced by horizontal unit ventilators which are provided with ventilation air from HRU-1 (see schematic for HRU-1). The supply and exhaust fans for HRU-1 run continuously and the outside air is conditioned to meet the discharge air temperature setpoint via modulation of heating and cooling valves in sequence with operation of the system s heat wheel (sensible and latent heat). During the occupied mode, the horizontal unit heater fan runs continuously and the heating and cooling valves modulated to maintain the space setpoint. During the unoccupied mode, the fan is cycled and the heating valve is modulated to maintain the night setback space temperature setpoint. Vertical Unit Ventilators To service the western side of the building, vertical unit ventilators are used because of the lack of space to run duct work from the AHUs on the roof to the west side of the building. During the occupied mode, the supply and exhaust fans run continuously. When the outside air is below 75 F, the mixed air dampers and the variable speed exhaust fan modulate in sequence with the heating valve and cooling valve to maintain the space setpoint. When the outdoor air temperature is above 75 F, the mixed air dampers and the unit s exhaust fan go to a minimum outside air position, the heating valve is locked out and the cooling valve modulates to maintain the space setpoint. During the unoccupied mode, the supply fan is cycled with the outside air dampers closed, the unit s exhaust fan is turned off and the heating valve is controlled to maintain the night setback space temperature setpoint. Existing Conditions Evaluation 7
Garage Exhaust Fan The garage exhaust fan is provided with a VFD control and during the occupied mode runs continuously at its minimum. A carbon monoxide sensing system monitors the level of CO in the air and when the CO level rises above the setpoint, the VFD increases the speed of the fan. When the CO level falls below the setpoint, the fan returns to its minimum speed. A make-up air heater, consisting of a vertical face-andbypass heating coil, is used to maintain a discharge air temperature of 60 F. Air flows through the coil only when needed and is induced by the garage exhaust fan (EF-1). Operating History of System The construction of the CAPA building is still ongoing with an expected date of completion in May of 2003, and as a result, there is no data on the operating history of the system. System Critique The CAPA building s mechanical system is a collection of various design strategies and techniques that utilizes a variety of HVAC equipment to meet the design requirements of the building. Much of the system s complexity is cause by site conditions and restrictions that the mechanical designer had to work around. If these restrictions were not in place on the design, the system could be simplified. The total construction cost of the mechanical system for the 175,140 square foot project is approximately $ 4.2 million, or approximately $24/square foot. One of the key design issues was the maintainability of the system which was given much thought and consideration. Through previous assignments, it was found that the design has sufficient access to system components as to make the system easily maintainable, and also the design supplies adequate ventilation to properly address indoor air quality issues. When all aspects of the design requirements and restrictions are considered, I feel that the design of the system satisfactorily meets the demands of the building. Existing Conditions Evaluation 8