Existing System Evaluation Executive Summary The MBNA Career Services Center is a 44,000 square foot, 4-story office type building at Penn State University, University Park Campus. The building is located across from the outdoor pool along Bigler Road, and construction was recently completed in July 2002. The MBNA Center has brought all the career related services offered by the University that were previously spread throughout the campus to one convenient location. The mechanical system at the MBNA Career Services Center consists of a Variable Air Volume (VAV) system that receives conditioned air from three air-handling units. A steam to hot water system provides heating for the building; steam for this system is provided by Penn State s campus wide steam distribution system, which is a byproduct from their power plant. Chilled water for cooling is provided by the chiller plant in the Computer Building. The reasoning behind the decision to pipe chilled water from an adjacent building will be discussed in more detail later in the report. Design Factors The design of the MBNA Career Services Center was influenced by several factors; the major influences being the low load requirements, site constraints and of course cost. These factors along with others directly impacted the decision to pipe chilled water from the Computer Building. Because of the relatively low tonnage requirements on the building, 133 tons, a water-cooled system would have been unfeasible and inefficient. An air-cooled system would have fit the requirements of the building better but integrating the air-cooled system with the site would have been difficult due to the limited available space surrounding the building. The architectural design of the building eliminated the possibility of placing a cooling tower on the roof, since half of the roof is sloped and the other part of the roof, which is flat and lower, can easily be seen from the third floor and contains sky lights. Noise and vibration would have also been issues if alternations had been made to place a cooling tower on the roof of the building. Another option considered by the mechanical engineers was to use rooftop unitary equipment since the tonnage requirements were low as well as the first cost of this type of system being 1
low. This option was not approved by Penn State since the life cycle duration and maintenance issues differ from their standards for mechanical systems. As with the design of any system, cost is a big factor to consider during the design phase of a project. By piping chilled water to the MBNA Center from the Computer Building, the cost of the cooling system was minimal since a cooling tower or air-cooled condenser would no longer be needed. General construction costs would have increased in order to provide additional space in the basement to house the equipment. Since the basement does not span the entire length of the building, additional excavation would have been required, again increasing the cost. As mentioned earlier, the tonnage requirements for the MBNA Career Services Center were low, roughly 133 tons total. The Computer Building already had the capacity to provide the MBNA Center with the cooling required, most of the time. If the Computer Building would request additional cooling, for instance on an extreme day, then the University can turn off the chilled water being sent to the MBNA Center. System Operation Heating Plant The MBNA Career Services Center contains a steam to hot water system. The building receives steam for heating from Penn State s campus wide steam distribution system. The heating system contains a steam to hot water heat exchanger, a duplex pump condensate return unit and two hot water pumps. The schematics on pages 8 and 9 show the major components and piping of the steam to hot water system, which is described in more detail below. Numbers in parenthesis () correspond to the numbered locations on the heating plant schematic. The steam enters the building (1), passes through a steam meter (2) and then reaches a PRV (Pressure Reducing Valve) station (3) where it is reduced from 150 psig to 10 psig. The PRV station contains two reducing valves in parallel, with the upper valve having a 1340 lbs/hr capacity and the lower valve having a 660 lbs/hr capacity; total capacity equaling 2000 lbs/hr. Condensate from the high pressure steam, collected at two points, before the steam meter and before the PRV station, prior to passing through the PRV station, and enters into a flash tank(4). 2
The steam leaves the flash tank as low pressure steam at 15 psig and combines with the low pressure steam leaving the PRV station (5). The steam then enters a TCV (temperature control valve) station (6) where it is further reduced from 10 psig to 2 psig in a parallel layout similar to the PRV station. The 2 psig steam flows into a shell and tube heat exchanger (9) which transfers the heat energy to the hot water loop. Condensate from the PRV station, flash tank, TCV station and heat exchanger is collected in a receiver tank (7). From there, the condensate is taken out of the building through the pump discharge line (8). The hot water leaves the heat exchanger and flows into a 190 gallon capacity air separator (10). Make up water at 30 psig is introduced into the system after the air separator. A diaphram type expansion tank (11) provides relief for the make up water. The hot water is then pumped to the heating coils by two 130.6 gallon per minute pumps with variable frequency drives (12 &13). Unit heaters are located at entranceways to provide immediate heat to occupants upon entering the building. Cooling Plant The MBNA Career Services Center does not have an internal cooling plant as mentioned earlier. The chiller plant in the Computer Building, which is located adjacent to the MBNA Center on the north side, provides chilled water for the building. Details regarding the chiller plant within the Computer Building are unknown at this time. The cooling system at the MBNA Center contains two chilled water pumps, one duty and one standby. DP switches located across each pump determine pump status. The schematic on page 10 shows major components and piping of the cooling plant. The numbers is parenthesis () below correspond to the numbered locations on the chilled water schematic. Chilled water enters the building (1) and is combined with make up water at 30 psig (2). Relief for the make up water is provided by a diaphram type expansion tank (3). The chilled water is 3
then pumped to the cooling coils by two 265 gallon per minute pumps with variable frequent drives (4 & 5). Air Distribution System Three air-handling units within the MBNA Career Services Center distribute supply air throughout the building. The following items are typical to all air-handling units. Smoke Detectors - located in the supply and return air ducts. In response to smoke detection the outside air damper closes and the supply and return fans stop. The fans will remain off until the operator resets them. Filter Alarm. A pressure switch across the filter bank measures static pressure. When the pressure reaches 1 w.c. an alarm is sounded as an indication that the filters are dirty and need replaced. Freeze Protection. A low-limit thermostat set at 35 degrees F is used to sense temperatures close to freezing. If temperatures are below this set point, a signal is sent to the DDC (Direct Digital Control) system. The supply fan stops, outdoor air and relief air dampers close and the heating valve is fully opened with the dampers in the full face position. Controls must be reset manually. Two air handling units are located in the basement of the building and the third is located in a mechanical room on the third floor. For detailed information on each of the air handling units, please see the Air Handling Unit Schedule on page 11. The smallest of the three air handling units is AHU-1 with a total cfm of 3500. The function of AHU-1 is to provide ventilation air to the fan coil units located in the program space area on the first floor. The supply air temperature control is based on the following: Outdoor Air Temperature Supply Air Temperature 0 72 60 65 95 60 There are two modes of operation for AHU-1, either cooling mode or heating mode. When the fan turns on, the outdoor air damper, which has two positions, opens to a fixed position. If the 4
outdoor air temperature is greater than 65 degrees F, the air-handling unit is in the cooling mode and the heating coil valve is closed. If the outdoor air temperature is lower than 60 degrees F then each of the two heating coil valves is modulated to maintain the supply air temperature to the two corresponding zones. The distribution of supply air to the remaining rooms on the first floor along with the second floor is provided by AHU-2, with a volume flow rate of 28,000 total cfm. Supply fan operation is based on a time of day schedule programmed into a DDC system. The typical schedule for operation is 8a.m. to 5p.m., Monday through Friday. A differential pressure switch is located across the fan to determine the status of the fan; an alarm sounds if status is lost. When the supply fan stops, the outdoor air and relief dampers close. The relief fan operates at two speeds; a low speed when the outdoor air flow is 50% or greater and a high speed when the outdoor air flow is 75% or greater. Occupied supply air temperature control is based on 72 degrees F space temperature and is reset in a cascade control manner. Economizer dampers, a cooling coil and a heating coil maintain the temperature of the supply air. The four modes of operation for AHU-2 are described below. The first three modes are used when the outdoor air temperature is less than the temperature of the relief air. 1. Dampers only. This mode operates between minimum outdoor air and 100% outdoor air. At minimum outdoor air, if the mixed air temperature is below the set point it will switch to mode 2 (heating). At 100% outdoor air, if the mixed air temperature is above the set point, it will switch to mode 3 (cooling/economizer). 2. Heating. When the outdoor air temperature is below 38 degrees F, the heating coil valve opens fully and the face and bypass dampers adjust to maintain the mixed air temperature. If the outdoor air temperature is above 38 degrees F, the face and bypass dampers are full open and then heating coil valve changes to maintain the mixed air temperature. 3. Cooling/Economizer. With 100% outdoor air and the outdoor air temperature higher than the mixed air temperature, the cooling coil is opened to maintain the set point of the supply air. 5
4. Cooling/Minimum Outdoor Air. This mode is used when the outdoor air temperature is greater than the relief air. If the chilled water is available, outdoor air damper is opened to a minimum and the cooling coil is modulated to maintain the set point of the supply air. A warm up cycle is used when the temperature within a zone is below 68 degrees F and the temperature of the outside air is below 55 degrees F. During warm up the outdoor air dampers are closed completely and the return air damper is opened fully. Once the zone space temperature reaches 72 degrees F the cycle is complete. On the third floor of the MBNA Career Services Center is AHU-3, which provides 7900 cfm of air to spaces on the third floor. AHU-3 is a 100% outdoor air unit. The set point reset is based on the following: Outdoor Air Temperature Supply Air Temperature 0 72 60 65 95 60 The supply air temperature is maintained through two modes of operation; heating and cooling. The telecommunications equipment room and the two server rooms located with in the MBNA Career Services Center are conditioned with their own air conditioning units. Packaged controls regulate the temperature and humidity within each of these rooms. The DDC system monitors these conditions and sounds an alarm if the temperature or humidity is beyond the limits. The units will typically run all the time, but the DDC system does provide start and stop capabilities. Once the supply air leaves the air-handling units, variable volume boxes with reheat regulate the amount of air entering the occupied spaces throughout the building. A DDC terminal box controller controls these boxes. The box opens to its maximum cfm set point during warm up mode in the morning. The VVB damper closes until it reaches the set minimum cfm position when the space temperature is below the set point. If the space temperature decreases further below the set point then the reheat valve opens to provide the amount of heat needed to meet required temperature. 6
Critique of System Overall, the mechanical system at the MBNA Career Services Center resulted in a low cost budget on the construction side as well as the design side. This reduction in cost is mainly due to the existing steam heating system provided by Penn State University and the extra cooling capacity available from the Computer Building. Since neither a heating plant nor a cooling plant were needed within the building, the air handling units take up a majority of the mechanical space, followed by the ductwork. Although out of the total 44,000 GSF building, the mechanical room area is only about 1920 square feet. Even though piping chilled water from the Computer Building saved money, there is the potential for problems to arise. Such as, the chiller plant in the Computer Building being oversized enough to handle its own cooling loads as well as an additional capacity of roughly 133 tons from the MBNA Center. As mentioned earlier, the chilled water to the MBNA Center can be shut off when the cooling loads on the Computer Building are higher than normal. In my opinion if this is the case then the capacity is not really high enough to have both buildings on the same system. According to my calculations from Technical Assignment M-1, Ventilation Compliance Evaluation, I found the outdoor air specified to be marginally insufficient when using the intermittent occupancy factor. For instance, the amount of outdoor air required for the first and second floor spaces was 13,951 cfm and according to the air handling unit schedule (page 11), the total outdoor air supplied by AHU-1 and AHU-2 equals 10,878 cfm. Although these two figures are not extremely different, I plan to double check my calculations to make sure the building is really lacking this much ventilation air, since it is so important to the health of the occupants to have adequate ventilation air. 7
TCV STATION 10 PSIG TO 2 PSIG 2000 LBS/HR TOTAL CAPACITY 6 3 5 PRV STATION 150 PSIG TO 10 PSIG 2000 LBS/HR TOTAL CAPACITY 2 STEAM METER 1 SEE HOT WATER SIDE SCHEMATIC FOR CONTINUATION SHELL & TUBE HEAT EXCHANGER 9 4 FLASH TANK 2000 LBS/HR CAPACITY 1,916,731 BTU/HR OUTPUT 1984 LBS/HR @ 2 PSIG STEAM PRESSURE STEAM CONDENSATE 8 STEAM SIDE SCHEMATIC 6 GALLON TANK MBNA CAREER SERVICES CENTER THE PENNSYLVANIA STATE UNIVERSITY RECEIVER 7 PUMP AIR SUPPLIED PRESSURE POWER PUMP CONDENSATE METER ANDREA BOROWSKI TECH. ASSIGN. M-3 NOVEMBER 11, 2002 8
MAKE UP WATER PRV STATION SET AT 30 PSIG THERMAL BREAK 10 AIR SEPARATOR EXPANSION TANK DIAPHRAM TYPE TANK VOL = 33.6 GAL PRESSURE TO 30 PSIG 11 SEE STEAM SCHEMATIC FOR CONTINUATION TO HEATING COIL PUMP 130.6 GPM W/ VFD 12 13 PUMP 130.6 GPM W/ VFD 190 GAL CAPACITY 9 SHELL & TUBE HEAT EXCHANGER 1,916,731 BTU/HR OUTPUT 1984 LBS/HR @ 2 PSIG STEAM PRESSURE FROM HEATING COIL CHEMICAL FEEDER SEE STEAM SCHEMATIC FOR CONTINUATION 9 HOT WATER SIDE SCHEMATIC MBNA CAREER SERVICES CENTER THE PENNSYLVANIA STATE UNIVERSITY ANDREA BOROWSKI TECH. ASSIGN. M-3 NOVEMBER 11, 2002
CWS CWR FROM COOLING COIL EXPANSION TANK DIAPHRAM TYPE TANK VOL = 7.8 GAL PRESSURE TO 30 PSIG SET AT 30 PSIG 3 2 MAKE UP WATER PRV STATION 1 TO COOLING COIL PUMP 5 4 PUMP MBNA CAREER SERVICES CENTER COMPUTER BUILDING 265 GPM W/ VFD 265GPM W/ VFD 10 CHILLED WATER SCHEMATIC MBNA CAREER SERVICES CENTER THE PENNSYLVANIA STATE UNIVERSITY ANDREA BOROWSKI TECH. ASSIGN. M-3 NOVEMBER 11, 2002
AIR HANDLING UNIT SCHEDULE SYMBOL AREA SERVED SUPPLY FAN DATA CW - COOLING COIL DATA HW - HEATING COIL RELIEF FAN DATA TOTAL % OA EAT LAT TOTAL EAT LAT TOTAL CFM CFM TYPE HP GPM EWT LWT GPM EWT LWT CFM TYPE HP DB WB DB WB MBH DB DB MBH AHU-1 PROGRAMS 3500 76.4 3520 FC 3 86.5 73.9 54.5 54.0 238.3 39.7 44 56 0.0 55 159.8 10.5 180 150 3520 FC 2 AHU-2 1st&2nd FLRS. 28000 29.3 27150 AF 30 79.4 67.2 55.0 54.5 1088.0 181.4 44 56 13.3 55 442.1 29.5 180 150 27150 AF 15 AHU-3 3rd FLOOR 7900 15.3 8000 FC 7.5 77.3 65.0 55.0 54.5 260.7 43.5 44 56 39.9 55 46.4 3.0 180 150 7900 IN-LINE 3 SYMBOL P-1 & P-2 SYSTEM CHILLED WATER P-3 & P-4 HOT WATER OPERATION DUTY/ STAND-BY DUTY/ STAND-BY PUMP SCHEDULE GPM FEET EFF MOTOR BHP HD % HP RPM 0 70 - - 200 66 69.9 4.2 7.5 1150 300 57 73.4 5.3 0 54 - - 100 52 63 2.1 5 1150 160 46 69 2.8 EMERG. POWER NO YES SHELL & TUBE HEAT EXCHANGER HOT WATER SIDE (TUBES) STEAM SIDE (SHELL) TEMP (F) TEMP (F) PD (FT) ENTERING LEAVING GPM IN OUT CTRL VALVE CTRL. VALVE 130.6 150 180 7.7 2 0 ROOMS SERVED TELECOM & SERVER RMS COOLING COIL HUMIDIFIER NOMINAL FAN CONDENSER FAN TOTAL CAPACITY TONS CFM BTUH (LBS/HR) HP CFM 2 800 22500 COMPUTER ROOM AIR CONDITIONING UNITS 4.3 0.2 2200 HIGH AMB. TEMP COMPRESSOR LOW AMB. TEMP REFRIG. 95 F -30 F R-22 EQUIPMENT SCHEDULES MBNA Career Services Center Tech. Assign. M-3 November 11, 2002