Description of All Alternatives Considered-

Transcription

1 Description of All Alternatives Considered- Energy efficiency is an area where the Tubman design can be improved. The design heating load is MBH and the design cooling load is MBH or 142 tons. There are (2) 1040 MBH boilers which are slightly oversized. There are (2) ton air-cooled chillers or 243 tons of cooling. Approximately 70 tons of that cooling is for areas where exhibits are either displayed or stored, so there is redundancy for those critical spaces The first alternative investigated was resizing the chillers so there is a smaller chiller for off-hours cooling and a larger one for occupied mode. This would make sense since the museum will have low load profiles during the evenings when only the exhibit spaces needed to be cooled. The second alternative was adding a cooling tower. The chillers could be switched from air cooled to water cooled. Water-side economizers can be installed to allow efficient winter cooling. Recovery of condenser heat is efficient way to provide reheat for dehumidification, which reduces energy cost. Although all of these advantages are good, the ultimate problem is the lack of a cooling tower in the original design. Based on conversation with the engineer who designed the building, the owner did not want a cooling tower on top of the building distracting from the large dome shape on the top of the roof. There is no where on the ground around the footprint of the building to place a cooling tower. The third alternative was switching from constant speed pumping to variable speed pumping. There are many advantages to such system. They are: improved efficiency (motor and pump) and consequently energy savings; reduced system noise; improved control of system flow to respond to flow and pressure requirements of the system; extended motor life due to soft stops & starts which puts less wear and tear on the parts of the pump; control valve in the bypass ensures that neither of the chillers would become starved during a low load situation, as flow is diverted directly from the supply back to the chillers. I choose to look at the first and third alternatives based on the potential energy savings and feasibility.

2 Description of Existing System- The following is a sequence of operation of the air side and water side of the HVAC system. AHU-1A & 1B: These air handling units are constant volume and serve the gallery spaces. Therefore, the system is always running (occupied mode) since the artifacts always need to have a very controlled environment. Occupied Mode- The fan runs continuously throughout the cycle after the supply and return dampers are closed. The discharge air temperature is set at 50.5 F leaving the cooling coil. In order to maintain this temperature, the control valve modulates the chilled water running through the cooling coil. The variable frequency drives (VFD) on the supply fans shall modulate in order to maintain the necessary duct static pressure which the constant volume terminal boxes need to function properly. CO 2 Demand Controlled Ventilation- The minimum outdoor air damper is open for all hours of operation, which is set at 2,000 CFM for the base ventilation rate. The maximum outdoor air set point is 15,000 CFM. CO 2 demand controls the secondary outdoor air damper. The return air and outdoor air dampers are modulated as required to satisfy all the CO 2 space sensor requirements. The maximum CO 2 set point is 900 PPM. The DDC system monitors the outdoor air CO 2 to ensure permissible levels. This value needs to be investigated because it is higher than the allowance given in Standard 62, which states the maximum is 700 PPM plus ambient CO 2.

3 AHU-2: This air handling unit is variable volume and serves the offices and meeting spaces. Occupied Mode- The unit runs continuously under static pressure control. It has an economizer cycle for free cooling. When the outdoor air temperature is below 55 F, the secondary outdoor air damper opens and the return air damper closes while maintaining the same discharge air temperature. When the outdoor air temperature is above 55 F, the secondary outdoor air damper closes and the return air damper will fully open since there is no economizing possibility. The cooling coil valve will modulate open. When cooling is required the following steps will be taken: 1) Utilize outdoor air for cooling. 2) When the return air damper is closed, if the cooling coil leaving air temperature raises 3 F above the set point for 60 seconds, the control valve for the cooling coil opens allowing chilled water flow. 3) When the outside air temperature rises above the economizer set point, the secondary outside air damper will close 100% and Unoccupied Mode- The outdoor and exhaust dampers close and the supply and return fans cycle as required keeping the space at 85 F during the summer and 65 F during the winter. CO 2 Demand Controlled Ventilation- The minimum outdoor air damper is open for all hours of operation, which is set at 1,000 CFM for the base ventilation rate. The maximum outdoor air set point is 4,000 CFM. CO 2 demand controls the secondary outdoor air damper. The return air and outdoor air dampers are modulated as required to satisfy all the CO 2 space sensor requirements. The maximum CO 2 set point is 900 PPM. The DDC system monitors the outdoor air CO 2 to ensure permissible levels. This value needs to be investigated because it is higher than the allowance given in Standard 62, which states the maximum is 700 PPM plus ambient CO 2.

4 Hot Water System: The hot water boiler plant runs continuously year round. The base hot water supply temperature is 140 F and maximum is 160 F. Hot Water Pumps- The first pump is turned on by the automatic temperature control (ATC). The second pump starts if, when the first pump is operating, the hot water return temperature drops 25 F below the hot water supply set point for 10 minutes. The second pump turns off when the hot water return temperature is 10 F below the hot water supply temperature set point for 10 minutes. Chilled Water System: The ATC in unison with the DDC controls all aspect of cycling the chillers. When cooling is called for, the control valve at the chiller opens and the associated chilled water pump turns on. When flow through the piping is proven by evaporator differential pressure, the chiller starts and runs. The chiller shall operate to maintain 45 F chilled water supply temperature. The lead/lag chiller shall be switched by the building management system. Both chillers and associated pumps shall be programmed in a leadlag scenario. Both chiller 1 and 2 can be turned off. The second chiller is turned off when both chillers are running at below 40% of capacity for 10 minutes. The drawings do not clearly state what is being measured to determine the percentage of capacity of the chillers. This control system must be investigated. The following is a summary of the components of the chilled water system: (2) Trane (Model RTAA125) air-cooled water chillers. - Capacity = tons - Flow Rate = 290 GPM - Entering Water Temperature (EWT) = 55 F - Leaving Water Temperature (LWT) = 45 F - Screw compressor

5 (3) Bell & Gossett pumps (Model ½ BB) [Note: One of the pumps is stand-by] GPM - 55 ft. head ft. NPSHR impeller RPM BHP motor HP Although the controls at the loads (the three air handling units) are not shown on any of the construction drawings that I possess, it appears to be a constant primary-only system. Two of the air handling units are constant volume and serve the gallery spaces. Therefore, the system is constantly running since the artifacts continuously need to have a very controlled environment. The other unit air handling unit is variable volume and serves the offices and meeting spaces. If the load goes to half and it is all on one air handling unit (such would be the case at night when the offices and other spaces were empty), with the current system the water separates into both branches of the supply piping, but goes straight through the bypass for the air handling unit that doesn t need chilled water. The other air handling unit is potentially starved when this situation arises.

6 Description of Proposed Redesign (Mechanical Considerations)- The three-way valves are replaced with two-way valves at the air handling units with two-way valves costing less than three-way valves. There is also less piping that needs to be around the load. The omitted fittings and welds also increase the cost savings. A flow meter could be installed instead of a differential pressure loss sensor around the evaporator. There is currently a combination flow meter/shutoff/balancing valve (circuit setter) shown on the plans. There is no manufacturer listed or any accuracy measurements given for it. This information would have to be checked before deciding whether the flow meter would have to be replaced in the new system. Calculating the actual pumps savings- There are several different techniques available for calculating the energy savings of variable speed pumps. I plan on using the following to determine the kwh saved switching to variable speed pumps. Carrier s Hourly Analysis Program (HAP) Bell & Gossett ESP-Plus Online Program EES Simulation Determining the payback period of time to justifying the additional cost will be performed using Microsoft Excel. Calculating the actual chiller savings- I plan on using Carrier s Hourly Analysis Program (HAP) to determine savings switching to a smaller and a larger chiller than the current two, equally sized chillers.

7 Preliminary Research Biography- ASHRAE Applications 1999, Chapter 20, Bernier, M., B. Bourret Pumping Energy and Variable Frequency Drives. ASHRAE Journal December 1999: Bahnfleth, W., E.Peyer Comparative Analysis of Variable and Constant Primary- Flow Chilled-Water-Plant Performance. HPAC Engineering April 2001: Coad, W A Fundamental Prospective on Chilled Water Systems. HPAC Heating/Piping/AirConditioning August 1998: Ellis, R., Commissioning a Museum and Archival Storage Facility. ASHRAE Transactions Pt. 1, 1996, p Gill, N Adjustable Speed Pumps: Your Control Valve Alternative? Hartman, T., Library and Museum HVAC: New Technologies/New Opportunities Part 1. HPAC Heating/Piping/AirConditioning April 1996: Hartman, T., Library and Museum HVAC: New Technologies/New Opportunities Part 2. HPAC Heating/Piping/AirConditioning May 1996: 63, 64, 67, 68, 72 & 103. Hegberg, R Converting Constant-Speed Hydronic Pumping System to Variable- Speed Pumping. ASHRAE Transactions Pt ASHRAE Winter Meetings Technical Papers: Pacific Gas and Electric Company CoolTools Chilled Water Plant Design and Specification Guide.

8 Martino, F Elusive Energy Savings: Centrifugal Pumps and Variable Speed Drives. Rishel, J The History of HVAC Variable-Speed Pumping. ASHRAE Transactions No. 1, 1995 ASHRAE Winter Meetings Proceedings: R.S. Means 2002 Mechanical Cost Data: pg 194, 240. Schwedler, M., B.Bradley An Idea for Chilled-Water Plant Whose Time Has Come Variable-Primary-Flow Systems. Trane Engineers Newsletter Volume 28, No. 3.