Mechanical System Redesign. Dedicated Outdoor Air System. Design Criteria

Transcription

1 Mechanical System Redesign Dedicated Outdoor Air System Design Criteria The outdoor air conditions used were for Philadelphia, Pennsylvania IAP at a 0.4% occurrence. The supply air conditions were developed using an exhaustive search method that optimized the cooling capacity of the ventilation air and the capacity of the cooling panels at the same time. With these supply air conditions, the room air conditions provide a high level of thermal comfort. Because the cooling panels transfer heat through radiation and convection, the room feels comfortable even at a higher dry bulb temperature. Table 3-Outdoor and Space Conditions Dry Bulb ( F) Humidity Ratio (gr/lbm) Dew Point ( F) (for supply air) Outdoor Air Conditions Supply Air Conditions Room Air Conditions Because the building originally had four AHUs, each serving one quadrant of the building, four new AHUs will each serve the same quadrants. The design criteria for the Pinnacle units, described in the next section, are as follows: Table-4-AHU Criteria Unit Name Pinnacle Unit Quadrant Served Airflow rate (cfm) Designation AHU-1 PVS-09 Northwest 4,000 AHU-2 PVS-09 Southwest 5,600 AHU-3 PVS-18 Northeast 9,700 AHU-4 PVS-09 Southeast 6,650 17

2 The specific size of AHU is based on air flow rate. Refer to appendix for the Pinnacle Unit Selection Table used to determine the size of each AHU. Refer to appendix for calculations of ventilation air. The amount of ventilation air is calculated using ASHRAE Standard minimum outdoor air requirement. The airflows are different from the original design because the multiple spaces equation of Standard 62 is not used in this design. Table 3 summarizes these requirements: Table 5-Ventilation Rates Space Occupancy Load Ventilation People /1000 ft 2 CFM/Person Conference room Office space 7 20 Cafeteria Seating Area Reception Area The airflow rate of AHU-3 is higher than the other units because it serves the servery/dining room. Additionally, a flat maximum occupancy of 120 people is used in the dining room in modeling the building. Pinnacle Ventilation System The SEMCO Pinnacle Ventilation System AHU (PVS) was used to replace the existing AHUs. This is because the Pinnacle System can produce low dew point space temperatures economically regardless of the temperature of the chilled water suuplied to the cooling coils. The PVS is comprised of a total energy wheel, a cooling coil, a passive 18

3 dehumidification wheel, a post-cooling coil, a supply fan, and a return fan. Because the PVS has two energy recovery wheels, it is advantageous to use in this application. The AHU components are described below, but the actual design was performed by selection software provided by SEMCO. The passive dehumidification wheel removes moisture more effectively than dessicant-based cooling or even a dual-wheel energy recovery system. Thus, it can produce the low dew point space temperature that is needed to decouple the space sensible and latent loads. Figure 5 demonstrates its dehumidification capacity as a function of wheel speed. This shows that the amount of dehumidification can be altered by controlling the speed at which the wheel rotates. Figure 5 19

4 The cooling coil, though part of the overall system, was selected using the USA Coil software. The conditioning of the air in the PVS is not dependent on the chilled water temperature. This makes it ideal for this particular application because the chilled water system serves two buildings. The controls are much simpler if the chilled water temperature is uniform throughout the system. Figure 6 displays the cooling coil selection for AHU-1. Refer to the appendix for the cooling coils of the other three AHUs. Figure 6 Normally, a sensible wheel is used to recover sensible heat from the return air to increase the supply air temperature. The supply air temperature needs to be increased because overcooling can occur if the space loads are too small. This can happen when the space is unoccupied. When a space is unoccupied, the humidity can be controlled by reheating the air after it leaves the cooling coil. However, the PVS passive 20

5 dehumidification wheel must raise the temperature of the supply air to achieve the desired humidity ratio. Consequently, the system needs a sensible cooling coil to postcool the supply air. The post-cooling coil was also selected using the USA Coil software. Figure 7 displays the coil selection for the post-cooling coil for AHU 1. Refer to the appendix for the post-cooling coils of the other three AHUs. Figure 7 Air System Modeling Cooling loads for the building are modeled using Carrier s Hourly Analysis Program (HAP). The air system is modeled as a two-pipe fan coil unit with common ventilation. The ventilation rates were those calculated in the appendix. This HAP model provided the total sensible load for each space. This information was used to design the radiant panel system discussed in the next section. The program also calculated the cooling load on the chillers in the central plant. Refer to the energy comparison section for a discussion on the reduction of cooling load on the chillers. 21

6 The most important HAP output is the breakdown of the total cooling load into sensible and latent components. Refer to the appendix for the DOAS air system output. This shows that the air system can adequately handle the entire modeled latent load using the specified supply air conditions. There are several ways that the AHU could be modeled using the SEMCO modeling software, but because the space latent load is handled entirely by the DOAS, the air system is modeled to determine the performance data for peak space latent loads. Figure 8 shows the model of AHU-1. Refer to appendix for the models of AHU-2, AHU- 3, and AHU-4. It is clear that the PVS is capable of producing the required supply air and room conditions. Figure 8 also shows the energy savings that are achieved using the PVS. While this shows that the PVS saves energy, it compares the DOAS system to a conventional system with the same supply air and room conditions. The current system uses a supply air condition of saturated 55 F air, and a room condition of 75 F and 55% RH. Because of the difference in conditions, more accurate energy savings are predicted using HAP. Refer to the energy comparison section. 22

7 Refer to appendix for AHU-1 schematic. 23

8 Radiant Cooling Panel System Radiant cooling panels are used in every space in the building to remove sensible space loads that the DOAS cannot handle. Invensys Building System panels are selected using their Magic 6 product selection software. Because the reflected ceiling plan of the building is already set up in a 2 4 grid, 2 4 panels are selected. To maximize the panels cooling capacity, the panels are free-hanging at 8-0, or 1-6 below the original ceiling line. The radiant panel system is also designed to take up at most 50% of the total ceiling area to allow for other ceiling fixtures, such as lights, diffusers, and sprinkler heads. For each quadrant, a pump delivers water to the radiant panel control closets on each floor. In the control closets, one control valve for each zone controls the inlet temperature of the water in one of the spaces. Design The total space sensible load is calculated using the HAP model. A Microsoft Excel spreadsheet (see appendix) is used to determine how much of the space sensible load could be handled by the DOAS system. The remainder of the sensible load is met by the radiant panels. The radiant panel inlet water temperature is chosen somewhat arbitrarily, but the mean temperature difference is chosen to provide maximum capacity for each panel. Table 4 summarizes the radiant panel design choices. Refer to appendix for Magic 6 software selection outputs. 24

9 Table 6-Radiant Panel Design Criteria Room Dry Bulb Temperature ( F) 78 Panel Inlet Temperature ( F) 52 Panel Outlet Temperature ( F) 60 Mean Panel Temperature Difference ( F) 22 Total Capacity (Btu/ft 2 ) 33.6 Pressure Drop (psi) 2.8 Water Velocity (fpm) 85 The air side temperature difference ( T) is only 20 F, compared to a normal DOAS T of about 30 F. This means that the DOAS cooling capacity is relatively small, because the cooling load is directly proportional to the T of the standard air by the equation Q=1.08 scfm T. Consequently, the radiant panels have to remove more of the space latent load. The way that the building envelope is currently designed allows more sensible solar and envelope load into the space than can be handled by the DOAS system and an appropriate amount of radiant panel area (<50% of the total floor area). To overcome this problem, half of the windows in the exterior private and open offices were replaced with spandrel panels that were modeled as equal in performance with the wall construction. This exchange is discussed in the cost comparison section. This envelope alteration did not completely solve the problem in the exterior offices. In the perimeter spaces, more panel area, about 60% of the ceiling area, was able to completely handle the sensible load. In corner offices and some of the perimeter offices, the amount of air supplied to the space was increased slightly to overcome the sensible load. Refer to the appendix for the complete design of every space. To demonstrate that the radiant panel system can work in every space, figures 9 through 13 show sample radiant panel layouts for each type of space. These select spaces are on the second floor and are served by either AHU-1 or AHU-2. In each figure, the 25

10 blue 2 4 panels are the radiant panels. The radiant panels in the interior offices are able to handle the sensible load, utilizing less than 50% of the ceiling area. Figure 9 Figure 10 26

11 The percentage of ceiling area that the radiant panels cover is about 55%, but there is still adequate room for other ceiling fixtures. The ventilation air is increased to 30 cfm. Figure 11 The radiant panel area in the corner office is about 60% of the ceiling area, but there is still adequate room for other ceiling fixtures. The ventilation air in this room is increased to 60 cfm. This is a large increase, but the office is oriented to the southwest, has a large envelope area, and thus receives a higher solar load. 27

12 Figure 12 The radiant panels in the conference room are able to handle the space sensible load. The panels take up less than 50% of the ceiling area. This is very important in the conference room, because there are extra ceiling fixtures, such as dimmable lights and a projector. 28

13 Figure 13 The radiant panels in the open office area are able to handle the space sensible load, utilizing less than 50% of the ceiling area. Panel Piping For modeling purposes, the building is split into four main zones that correspond to the areas that each AHU serves. These zones are divided into mini-zones that correspond to a particular use group, such as perimeter enclosed offices with south-facing exposure. The purpose of this grouping is to separately control the radiant panel water flow to each mini-zone. Each mini-zone has its own control valve in a control closet that 29

14 can easily be manually adjusted and maintained. The location of the control valves is not only advantageous to maintenance. If the control valves were located above the ceiling in the space, a leak could drip into the space. Locating the control valves in a central closet, however, reduces the risk of a leak developing above the space. Figure 14 on page 32 shows the location of the control closet for AHU-2 on the second floor. The valves are shown outside of the closet for clarity. One pump serves the control closets of each AHU, and these pumps are located in the basement. It would be optimal to place them in the same quadrant on one of the floors to be closer to the control closets, but the pumps take up floor space and have the potential to leak. While detailed controls for the piping system are out of the scope of this report, it is necessary to discuss the basic control sequence. The water for the panels comes from the chilled water system, which supplies chilled water at 44 F. A three-way valve is used to mix returning 60 F water with the chilled water supply to maintain the required 52 F inlet water temperature. One concern about locating chilled water piping in a space is what happens when the air system that normally controls condensation is turned off at night and humidity is allowed to build up. In this case, condensation would occur when 52 F water was initially pumped back into the system in the morning. To prevent this, the three-way valve can control the water temperature to ensure that is above the dew point of the space. Another control valve is placed farther down the supply line and is controlled by a temperature sensor located at the inlet of the panel system of a typical space. Condensate sensors in each room sense any condensation from the panels in the room. The 30

15 condensate sensors are located at the inlet of the piping, near the ceiling, to prevent tampering. If the condensate sensor is tripped, the control valve in the control closet for that mini-zone shuts down the flow to that mini-zone. Figure 15 on page 33 gives an example of how one mini-zone could be controlled. 31

16 Picture not available due to security reasons. Figure 14 32

17 Picture not available due to security reasons. Figure 15 33

18 Mechanical System Energy Analysis The new DOAS/Radiant system was compared to the current VAV system in terms of monthly electrical energy usage and chiller plant load. The load breakdown of each system was also compared. Table 7 summarizes the chiller plant load of each system from the Phase 2 building only. The original VAV system serves the original building. Table 7-Chiller Plant Loads System Chiller Plant Load (tons) Original VAV System 541 DOAS/Radiant Parallel System 122 Figure 16 shows the cooling load breakdown of the original VAV system. Note how small the envelope load is in comparison to the Outdoor Air Load. Figure 17 shows the cooling load breakdown of the DOAS/Radiant System. Here, the envelope load (including solar load) is a much higher percentage of the overall load because the OA load does not factor in to the cooling load estimate. This also demonstrates why eliminating one-half of the office windows contributes so strongly to the overall load on the panels. 34

19 VAV System Cooling Load Breakdown Envelope 3% Electric 32% OA Load 55% Occupant 9.74% Figure 16 DOAS/Radiant Cooling Load Breakdown Envelope 20% Occupant 13% Total Electric 67% Figure 17 35

20 Mechanical System Cost Comparison The first cost and monthly operating costs of the original VAV and DOAS system were compared. These estimates do not take into account parts of the mechanical system that did not change with the DOAS/Radiant system redesign. Because the chiller plant serves both Phase 1 and Phase 2 buildings, the central plant configuration cannot be altered. However, because the DOAS/Radiant system reduces the Phase 2 building cooling load by so much, one of the water cooled electric chillers can be downsized from 650 tons to 350 tons. This also means that one of the cooling towers can be downsized to match the condenser water flow from the smaller chiller. The main first cost savings, however, come from the reduction in ductwork. Table 8 shows a summary of the first costs of each system and the amount of savings that the DOAS/Radiant system produces. Table 8-First Cost Comparison Item Unit Cost-VAV Units-VAV VAV Unit Cost-Parallel Units-Parallel Parallel Savings 650 tonabs Chiller $321,000 1 $353,100 $321,000 1 $353,100 $0 650 ton WC Chiller $229,500 3 $757,350 $229,500 2 $504,900 $252, ton WC Chiller N/A N/A $132,000 1 $145,200 -$145, ton CT $26,796 4 $117,902 $26,796 3 $88,427 $29, ton CT N/A N/A $16,433 1 $18,076 -$18,076 Ductwork 1 $4/ft $744,000 $1/ft $186,000 $558,000 AHU-1 $77,808 1 $85,589 $24,378 1 $26,816 $58,773 AHU-2 $77,808 1 $85,589 $26,366 1 $29,003 $56,586 AHU-3 $77,808 1 $85,589 $35,903 1 $39,493 $46,096 AHU-4 $77,808 1 $85,589 $28,372 1 $31,209 $54,380 Radiant Panels 1 N/A N/A N/A $13/ft ft 2 $636,750 -$636,750 $255,733 All cost estimates are from RS Means, 2003, unless noted otherwise 1: Cost estimate from Mumma. "Ceiling Panel Cooling Systems," ASHRAE Journal: ASHRAE, November

21 The DOAS/Radiant system also saves operating costs. An energy analysis is performed for both VAV and DOAS/Radiant systems using the HAP which shows that the DOAS/Radiant system also has a lower operating cost. Table 9 summarizes these figures, while Figure 18 compares each system visually. Table 9 also compares the energy usage for each system. Table 9-Monthly Electric Operating Costs Electric Cost Electric Usage (kwh) Month DOAS VAV Month DOAS VAV Jan $37,569 $45,143 Jan 540, ,443 Feb $36,159 $41,649 Feb 517, ,503 Mar $41,326 $49,954 Mar 609, ,669 Apr $42,668 $52,720 Apr 616, ,326 May $45,969 $56,471 May 654, ,653 Jun $52,421 $64,663 Jun 768, ,262 Jul $56,219 $68,312 Jul 839, ,718 Aug $54,478 $67,740 Aug 805, ,599 Sep $51,367 $63,633 Sep 744, ,787 Oct $44,134 $53,844 Oct 630, ,301 Nov $42,248 $51,454 Nov 599, ,471 Dec $40,651 $48,280 Dec 596, ,487 Total $545,207 $663,860 Total 7,924,132 9,127,219 Savings $118,653 Savings 1,203,087 The spandrel panels that take the pace on about one-half the windows in the office spaces could present a higher first cost. However, a comparison of costs from Means 2003 Facilities Construction Cost Data shows that the cost of an insulated spandrel panel is about the same as a tinted, doubled glazed window. 37

22 Electric Operating Cost Comparison $75,000 $70,000 $65,000 DOAS/Radiant System VAV System $60,000 $55,000 $50,000 $45,000 Dollars $40,000 $35,000 $30,000 $25,000 $20,000 $15,000 $10,000 $5,000 $0 Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Figure 18 38

23 Conclusions Because the mechanical system for this building cannot adequately meet the intent of ASHRAE Standard , a DOAS system, coupled with radiant cooling panels was implemented in the building. This new system reduces the cooling load on the chiller plant, reduces first cost and reduces electric operating cost. Because of these savings, this new system should be implemented. The DOAS system provides the correct amount of fresh air to each space to meet the intent of ASHRAE Standard and provide a comfortable indoor air environment. Four SEMCO PVS AHUs are used to condition the outdoor air using the common chilled water temperature so that the central chiller plant does not have to be altered. The DOAS system is able to completely handle the space latent load and some of the space sensible load, so that the radiant ceiling panels only have to take care of the excess sensible load. Radiant panels in every space of the building take care of the excess sensible load while usually taking up less than one-half of the ceiling area, leaving the rest of the ceiling for other ceiling fixtures such as lights, diffusers, and A/V equipment. The radiant panel chilled water flow is controlled at control closets on each floor of the building in every quadrant. Each control closets contain valves for mini-zones that the closet maintains. These control closets utilize space that was formerly vertical duct space and provide a maintenance friendly area for control. Though the DOAS/Radiant system saves money in both first and operating costs, there are a few reservations with the system. First of all, in some spaces, namely the 39

24 corner offices, the radiant panels take up so much of the ceiling that the ceiling height should be lowered to the panel height of 8-0 so that the ceiling has a more uniform height. This is a workable solution, but the occupants of the corner offices may feel that their offices are smaller because of the lower ceiling height. Additionally, in these areas it may be difficult to locate a ceiling position for the sprinkler heads because the lights, diffusers, and panels take up most of the ceiling space. Integrating the fire suppression system with the radiant panel piping system may solve this problem, but that is out of the scope of this redesign. Overall, this DOAS/Radiant system provides a thermally comfortable, cost effective mechanical system. 40