The Evaluation and Design of the Ventilation System within Mansfield Dam (Lake Travis)

Transcription

1 The Evaluation and Design of the Ventilation System within Mansfield Dam (Lake Travis) SUPPLEMENTAL REPORT Submitted to: Andy Sumner, P.E., Senior Engineer, Dam and Hydroelectric Division Lower Colorado River Authority P.O. Box 8, Buchanan Dam, Texas Prepared by: Clark Hughes, Team Leader Stephen Johnson Matthew Payne Mechanical Engineering Design Projects Program The University of Texas at Austin Austin, TX Summer 1997

2 I. Introduction This report was written to supplement the material presented in the design team's original report entitled "The Evaluation and Design of the Ventilation System within Mansfield Dam (Lake Travis)." The design team has investigated additional solutions to the humidity problems inside Mansfield Dam. The design team presents a closed loop system concept variant. The closed loop system uses doors to seal the main gate operating galleries. Four dehumidification units are used to remove moisture from this closed loop, and a fan circulates air through the loop. This system will cost approximately $36,700 to install and will have annual operating costs of approximately $6,800 per year. The closed loop system will save LCRA up to $590,000 over a period of 15 years. The report concludes with the design team's recommendation that LCRA install the closed loop dehumidification system to eliminate high humidity levels inside Mansfield Dam. The closed loop system offers the simplest solution to dehumidifying the tunnels inside the dam and offers LCRA the most return on its investment. II. Closed Loop System The design team has investigated a closed loop dehumidification system that recirculates dry air through the main gate operating galleries instead of installing a system that uses 100% outside air. The closed loop dehumidification system utilizes four steel doors to seal the gate operating galleries. Four small vapor-compression

3 dehumidification units remove moisture from the air in this closed loop. A fan circulates air and prevents wet stagnant areas in the loop. In developing the four systems presented in the team's original report, the design team assumed that LCRA wanted a system that would use 100% outside air to dehumidify the gate galleries inside Mansfield Dam. The team designed the four systems outlined in the original paper to dehumidify the gate galleries as well as provide cool dry air to the power house. After some discussion with LCRA, the team determined that cooling the power house was not a primary goal and decided to investigate a system that would only accomplish the primary goal of dehumidifying the main gate operating galleries [Sumner, 7/11/97]. Based on this decision, the team has designed a closed loop dehumidification system to control moisture in the main gate operating galleries. The team believes that a closed loop dehumidification system is a superior alternative to the 100% outside air, water cooled, vapor-compression system recommended in the original report. 2.1 System Description The closed loop system uses doors to close off the circuit of tunnels that houses floodgate equipment. Figure 1 shows the locations of these doors. Door 1 closes the gate galleries to ambient air that enters the dam from the tunnel that leads to the dehumidification bunker. Door 2 seals the gate galleries from an open passage that leads to a sump pump in the lower drainage galleries. Door 3 seals off air that may enter the gate galleries from a circular stairway leading to the drainage galleries. Air from the 2

4 lower galleries is very humid and should be kept out of the gate operating galleries. Door 4 seals the gate galleries off from the dam entrance., Sealed doorways Dehumidification Bunker Figure 1. Door placement in closed loop. The final step in sealing the tunnels housing the gate operating equipment is to seal the vertical ventilation shaft located between floodgates two and three. The three other shafts in this circuit of tunnels have already been boarded shut. Sealing this shaft would limit the introduction of humid air from the upper galleries. 3

5 Sketches of the outlines of the doors are shown in Figure 2. Dimensions for these doors and a discussion of manufacturing and installation costs follow Figure 2. R H Y W Outline of doors 1, 3, and 4. X Outline of door 2. Figure 2. Outlines of metal doors. The doors would be made out of a corrosive resistant steel. One possibility is 1.27 cm (1/2 in) thick ART 1. This material costs approximately $ per square meter ($13.57 per square foot) [Skotz, 8/6/97]. Table 1 on the following page gives the approximate dimensions and costs of the doors. The costs were quoted from Skotz, Inc., an Austin welding company. Skotz, Inc. would use a welding crew of two men working 134 hours at $38 an hour to manufacture and install the doors. The closed loop dehumidification system works like a commercial air conditioning unit in that the same treated air is cycled through the system, and small amounts of outside air can be brought into the system for ventilation. Dehumidification equipment, which includes a fan, directs the flow of treated air through the tunnels of the gate galleries in a continuous cycle. Additional air can be introduced into the system by 4

6 adjusting openings in doors one and four. Table 1. Dimensions and costs of sealable doors. Door 1 Door 2 Door 3 Door 4 Total Dimension-W 1.52 m 1.52 m 1.98 m (60 in) * (5 ft) (78 in) * Dimension-R 0.76 m 0.76 m 0.99 m (30 in) * (30 in) (39 in) * Dimension-H 2.13 m 2.13 m 3.51 m (84 in) * (84 in) (138 in) * Dimension-X 2.24 m * (88 in) * * * Dimension-Y 2.87 m * (113 in) * * * $ Material $2,800 (204 ft 2 ) * * * * $ Labor $8,400 (110 hours) * * * * $ Installation $1,800 (24 hours) * * * * Total cost, $13,000 doors * * * * Treated air from the closed loop system remains in the tunnels, so the equipment can continue to treat the air after each pass through the gate galleries. Because the system is retreating the same air, it requires smaller dehumidification equipment with smaller volumetric flow requirements than dehumidification equipment treating 100% outside air. Moisture would continue to be drawn out of the same air in the closed circuit of tunnels. Moisture removed from the air by the units is discharged to nearby drainage trenches. Figure 3 shows several possible locations where there is enough room to put small vapor-compression dehumidification units. The four spaces in the upstream gate galleries 5

7 are each recessed ventilation shafts with an available width, depth, and height of 1.1 m x 0.9 m x 2.4 m (43 in x 36 in x 96 in). Also, a 0.8 m (32 in) diameter fan can be placed in the tunnel near floodgate 24 to facilitate flow through the circuit. Dehumidification equipment Fan Figure 3. Possible locations for dehumidification equipment. Four dehumidification units and one fan offer enough capacity to dry out the gate operating galleries in five and a half days. The closed loop dehumidifiers are each capable of removing approximately 40 gallons of water vapor per day [Vogt, 8/6/97]. Forty gallons corresponds to m 3 of water, which represents 150 kg of water per day per unit. Four units bring the amount of water removed from the circuit to 600 kg per day. The transient time is calculated in the same manner as in the original report. drying time = water on tunnel surfaces rate water is removed The original estimate of the total amount of water on the surfaces of the tunnel 6

8 walls was 3250 kg. Using this estimate, the drying time required for four dehumidification units is 5.5 days. The design team is recommending that LCRA use four CD425 dehumidifier units from EBAC and one Series 09, type LH3 Lo-Noise fan from Hartzell. Table 2 details the specifications of the closed loop system. The team believes that noise will be an issue with any fan or blower used in the dam. Therefore the team chose a Lo-Noise fan that generates 74 decibels [Hartzell, 1997]. The life expectancy of this system is approximately 15 years [Vogt, 8/6/97]. Table 2. Specifications, closed loop dehumidification system. CD425 Series 09, Type LH3 Flow Rate 0.83 m 3 /s (1750 cfm) 5.52 m 3 /s (11700 cfm) Length 1.09 m (43 in) 0.86 m (34 in) Width 0.48 m (19 in) 1.07 m (42 in) Height 1.19 m (47 in) 0.94 m (37 in) Weight 160 kg (353 lb) 95 kg (210 lb) Lowest Dew Point 0.6 C (33 F) NA Electrical Requirements 480 V 480 V Power Consumption 2.98 kw 746 W Noise NA 74 db A Unit Cost $5,680 $1,220 Yearly Operation Costs $1310 $320 The design team has considered several positions to install the closed loop system dehumidification system equipment. Figure 4 indicates these positions. The team recommends that LCRA place three CD425 units in the upstream gate gallery alcoves adjacent floodgates 2, 8, and 24. One additional unit should be placed in the downstream gate gallery alcove that is currently being used to store miscellaneous equipment. The 7

9 Lo-Noise fan should be placed in the upstream gate gallery by floodgate 24 so it blows across the three dehumidifiers. Dehumidification equipment locations Fan Figure 4. Recommended locations of dehumidification units. 2.2 Advantages and Disadvantages of the Closed Loop System There are advantages and disadvantages in using the closed loop system instead of one of the four systems presented in the team's original report. Because the air flow is confined to the gate galleries, the closed loop system will not provide dry air to any other part of the dam. LCRA currently uses air from the dam to cool the power house that houses the turbines and generators. The closed loop system would not provide dry air to the power house. A second disadvantage to the closed loop system is that door four limits access to the gate operating galleries. Major service of the floodgates requiring large machinery takes place about every 10 to 15 years [Fohn, 7/25/97]. In order to move large machinery 8

10 in and out of the galleries, door four may need to be temporarily removed in order to service the floodgates. The primary advantage of the closed loop system, though, is a very significant one. The closed loop system offers the same benefit of reduced maintenance costs at a lower price. Section 2.3 of this report presents an economic analysis of the closed loop system. 2.3 Economic Analysis of the Closed Loop System The design team evaluated the maintenance savings of the closed loop system based on its present worth. Figure 5 on the following page presents the economic analysis of the closed loop system. A discussion of the five parameters used to calculate cost follows: Initial System Cost: EBAC, Inc. quoted the cost of a CD425 dehumidification unit at $5,680. The cost of four units is $22,720 [Vogt, 8/6/97]. Hartzell quoted the cost of the Lo-Noise fan at $1,220 [Hartzell, 1997]. Skotz, Inc. can manufacture and install doors at a cost of $13,000 [Skotz, 8/6/97]. Based on these costs, the design team calculated the initial cost of the closed loop system: C initial C = C + C + C inital dehumidifer fan door = 4 $5, $1,220 + $13, 000 C initial = $36,940 9

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12 Installation cost: The team estimated the labor to install the closed loop system as one man-week. This includes placing the dehumidification units in their proper locations, installing wiring to provide power to the units, and routing condensate from the dehumidifiers to drainage trenches. Based on LCRA s cost of $62,550 per man year [Sumner, 7/17/97], the cost of one man week was calculated as follows: $62, 550 1year 1month 1 man week = = $1, 303 1man year 12 months 4weeks Annual Maintenance Savings: The closed loop systems offers annual maintenance savings of $65,505 per year. These calculations are presented in the original report. Annual Operating Costs: Two elements in the system consume energy: the dehumidifiers and the fan. Power consumption for each dehumidifier was calculated as follows [Vogt, 8/6/97]: Power = 460Volts 8amps = 3680Watts The fan is driven by a motor that consumes 745 Watts [Hartzell, 1997]. Based on these figures, the team calculated the total power consumed by the system: P = P + P total dehumidifier fan P = Watts + 745Watts = 15465Watts = 15. 5kW total 11

13 Based on this power consumption and LCRA s cost of electricity of $0.05 per kilowatt-hour [Sumner, 7/17/97], the team calculated the annual cost of the system: $ hours 365 days Annual Cost = kw = $6, kw hr 1day 1 year Annual Maintenance: The design team estimated the annual cost due to upkeep of the system at $500. This upkeep includes cleaning evaporator coils and changing filters. Based on these costs, the design team used the spreadsheet presented in Appendix E of the original report to calculate a present worth of $586,000 for a closed loop system that operates for fifteen years. Based on the cost of the system, a rate of inflation of four percent and a minimal acceptable rate of return of eight percent, the closed loop system will provide $586,000 in net savings to LCRA. Table 3 presents an economic comparison of the air cooled vapor-compression, water cooled vapor-compression, desiccant, and closed loop dehumidification systems. The systems are compared on the basis of their present worth over five, ten, and fifteen year operating lives. The present worth of a system is defined as the value of the total benefits and costs that a system will provide throughout its operational life. These calculations include adjustments for a rate of inflation of four percent and LCRA's minimal rate of return (MARR) of eight percent. One should realize that the dollar amounts presented in Table 3 represent the 12

14 return on LCRA's investment of purchasing the system. This return has already taken LCRA's MARR of eight percent into account. Thus, the figures in Table 3 represent the difference between the return on a dehumidification and an equivalent amount of capital invested at eight percent. For example, the purchase and installation price of the closed loop system is $38,000. From Table 3 the present worth of a closed loop system that operates for fifteen years is $586,000. Over a fifteen year operating life, the closed loop system is worth $586,000 more than the return on $38,000 invested at an eight percent interest rate. Table 3. Comparison of dehumidification systems. System PW for Five Years PW for Ten Years PW for Fifteen Years Time to Recoup Investment Air Cooled $133,000 $304,000 $445,000 2 years Vapor-compression Water Cooled $140,000 $316,000 $462,000 2 years Vapor-compression Desiccant $1,000 $73,000 $133,000 5 years Closed Loop System $211,000 $418,000 $586,000 1 year 13

15 III. Updated Decision Matrix Table 4 below presents the design team s revised decision matrix. Concept variants are compared on the basis of present worth, humidity, transient capacity, and comfort. Table 4. Decision Matrix. Present Worth Humidity Transient Capacity Comfort Total Possible Points LHS-9000 Desiccant System HCD-9000 EA Desiccant System Water Cooled Vapor- Compression Air Cooled Vapor-Compression Closed Loop System Present Worth Present worth is a measure of how much money each concept variant will save in terms of reduced maintenance costs. The closed loop system has the highest present worth of $586,000 and was awarded the full fifty points for the present worth category. Any concept variant with a present worth of zero at 20 years is given zero points. The 14

16 other concept variants are assigned points consistent with this scale. 3.2 Humidity In the updated decision matrix the LHS-9000 and HCD-9000 EA desiccant systems both score zero out of fifteen points for humidity. The design team based the humidity scores in the original report on "minimum attainable delivered air moisture levels". These humidity levels were based on a closed loop system that treated recycled air multiple times [Munters, 1996]. None of the systems presented in the original report recycled air, so none of the systems were capable of treating air multiple times. Thus, the humidity scores for the desiccant systems in the original report were too large. To correct this oversight, the team has taken the performance characteristics of the DES E/S desiccant wheel system to be representative for the LHS-9000 and the HCD-9000 EA desiccant systems. Although the DES system was eliminated due to its large size, the specifications for the DES system served as the team s best estimate of the condition of the air exiting the LHS and HCD systems. The updated decision matrix reflects the fact that these two dehumidification systems barely meet the minimum requirements of design humidity ratio given in the specification sheet in section of the original report. The desiccant systems both score zero because they will have exit humidity ratios of kg water /kg air (63 grains water /lb dry air ). The design team assumes that the vapor-compression systems will just meet the team's dehumidification system specifications. Thus, these systems will also receive no 15

17 credit for achieving lower-than-required moisture levels. Should a vendor offer a system that provides drier air, then such a system would receive a higher score for lower humidity. However, without such a quote, the design team assumed that the vaporcompression systems would just meet the team s specifications and would receive zero points in the humidity category. The closed loop dehumidification system scored ten points out of a possible fifteen points for humidity. Because the closed loop system recirculates air in the gate galleries and continues to dehumidify that air, the design team was able to use the lowest achievable dew point from Table 2. The humidity ratio corresponding to a dew point of 0.6 C (33 F) is kg water /kg air (28 grains water /lb dry air ). Using the methods outlined in section 5.1 of the original report, the score that corresponds with this humidity ratio is ten points out of fifteen. Humidity points are assigned to any system that can provide a humidity ratio below the specified value of kg water /kg air (63 grains water /lb dry air ). A system providing a humidity ratio of kg water /kg air is given zero points in this category, and two extra points are assigned for each kg water /kg air (7 grains water /lb dry air ) below that value. Since the closed loop system can achieve a humidity ratio of kg water /kg air (28 grains water /lb dry air ), it receives a score of ten. 3.3 Transient Capacity Because the closed loop system does not continuously introduce new air into the tunnel circuit, the design team concluded that the original scoring scheme for the transient 16

18 category needed to be redefined. The original scoring was based solely on flow rate, which is not relevant to a closed loop system. The new scoring system is based on the time required to dry the dam, which is a more representative metric of a system s performance in this category. The design team decided that twenty days is the longest acceptable drying time for any dehumidification system. A system requiring twenty days to dry the dam would receive no points. A system that could dry the dam in zero time would receive the full twenty points. Each of the vapor-compression and desiccant systems were capable of drying the gate gallery tunnel sections in approximately four days, as shown in Section of the original report. These systems each receive 16 points in this category. The closed loop system can dry out the gate galleries in 5.5 days and scores 14.5 points for this category. 3.4 Comfort The scoring scheme for the comfort category is unchanged from the original report. Comfort is based on the output temperature of each dehumidification system. Because the closed loop dehumidification units provide no control over output temperature, the closed loop system receives zero points in this category. Each of the other systems receive full credit because they are all capable of controlling their output temperatures to within ASHRAE standards. 17

19 IV. Conclusion and Final Recommendation The decision matrix indicates that the vapor-compression systems and the closed loop system are superior to the two desiccant systems. The matrix does not, however offer much resolution between the vapor-compression systems and the closed loop system. The design team recommends the implementation of a closed loop system based on cost, simplicity, and ease of installation. The vapor-compression cycles each offer the added benefit of providing cool air to the power house, but they also carry with them significant uncertainties regarding installation costs. Should LCRA elect to pursue a vapor-compression system, a preliminary embodiment design of such a system is presented in Appendix A. LCRA must decide if the benefit of cooling the power house is worth this added economic uncertainty and the higher installation and operational costs of the vapor compression systems. 18

20 References ATS, Inc., Desicair Desiccant Dehumidifiers, Frederick, Maryland, Fohn, Bob, interview, telephone conversation, July 25, Hartzell, Hartzell Rotating Stock, Hartzell Fan, Inc., Piqua, Ohio, Munters, HoneyCombe Dehumidifiers, Modular Units, Cargocaire, Amesbury, Massachusetts, Skotz, Robert, interview, telephone conversation, August 6, Sumner, Andy, interview, site visit Mansfield Dam, Austin, Texas, July 11, Vogt, Bruce, fax concerning CD425 specifications, EBAC, from (512) to Clark Hughes at (512) , August 6,

21 Appendix A: Vapor Compression Embodiment The design team developed an embodiment of a vapor compression system to serve as a guideline for LCRA to use when soliciting bids from manufacturers, should LCRA choose to install a 100 percent fresh air system. This vapor compression dehumidification system employs special features that significantly increase system efficiency and lower operating costs. Figure A1 shows the schematic of the design team s embodiment of the vapor compression dehumidification system. The following paragraphs detail the operation of the various components. cooling water in T = 18 C cooling water out reheated air to dam T = 23 C ω = T = 25 C 3 T = 25 C condenser 2 condenser 1 2 P cooled, dry air T = 10 C ω = P 5 T = 2 C evaporator 1 T = 38 C ω = warm air in condensate out Figure A1. Vapor-compression embodiment schematic. A1

22 The working fluid in this system is R-12, a common refrigerant. Other refrigerants may be substituted for environmental reasons, but such a change would alter the operating parameters and would require that the system be redesigned. The performance would not, however, be significantly affected by such a design change. This vapor-compression cycle is identical in principle to the standard vaporcompression cycle presented in the primary report. The two pressures engineered into a vapor-compression cycle are dependent on the temperatures of the evaporator and condenser. The evaporator temperature is selected such that its temperature is low enough to cool the moving air to the proper designed dew point temperature. The pressure in the evaporator is the saturation pressure that corresponds to that designed temperature. The condenser temperature is chosen as low as possible, but the condenser temperature must be warmer than the cooling medium. The pressure in the condenser is the saturation pressure that corresponds to the designed condenser temperature. The work input required to run a vapor-compression cycle is strongly related to the difference in pressures between the evaporator side and the condenser side of the system. The magnitude of this pressure differential is tied to the temperature difference between the evaporator and the condenser. Therefore, the work input can be reduced by reducing the temperature differential between the evaporator and condenser. The system that the design team has developed takes advantage of this principle by using cool lake water rather than warm outside air to cool the condenser. This feature helps to enable this vapor-compression system to achieve approximately double the efficiency than can be obtained by a conventional system that uses outside air as the cooling medium for the A2

23 condenser. A second performance-enhancing feature designed into this vapor-compression system is the use of a reheat condenser (condenser 1) to eliminate the need for a resistive reheat unit. After the air is cooled by the evaporator, it is reheated by condenser 1 to 23 C (73 F) before it is passed into the dam. The use of this condenser to reheat the air introduces zero operating cost to the system and eliminates the need for the costly 67 kilowatt resistive reheat unit. This feature alone introduces a savings of roughly $30,000 in annual electricity costs. In addition to the second condenser, Figure A1 shows two other items that are not presented in the schematics in the original report. These components are the pressure gauges and the associated control mechanisms that regulate the pressures in the system. The pressure gauge at the throttling valve monitors the pressure on the evaporator side of the system. The signal from this gauge is used by the control mechanism to adjust the throttling valve as a means of regulating the evaporator side pressure. The pressure gauge at the compressor monitors the pressure differential between the two sides of the system. The control unit associated with this gauge fine tunes the compressor speed to maintain the design pressure differential. Table A1 is a state table for this vapor-compression system. The table shows pressure, temperature, enthalpy, entropy, and quality at the five states identified in Figure A1. One extra state, 2s, is included for purposes of calculation and is discussed below. A3

24 Table A1. State table for vapor-compression system. State P (kpa) T ( C) h (kj/kg) s (kj/kg K) x s 656 * * * 371 * * * * * 0.15 The state table identifies design parameters as underlined values. The arrows outline the flow of information to calculate the property value changes from state to state. Asterisks indicate property values that do not need to be calculated for the analysis. The state 2s represents a theoretical state which is the result of a perfect (isentropic) compressor. State 2 takes compressor efficiency into account - the efficiency used in the calculations to determine state 2 for this system is 65% [Howel, 1992]. A refrigerant flow rate of 2.0 kg/s provides 255 kw of cooling (Q L ) to the air as the air passes through the evaporator. The compressor work required to maintain this flow rate and the required pressure differential is 36 kw. The coefficient of performance (COP) for this system is then calculated as QL 255 COP = = = work input 36 The design team notes, however that the actual COP will be somewhat lower than 7.1 due to electrical and mechanical losses in the compressor as well as thermal losses 71. A4

25 throughout the system. A real system designed to the parameters outlined in Table A1 would probably have a COP of approximately 5. A COP of 5 represents considerable improvement over the COP values for standard vapor-compression systems as well as vapor-compression systems that utilize a water-cooled condenser. This improvement is a result of the fact that this system takes advantage of the low temperature of the lake water, whereas most systems use either warm ambient air or industry-standard 85 F cooling water. In summary, a vapor-compression system that utilizes self-reheat capability as well as a properly engineered water cooled condenser offers significant improvements in efficiency over existing systems. Should LCRA elect to utilize a 100 percent fresh air system, the information presented in this appendix should prove helpful during the vendor solicitation process. The design presented in this appendix is not complete, but the ideas are valid and should be incorporated into a final design. Reference Howell, John R., and Richard O. Buckius, Fundamentals of Engineering Thermodynamics, 2nd., McGraw-Hill, New York, A5