Saving Heating Costs In Warehouses

Transcription

1 2005, American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. ( Reprinted by permission from ASHRAE Journal, (Vol. 47, No. 12, December 2005). This article may not be copied nor distributed in either paper or digital form without ASHRAE s permission. Saving Heating Costs In Warehouses By Richard Aynsley, Ph.D., Member ASHRAE T he cost of fuels used for heating large shipping and receiving Twarehouses has increased dramatically. This increase has stimu- lated interest in destratifi cation as a means to improving heating method ignores the influence of infiltration, ventilation or other sources of heat within the space, such as lighting, people and machinery. More complex methods attempt to account for these influences. Heat losses due to ventilation and infiltration can amount to 20% to 50% of the total heat loss from a building. 1 Not accounting for this heat loss due to ventilation in estimating energy savings from destratification can lead to significant errors. What is needed is a relatively simple method that can produce estimates within 10% of measured values using readily available data. The purchase cost and operating cost of fans used to destratify the air also is needed to assess the cost benefit of installing and operating the fans. energy effi ciency. The simplest methods for estimating winter energy savings from destratification in shipping and receiving warehouses focus on the difference in air temperature between floor level and underside of the roof. They assume that after destratification the air temperature is uniform within the space. The estimate of energy savings is the difference between heat lost through the roof at the two temperature differences before and after destratification. This Simplified Estimation of Destratification Savings An example of the most commonly used method for estimating energy savings from destratification follows. Gas consumed by the forced-air gas furnaces during an 89-day period from February through April 2003 was 19,542 About the Author Richard Aynsley, Ph.D., is director of research and development at Big Ass Fan Company, Lexington, Ky. 4 6 A S H R A E J o u r n a l a s h r a e. o r g D e c e m b e r

2 dekatherm. During these 89 days, the heating degree days were 2,220 over a base of 65 F (18 C), and the average outdoor air temperature was 41 F (5 C). The indoor thermostat temperature was maintained at 65 F (18 C). Before destratification the air temperature at the underside of the roof deck 29.5 ft (9 m) above floor level was 89 F (32 C). The roof is estimated to have an average heat transfer coefficient of 0.13 Btu/h ft 2 F (0.74 W/(m 2 K)). The temperature difference before destratification, t t bd, is the difference between the underside of the roof deck, measured on-site, and the average outdoor heating season temperature from climatic records for the location. For the example, this is (89 41) or 48 F (9 C). The temperature difference after destratification, t t ad, is the estimated difference between the underside of the roof deck, taken as the average of air temperatures at floor and roof deck, and the average outdoor heating season temperature from climatic records for the location. For the example, this difference is (( )/2) 41 or 36 F (2 C). The heat lost through the roof before destratification,, is calculated using Equation 1. = (1) = , = 361,920 Btu/h Heat loss through the roof before destratification U = Average heat transfer coefficient for the roof (Btu/h ft 2 F) A = Area of roof (ft 2 ) t i o = Temperature difference through roof before destratification ( F) The heat lost through the roof after destratification, q ad, is calculated with Equation 2. = = , = 271,440 Btu/h (2) Heat loss through the roof before destratification U = Average heat transfer coefficient for the roof (Btu/h ft 2 F) A = Area of roof (ft 2 ) t t ad = Temperature difference through roof after destrattification ( F) The seasonal gas fuel saved, FS, due to destratification is calculated using Equation 3. FS (3) = 2136 (361, ,440)/(100, ) = 2761 dekatherm = Seasonal gas fuel saved due to destratification in dekatherm hrs hs = Heating season hours for the location Q g = Caloric value for natural gas fuel, typically 100,000 Btu/dekatherm η = The efficiency of the gas furnace, typically 0.7 This gas fuel saved can be expressed as a percentage of the gas used before destratification or 100 2,761/19,542 = 14.1%. This method does not account for heat losses due to ventilation, or infiltration through the building envelope, or indoor heat gains from lighting, people and machinery. For large, single-story buildings such as shipping and receiving warehouses, the roof surface typically offers the largest surface for heat loss. While the heat loss by conduction through the walls is relatively constant, the heat transfer by conduction through the roof can vary significantly. In winter, this is due mainly to the insulating effects of accumulated snow, wind, and radiant heat exchange with the sky. It is often difficult to assign a value for the thermal conductivity for envelope elements in existing buildings used in this method. Detailed construction drawings or specifications are rarely available, and often do not represent how the building was actually built. In any case, roof insulation often varies in thickness to provide a slope for rainwater drainage to outlets. Older insulation can be compromised by absorption of moisture, or by compaction of insulation over time. A Better Approach to Estimating Destratification Savings A better way exists to estimate winter heating savings due to destratification. Occupants of existing buildings usually have well-documented data on the amount of gas used during a heating season from their utility company invoices. These invoices also often have the degree days of heating associated with each billing period. If heating degree data is not provided on invoices, it can be obtained from local climate records along with the average outdoor air temperature during a particular period of time. These data can be used to calculate the overall rate of heat loss through the building envelope over a given period. Pignet and Saxena 2 recommend: Btu/h F (4) U = Seasonal gas fuel saved due to destratification in dekatherm A = Heating season hours for the location = Caloric value for natural gas fuel, typically 100,000 Btu/dekatherm t i = Average indoor air temperature ( F) t o = Average outdoor air temperature ( F) The amount of heat released inside the building from sources other than the furnaces, such as lighting, people and machinery, can be estimated as indicated earlier. This is important because, during the warmer winter months, if the thermostat setting is maintained, heat from sources other than the furnaces will provide a larger proportion of the heating to the building. The amount of heat released by various types of lighting D e c e m b e r A S H R A E J o u r n a l 4 7

3 fixtures, appliances and occupants can be calculated from data in Chapter 29 of the 2001 ASHRAE Handbook Fundamentals. 3 Heat from sources other than the furnaces, such as lighting, machinery and people, in the example warehouse during the sample heating season, is about 3.2 Btu/hr ft 2 (10 W/m 2 ). For a 24- hour, three-shift operation during the sample heating season of 89 days, the heat released is 396,441,600 Btu ( kj). From Equation 4: UA = (1,367,940, ,441,600) / ( ) = 49,010,600 Btu/h F The reduced heat load, q ad, due to destratification is calculated using Equation 5. q ad = (UA) (t ibd t iad ) (5) = 49,010,600 (77 65) = 588,127,200 Btu/h q ad = Reduced heat load after destratification UA = Lumped time-averaged heat loss rate for the building envelope (Btu/h F) t ibd = average indoor air temperature before destratification ( F) t iad = Average indoor air temperature after destratification (thermostat F) This is converted to the equivalent quantity of gas G q (dekatherm), by dividing by the caloric value of the gas per dekatherm and the heating system efficiency (0.7). G q Q g G = Equivalent quantity of gas Q = Caloric value for gas Roof/Ceiling η = Efficiency of the heating appliance Expressing this as a percentage of the gas used before destratification, or 100 8,402/19,542 = 43% Estimating Average Indoor Air Temperature A critical factor in destratification is the mixing of indoor air to an even temperature. When this is done, the difference between the temperature of air near floor level and at the underside of the roof deck is usually no more than 1 F (0.6 C). Before destratification, the difference between the temperature of air near floor level and at the underside of the roof deck can be up to 30 F (17 C). Most simple methods for estimating winter energy savings from destratification use a linear profile for air temperature with height above floor level. In reality the air temperature profile often is not linear. The vertical profile of air temperature inside a space is strongly influenced by the distribution and height of heat sources in the building. Most U.S. shipping and receiving warehouses use a number of natural gas-fired forced air furnaces mounted about 20 ft (6 Outlet (6) Height (H) H Floor m) above floor level. On-site measurements were made of the air temperatures in a 33 ft (10 m) high shipping and receiving warehouse with two forced air gas furnaces. These measurements indicated that air temperature was constant to at least 5 ft (1.5 m) below the roof deck (Figure 1 ). 4 shows various temperature profiles for heated spaces. On-site measurements suggest that the vertical profile of air temperature was similar to a D profile. Average indoor air temperature in a space with a D-type vertical profile of air temperature, after thorough mixing, can be estimated by taking the average of two temperature-weighted volumes of the indoor vertical profile of temperature above and below the horizontal part of the profile that aligns with the height of the furnaces above the floor. Whenever possible, it is advisable to measure the vertical profile of air temperature on-site. A New Method for Estimating Destratification Savings In a departure from the method suggested by Pignet and Saxena, 2 the writer suggests that averages of indoor air temperature, t iad, after thorough mixing should reflect the characteristics of the vertical profile of indoor air temperature. In the case study with forced air furnaces well below the underside of roof, this can be done by using Equation 7, to account approximately for the D type vertical profile of air temperature. = ((89 58, ) + (65 58,000 22))/ ( 58,000 ( )) = F 4 8 A S H R A E J o u r n a l a s h r a e. o r g D e c e m b e r H h H o Increasing Air Temperature E A: Heat supplied evenly and close to fl oor level. B: Heat supplied evenly in the space and air is well mixed. C: Heat is supplied evenly in the room with little air mixing. D: Heat is supplied from ceiling or upper part of walls with little air mixing. E: Heat is from a concentrated source in the upper part of the space with little air mixing after. 4 H is the ceiling height H h is the height of heater H o is the height of ventilation outlet Figure 1: Characteristic vertical profiles of indoor air temperature determined by location of warm air supply and mixing. 4 C B A D (7)

4 Month Gas Used in 2003 Gas Used in 2004 HDD in 2003 HDD in 2004 Before Destratification After Destratification Heating Season Heating Season (Dekatherm) (Dekatherm) February 10,358 6,259 1, March 5,798 4, April 3,386 2, Totals 19,542 12,702 2,220 1,960 Average 6,514 4, Table 1: Gas used and heating degree days at the facility. t iad = Average indoor air temperature (after mixing) F t ah = Air temperature above heaters ( F) A fl = Floor area of facility (ft 2 ) H ah = Height above heaters to roof (ft) t ts = Thermostat temperature setting ( F) H bh = Height below heaters to the floor (ft) The average seasonal rate of heat loss, UA, through the building envelope in Btu/ F before destratification can be calculated using Equation 4 t o, is the average outdoor air temperature for the sample heating season. The heat provided by the furnace during the sample heating season is 19,542 dekatherm times the caloric value of natural gas, 100,000 Btu/dekatherm, times the efficiency of the furnace This equals 1,367,940,000 Btu ( kj). Heat in Btu from other sources in the warehouse during the sample heating season is the heat release rate 3.18 Btu/h ft 2 (10 W/m 2 ), times the number of days in the sample heating season 89 converted to hours, times the floor area of the warehouse, 58,000 ft 2 (5388 m 2 ). This equals 393,963,840 Btu ( kj). Using Equation 4: UA = 1,367,940, ,963,840 / = 57,409,705 Btu/h F The reduced heat load, q ad, due to destratification is calculated using Equation 5. This is converted to the equivalent quantity of gas, EQG (dekatherm), by dividing by the caloric value of the gas per dekatherm and the heating system efficiency (0.7) using Equation 6. Expressing this gas fuel as a percentage of the gas used before destratification: 100 5,487/19,542 = 28%. The estimated savings over this sample heating season are 5,487 $0.8 = $4,390. The sample heating season used of 2,220 heating degree days (HDD) to a 65 F (18 C) base cannot be representative of longer term heating seasons for the local climate. An estimate can be made of the savings over a typical heating season, based on 30-year climatic data by expressing the heating demand for a heating season in terms of dekatherm/hdd. In this case the 30-year ASHRAE design season for the location is 4,871 HDD to a 65 F (18 C) base. These data are indicated for cities in the United States in the 1981 ASHRAE Handbook Fundamentals. Estimated savings for the ASHRAE heating season based on 30-year data are $4,390 4,871/2,220 = $9,632. Comparison of Estimated Energy Savings with Measured Savings The data used in the two estimates detailed earlier relate to a 58,000 ft 2 (5388 m 2 ) shipping and receiving warehouse in Middletown, N.Y. The owners of this facility collaborated in a study to evaluate the cost savings of destratification in winter. Gas consumption by the heaters along with heating degreedays (base 65 F [18 C]) were recorded for February, March, and April in 2003 before destratification fans were installed. Similar measurements were made during the same months in 2004 after destratification fans were installed. These data are shown in Table 1. The other facility-related data used here were provided by the facility owners. Given that each winter is unique, direct comparisons cannot be made. To overcome this, one can compare the quantity of fuel required to provide the same indoor condition divided by the severity of the winter season in heating degree days. In this case, for 2003, before destratification, this equals 19,542/2,220 = 8.8 dekatherm/ HDD. In 2004 after destratification, this equals 12,702/1,960 = 6.48 dekatherm/hdd. This represents a 26.4% reduction in gas used. The first method for estimating winter energy savings from destratification gave a savings of 14.1%. This was 46.6% less than actually measured in the case study. The second method by Pignet and Saxena, 2 gave a savings of 43%. This was 62.9% more than actually measured in the case study. The third method by Pignet and Saxena, 2 modified to better account for the vertical profile of indoor air temperature for gas furnaces mount on the walls gave a savings of 28%. This was 6.1% more than the measured value. This is within 10% of the measured value, a criteria often taken as acceptable for engineering estimates of this type given the limitations of the broad assumptions used. Cost of Operating Large Ceiling Fans for Destratification Energy-efficient, large, low-speed ceiling fans can be purchased principally for destratification in winter. A 58,000 ft 2 (5388 m 2 ) shipping and receiving warehouse would typically have five 24 ft (7.3 m) diameter fans operating quietly and continuously at approximately 20 rpm. Velocity at head-height 5 0 A S H R A E J o u r n a l a s h r a e. o r g D e c e m b e r

5 above the floor should be kept below 50 fpm (0.25 m/s) to avoid drafts. Cost of purchase and installation of the five large industrial ceiling fans would be approximately $30,500. At 20 rpm, each fan would use 0.1 kw of power. During the 89-day period of this study these fans would use or 1,068 kwh of electricity, costing approximately $0.06 per kwh, or $ Conclusions Three methods for estimating the winter energy savings from destratification were compared with field data for a three-month sample heating season. The first method was extremely simple but underestimated the actual savings by 46.6%. The second, more detailed engineering approach was developed by Pignet and Saxena. 2 This method failed to account for the vertical profile of indoor air temperature developed when gas furnaces are mounted high on the walls. This method overestimated the energy savings by 62.9%. The third method was the Pignet and Saxena 2 method, modified by the writer to account for vertical profiles of indoor air temperature. This method overestimated savings by 6.1%, but met the criteria of being within 10% of actual measured value. The benefit of the Pignet and Saxena 2 method is that it computes a lumped heat transfer coefficient for the building from seasonal heat loss based on heating energy used. This avoids difficulties in measuring, or otherwise determining, the heat transfer coefficient for the building envelope and other losses due to infiltration and ventilation. The modification suggested by the author to the Pignet and Saxena 2 method, better accounts for the influence of the type of vertical profile of indoor air temperature on the average indoor air temperature. Winter conditions vary from year to year. This complicates year to year comparisons of the efficiency of heating energy use. To overcome this difficulty, it is suggested that comparisons be made on the basis of fuel used per heating degree day when comparing fuel used over different heating seasons. References ASHRAE Handbook Fundamentals, Chapter 26, p Pignet, T. and U. Saxena Estimation of energy savings due to destratification of air in plants. Energy Engineering 99(1): ASHRAE Handbook Fundamentals, Chapter Andersen, K.T Design of natural ventilation by thermal buoyancy with temperature stratification. Proceedings of 6 th International Conference on Air Distribution in Rooms, ROOMVENT 98, 2: Advertisement formerly in this space. D e c e m b e r A S H R A E J o u r n a l 5 1