HEATING AND COOLING GENERATION AND DISTRIBUTION SYSTEMS

Transcription

1 HEATING AND COOLING GENERATION AND DISTRIBUTION SYSTEMS Central Vs Decentralized Systems Stephan Richter, Ph.D GEF Inginieur, Germany Scot Duncan Retrofit Originality Incorporated, USA Heating and cooling generation and distribution can be among the major contributors to the installations energy waste and inefficiency. With new construction and major retrofits projects as well as major utility modernization projects central systems are economical and shall be considered for the whole installation or a part of it if the heating density is higher than 40,000 MBtu/ (h sq. mile), and the cooling density is higher than 68,700 MBtu/ (h sq. mi.) or 5,750 tons/ (h sq. mile). When not proven otherwise, central plants shall be designed for combined heat and power (CHP) generation or tri-generation (heating, cooling and power generation), which has an enormous potential for increased thermal efficiency, fuel reliability and reduced environmental impacts. Central plants with multiple cooling units are preferred, to permit loss of the largest unit while maintaining at least 65% design capacity. Where the master plan calls for multiple buildings in an area, in the design provide for future expansion of the central plant. Water cooled compressors are preferred over air cooled systems and when feasible rejected heat shall be utilized. Heating Systems With utility modernization projects, existing heating systems currently using steam as a heating media shall be converted to variable-temperature-variable-flow medium (<270 o F) or low temperature (<190 o F) hot water. Thus reducing operation and maintenance costs, and allowing the use of less expensive, more efficient piping material. Systems with condensing boilers are to be designed with lower operating return hot water temperatures, i.e. <55 C (130 F), and use hot water reset to take advantage of the higher efficiencies of condensing boilers. Steam needs shall be evaluated and, when absolutely necessary, provided by local steam boilers. Use onboard steam generators on equipment requiring steam or a small steam boiler just for the year-round steam load. Hot water system experience fewer problems related to expansion and contraction, have fewer corrosion problems, and are much easier to control, all of which result in low maintenance costs. Use boiler with the thermal efficiency 90% E t. Solar-assisted systems shall be considered as alternatives or to compliment conventional boiler systems. Conversion of steam systems may require some changes in the pipe distribution and new requirements for heat exchange equipment at the customer interface and in the central heating plant. In new construction and steam to hot water conversion projects, a so-called indirect compact substation shall be used. Figure 1 shows an example of the building interface installation:

2 The main parts of the customer interface are: 1. DH control for the secondary side (Figure 1, component a) 2. Control valve (Figure 1, component b) 3. Differential pressure control, flow rate control (Figure 1, component c) 4. Heat meter (Figure 1, component d) 5. Plate heat exchanger (Figure 1, component e). Figure 1. Photo of a modern, state-of-the-art DH compact station. Both the DH control for the secondary side (a) and the control valve (b) adjust the secondary system flow according to the outside air temperature. Furthermore, the control valve is used to program a time-dependent adjustment, e.g., the day/night shift, the so called night-time heating reduction. The differential pressure control, flow rate control (c), is used to control the flow rate. Therefore, a certain flow rate limitation is fixed while the differential pressure is variable. When the differential pressure increases, the controller shuts according to its setpoint; similarly when the differential pressure decreases, the controller opens. The heat meter (d) is used both for billing and to control the flow rate. Typically, at OCONUS installations, the utility owns the heat meter while the customer owns the compact station. The plate heat exchanger (e) is shall be specified to decouple the primary DH distribution system from the secondary building side. This is important since the secondary building piping cannot bear up the relative high temperatures and pressures of the primary DH side. Hot water in the building can be supplied either to radiators or to coils of the air heating systems. An 2

3 admix control reduces the flow temperature in the secondary loop according to the ambient temperature. The secondary loop can handle different control programs, e.g., for weekend or nighttime heating reduction. With the central heating system, domestic hot water preparation is also an admix operation controlled by the DH control unit for the secondary loop (e). In this loop, the lowest temperature is limited by hygienic conditions. Thus, the lowest flow temperature in the DH system is limited to 160 F (70 C) since the domestic hot water must have a temperature higher than 140 F (60 C). The flow temperature must periodically be raised to 175 F (80 C) to boost the domestic hot water to 160 F (70 C) (the required temperature to kill legionella) for thermal disinfection. Pipes for Hot Water Distribution Systems. Pre-insulated bounded pipes (Figure 2) shall be used for both medium and low temperature hot water systems. These pipes consist of a steel medium pipe and a plastic (i.e., polyethylene) jacket pipe. The insulation between the two pipes is made from a polyurethane (PUR) heat insulation foam. The pipes are pre-insulated in the factory and the PUR foam is a rigid material that bonds the outer jacket with the internal (medium) pipe. Because of the adverse consequences to the energy efficiency and pipe integrity of leaks, the pipes will include an integral leak detection and location system. Using these pipes will reduce the number of manholes and the size of the manholes which are currently about 1515 ft. In addition, the manholes can be covered by an iron cap. Currently the existing manholes are open due to ventilation requirements. Thus surface water and rain can easily flood the manholes and reduce the lifetime of the pipes due to external corrosion. Figure 2. Photo of pre-insulated bounded pipes (pipe on the left is unused and is equipped with a leak detection system; pipe on the right was in use for about 30 years in a DH system with sliding flow temperatures [about 80 C/130 C]). The most important limitation of the pipe is its maximum temperature restriction of 285 F, which minimizes the aging of the PUR foam caused by exposure to the high temperatures. 3

4 Figure 3. Trench/canal for a buried pre-insulated pipe The pipes are buried in frost-free depth in an open trench (Figure 3). After the laying of the pipe with a length of some 15 to 30 ft, the single pipes are connected through welding. Those weld joins are tested with radiation and evacuation tests. Afterwards, the PE jacket pipes are connected with shrinking bushings. Finally, the space between the medium pipe and bushings is foamed in place. Figure 4 shows different precast fittings, elbows and branches. Finally, the trench is filled with sand and compressed to bury the pipes. When the pipes are completely buried, the trench is further filled and prepared for the desired surface, which may be a street, pathway or grassland. Figure 4. Pre-cast fittings and elbows of pre-insulated bounded pipes. It is absolutely critical that QC and QA be provided during the pipe installation to ensure the proper installation. Key issues are the welding quality, bevel seams, the bushings and the foaming in back, the sand bed, the proper connection of the leak detection system and the expansion cushions. 4

5 HOT WATER SYSTEMS The use of hot water has a significant impact on energy consumption. Therefore, it is essential to reduce hot water use. The following should be standard procedure: The following limits of Hot Water temperatures shall be set at points of use: 1. Administrative use or general cleaning: 35 C /95 F. 2. Shower facilities: 43.3 C / 110 F. 3. Automatic dishwashing in dining facilities: 60 C / 140 F. 4. Final rinsing of dishes and kitchen utensils in dining and diet kitchen: 82.2 C / 180 F. 5. Hot water is not authorized in the following areas: a. Retail areas, except for food handling areas. b. Warehouses. To maximize water use efficiency and energy conservation: Shower heads with a flow rate of less than 2.5 gpm shall be used. There are two basic types of low-flow showerheads: aerating and laminar-flow. Aerating showerheads mix air with water, forming a misty spray. Laminar-flow showerheads form individual streams of water. In a humid climate, use of a laminar-flow showerhead is preferable because it won't create as much steam and moisture as an aerating one; For dishwashing, use low-flow pre-rinse spray valves with a flow rate of 1.6 gallons per minute or less, and a cleanability performance of 26 seconds per plate or less, based on the ASTM Standard Test Method for Performance of Pre-Rinse Spray Valves. Cold water shall be used in lieu of hot water whenever possible. COOLING SYSTEMS ASHRAE Standard provides mandatory efficiency requirements for cooling equipment (section 6.8). Equipment must meet or exceed the seasonal energy efficiency ratio (SEER) or energy efficiency ratio (EER) for the required capacity. The cooling equipment should also meet or exceed the integrated part-load value (IPLV) where applicable. However, requirements to use in Federal Buildings only Energy Star or FEMP designated products are considerably more stringent than Standard ( For central refrigeration air-conditioning systems, provide freeze protection for all exposed piping and components for outdoor packaged chiller units. Air handling units of the single zone building cooling systems in hot and humid climates (zones 1a, 2a, and 3a) shall be designed with a reheat coil to ensure that the supply air temperature is above the dew point. Variable temperature chilled water single zone units are also not acceptable without a reheat coil. Use the following chilled water systems design and retrofit guidance to achieve better performance and energy efficiency. 1. Use Variable Frequency Drives (VFD s) for the Primary Chilled Water pumps above 5 HP 2. A VFD allows the equipment speed to be controlled to match the need of the loads it serves, rather than running at full speed any time it is running. Installation of VFD s on the primary chilled water pumps will allow the speed of the pumps to be varied in response to changes in the cooling loads and chilled water 5

6 system temperature differentials. Since the power demand of the primary chilled water pump motor varies approximately to the 2.5 power with speed (it is not quite a cubic relationship in the installed world), reducing the pump speed to 70% when the chiller load is around 70% will result in a primary chilled water pump energy savings of approximately 55% to 60%. At low loads, reducing the speed to 50% when the chiller load is around 40% or less will result in a primary chilled water pump energy savings of approximately 80%. Since the chillers spend the majority of their time operating below 70% of their design cooling capacity, the system energy savings can be substantial. 3. Use Variable Frequency Drives (VFD s) for the Condenser Water Pumps A VFD allows the equipment speed to be controlled to match the need of the loads it serves, rather than running at full speed any time it is running. Installation of VFD s on the condenser water pumps will allow the speed of the pumps to be varied in response to changes in the cooling loads. Since the power demand of the condenser water pump motor varies approximately to the second power with speed, (due to the fixed minimum lift conditions of an atmospheric cooling tower, the savings does not correspond to a cubic relationship) reducing the speed to 70% when the chiller load is around 70% will result in a condenser water pump energy savings of approximately 50%. At low loads, reducing the speed to 50% when the chiller load is around 50% or less will result in a condenser water pump energy savings of approximately 75%. We typically do not recommend taking the condenser water pump speed down below approximately 50%, as the savings are minimal below that level, and we need to pay attention to the minimum allowable flow rate over the cooling towers, which is typically in the 50% of design flow range. The pump energy savings ratio for the condenser water pump is slightly different than it is for the primary chilled water pumps, as the condenser water pumps have a fixed minimum lift required to move water up over the top of the cooling towers, and there is no static regain for water falling through a cooling tower as there is with primary chilled water pumps operating in a closed loop. Since the chillers spend the majority of their time operating below 70% of their design cooling capacity, the system energy savings can be substantial. 4. Cooling Load Based Optimization. Cooling load based optimization strategies should be incorporated into the control routines for the CHWS temperature set-point, the chilled water differential pressure setpoint and the condenser water temperature setpoint (for water cooled equipment) to reduce chiller and pumping system energy waste, while improving the control system response at the cooling loads. The load based optimization strategies should also incorporate chiller staging routines that take best advantage of the installed equipment. Self tuning loops should be incorporated that will adjust the system to accommodate continuous changes in the load and weather. One such system which incorporates these concepts is the Load Based Optimization System (LOBOS), which takes data from the air handling units that are controlled from the same DDC control system as the chiller plant, and uses this data to raise the chilled water supply temperature as high as possible, to save chiller energy, while lowering the chilled water system differential pressure as low as possible, to save pumping system energy. These resets are accomplished while still maintaining the required supply air temperatures at the air handling units feedback from the cooling loads is required for best system performance. The system also resets the condenser water supply temperature setpoint to reduce chiller energy by lowering the condensing pressure of the refrigerant, without wasting cooling tower fan energy. Increasing the chilled water supply temperature (within reason, comfort and humidity control limitations) can reduce energy consumption on constant speed centrifugal chillers by up to 20%, and by up to 40% or more on variable speed centrifugal chillers. 6

7 Reducing the chilled water system differential pressure (within reason, comfort and humidity control limitations) can reduce energy consumption on variable speed chilled water pumping systems by up to 50% at light loads. Differential pressure is what forces cold water from the chiller plant through the chilled water distribution piping system to the air conditioning systems to provide cooling to the end use loads. A system does not need the same differential pressure when it is 70 F and dry outside as it does when it is 90 F and raining outside. A reset strategy based on the needs of the end use loads can reduce pump energy by 40% or more, depending upon the diversity of the loads. Efficiency and load based reset strategies should be incorporated into the control routines for the cooling towers. Reducing the condenser water supply temperature (within reason and equipment limitations) can reduce energy consumption on constant speed centrifugal chillers by up to 30%, and by up to 60% or more on variable speed centrifugal chillers. An additional benefit associated with resetting the chilled water supply temperature and differential pressure is that cooling coil control valve response and thus system temperature control are much better when these strategies are incorporated. When cooling loads are light but the differential pressure setpoint is at 40 PSID, and the CHW supply temperature is at 42 F, most of the cooling coil control valves will be operating at or near shutoff. Valves operating near shut off provide very imprecise temperature control, resulting in large swings in the leaving air temperature from the air handling units, and over- and under-dehumidification. If the CHW supply temperature is increased and the differential pressure is decreased, the valves will open up and operate closer to mid-stroke or even further open, where their control can be very accurate. Figure 5 shows the trend of supply air temperature vs. supply air temperature setpoint Figure 5. Supply air temperature vs. supply air temperature setpoint. Red = Supply air temperature, Green = Supply air temperature setpoint During the day, the CHWS temperature setpoint and differential pressure setpoint are reset based on the loads. At approximately 19:00, the system is operated in a fixed temperature and DP setpoint mode. It can be seen that the supply air temperature control stays within approximately 0.5 F until 19:00, when the swings exceed 5 F, due to the CHW control valve at the AHU being near shut off. 7

8 Optimized chiller equipment staging can reduce the energy consumption penalty imposed by running too much constant speed equipment by over 50%. Figure 6 shows an example of Load Based Optimization System (LOBOS) reset strategies in operation. Temp Scale kw/ton Scale Condenser water return temperature Condenser water supply temperature Chiller kw per ton of cooling Chilled water return temperature Chilled water supply temperature Figure 6. Results of a test run to see what effects changing the condenser water temperature setpoint would have on VFD chiller system efficiency. The red line is the condenser water temperature with a system using the Load Based Optimization System (LOBOS) control system, which determines less wasteful, more efficient operating points for the HVAC system based on the actual cooling loads of the facility. The blue line is the chiller efficiency in kw per ton. The green line is the chilled water supply temperature. The test shown in the trend log screen shot, shows that the chiller energy efficiency is running at approximately 0.33 kw per ton, prior to the start of the test. This is excellent efficiency, as most chillers installed today operate between 0.60 and 1.5 kw per ton of cooling. With LOBOS, the condenser water temperature is running at approximately 68 F, while the chilled water temperature is running at approximately 50 F. We manually raised the condenser water setpoint to 80 F from the automatically controlled setpoint of 68 F to determine what effect a normal operating strategy would have on chiller system performance. Many facilities routinely operate their condenser water systems at between 80 F and 85 F, which are the typical design points for chillers when they leave the chiller factory. As can be seen, the chiller efficiency was made dramatically worse when the CDWS temperature was raised by 12 degrees, increasing from 0.33 kw per ton to 0.45 kw per ton, using 36% more energy to deliver the same amount of cooling. The chiller energy increased from 332 kw to 452 kw, an increase of 120 kw. There was a savings in cooling tower energy of approximately 40 kw since we raised the condenser water temperature setpoint, but the net effect on the system was an increase of 80 kw, or approximately 21% more energy to provide the same level of cooling. When the condenser water temperature was released back to LOBOS operating controls, the efficiency was returned to the previously obtained levels. This also shows that a facility can be designed with excellent cooling equipment, but if it is operated and controlled in a normal manner, the efficiency can suffer in a rather dramatic fashion compared to the potential efficiency. 8

9 LOBOS Control Screens Chiller Plant Tuning Graphic - Simple Four adjustments - how fast the Differential Pressure and Chilled water supply temperature are adjusted when the loads are increasing and decreasing. AHU Tuning Graphic - Simple Six adjustments Setpoints for fan speed and return air temperature setpoint, and how fast the static pressure and supply air temperature setpoints are adjusted when the loads are increasing and decreasing. 9

10 AHU Setup Screen This screen lets you set the level of importance for each AHU a lab unit gets greater weight than a less critical unit, the further out units get more weight than the close in units, and big AHU s get more weight than smaller AHU s. Use High Efficiency Dehumidification Strategy. When air handling units are being replaced, or where there is a need for dry, non-saturated supply air, the installation of High Efficiency Dehumidification Systems, in either Dedicated Outdoor Air System (DOAS) or Variable Air Volume (VAV) design iterations should be utilized. Although the airside of the HVAC systems is not a direct chiller plant energy efficiency project, the design and operation of the airside has a substantial impact on the performance of the chiller plant. The chiller plant is operated to respond to the needs of the end uses, (the air handlers serving the occupied spaces) so if the end use air handlers are inefficient at transferring heat due to their design or small cooling coils, colder water and higher flow rates of chilled water will be required. This can create a very inefficient cooling system, and can contribute directly to the Low Delta T Syndrome. Air distribution systems that have normal designs and cooling coil sizes have a detrimental effect on chiller plant performance. For example, a cooling coil that requires 42 F chilled water supply temperature and provides a 54 F return water temperature to meet cooling loads and provide adequate dehumidification will cause the chiller plant to use 25% to 40% more energy than a cooling coil selected to provide the design supply air temperature and the design dehumidification load with 50 F chilled water supply and 70 F chilled water return will require. If a HEDS designed AHU is utilized, the chiller plant savings will increase by another 25% or more. When design dehumidification loads exist, there is typically the need for some form of reheat energy to temper the air entering the spaces. Reheat energy typically comes from a new source of energy, typically a boiler plant connected to steam or hot water sourced re heat coils, or electric strip reheat coils. Cold, saturated air entering a humid space can cause condensation to form on surfaces that it comes in contact with, creating wet spots and all of the problems that go along with water on surfaces in occupied areas, so some form of re-heat or a method to decrease the relative humidity of the air entering the space is required. 10

11 For a system such as a DOAS design, or a VAV system serving a barracks facility, the design day chiller plant and boiler plant energy savings associated with a HEDS system can be in excess of 50% when compared to a typical design. High Efficiency Dehumidification System Supply air temperature is 10 F above the dewpoint temperature. For DOAS-HEDS, the supply air temperature can be 20 F above the dewpoint temperature. Variable Volume System Performance Comparison "Normal HVAC" vs. "HEDS" Entering Conditions Leaving Conditions % Design CFM DB WB DB Dewpoint 100% % % % % Design CFM Normal AHU System Chiller Plant Load + Reheat Energy (BTUH) High Efficiency Dehumidification System (HEDS) Chiller Plant Load + Reheat Energy (BTUH) HEDS % Energy Savings 100% % 75% % 50% % 25% % 11

12 DOAS Performance Comparison "Normal DOAS" vs. "HEDS-DOAS" Entering Conditions Leaving Conditions CHWS Temperature at Unit DB WB DB Dewpoint 45 F CHWS Temperature at Unit Normal DOAS System Chiller Plant Load + DX Subcooling + Reheat Energy (BTUH) High Efficiency Dehumidification System (HEDS-DOAS) Chiller Plant Load + DX Subcooling + Reheat Energy (BTUH) HEDS % Energy Savings 45 F % High Efficiency Dehumidification System Energy Savings vs. Design 100% reheat to 65F/68F Typical for DOAS, Barracks, labs, manufacturing, underfloor air distribution systems, etc. High Efficiency Dehumidification System Energy 100% Reheat to 65F/68F 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Percent of Design Airflow Percent of Cooling and Re-heat Energy Saved 12

13 High Efficiency Dehumidification System Energy Savings vs. Design 25% reheat to 65F/68F Typical for facilities that need minimal reheat. High Efficiency Dehumidification System Energy 25% Reheat to 65F/68F 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% Percent of Design Airflow Percentage of Cooling and Reheat Energy Saved Install a Continuous Commissioning, Monitoring and Verification System to Maintain Savings Persistence of the Chiller Plant and HVAC Systems. Essentially, this consists of installing accurate temperature sensors on the inlet and outlet of each chiller and heat recovery unit evaporator and condenser, installing an accurate low range differential pressure transducer across the evaporator of each chiller and heat recovery chiller and using the differential pressure to calculate the flow rate through the chiller, installing a kw transducer on each chiller, installing kw transducers on each pump and cooling tower fan and air distribution fan, or, if they are powered by Variable Frequency Drives, obtain the kw from the VFD over an RS 485 or other network connection. Air handlers that are equipped with economizers must also be equipped with return air and mixed air temperature sensors, and accurate supply air temperature sensors. If there is budget available, large AHU s should have DP transducers installed across the coils, and inlet and outlet water temperature sensors so that tonnage at the AHU s can be calculated as well. Software would then be implemented to calculate the efficiency of each piece of equipment, and then trends and alarms would be set up so that there would be a historical database of equipment and system performance. The alarms would be triggered when the system or equipment started operating outside the boundaries set during system startup. Install Water Source Heat Pumps (WSHP s) To Augment the Capacity of the Hot Water Boiler, and To Reduce the Cooling Load on the Existing Chiller Systems When Heat Is Required. A 20 ton WSHP can deliver approximately 320,000 BTUH of heat when it is loaded up. In many cases, the chilled water temperatures that are delivered to the facilities may be too high to adequately cool the facility, and may also be too high to adequately dehumidify the building. In these cases, small WSHP s can be used to reduce the CHWS temperature entering the building while simultaneously dumping the excess heat into a water storage tank for the domestic water loads. In cases like these, dry coolers should be installed to act as a heat rejection source for the WSHP when the domestic water system can no longer accept any more heat, but there is still a need for the cooling system capacity/temperature to be augmented. Develop an HVAC and Control System Design Criteria for New and Retrofit Projects. 13

14 A design standard should be developed with substantial input from the operating staff, and from Subject Matter Experts familiar with dehumidification and control system design and function in the field. Once implemented, the savings associated with standardized system designs and installations will result in millions of dollars per year in avoided energy, maintenance and biological remediation costs. A design standard will also lessen the costs to perform system design and engineering reviews, as standardized designs will be able to be utilized by the local design engineering firms. This will have the added benefit of educating these firms for work in the private sector, allowing their clients to reduce the lifecycle costs of their facilities, and helping America to reduce energy waste and remain competitive with other nations. AHU and Their Cooling Coils Retrofit Strategies 1. The AHU should be configured in a blow thru configuration, rather than a draw thru configuration. The fans should be upstream of the cooling coils, blowing through them (see Table 1). 2. Provide cooling coils that meet the desired leaving air temperatures with 50 F entering chilled water temperature and as close to 70 F leaving chilled water temperature as possible. 3. The maximum cooling coil face velocity should be 350 feet per minute. For blow through and 300 feet per minute for draw through. 4. The desired fans are direct drive plug fans. 5. The cooling coils should be 8 rows deep. 6. The cooling coils should have 12 fins per inch. 7. The maximum leaving air temperature should be 55 F. 8. The minimum fin thickness is 0.008" 9. The minimum tube wall thickness is 0.028" 10. The minimum tube diameter is 5/8" 11. The coils shall be equipped with individually replaceable return bends of no less than 0.035" thickness 12. Hairpin return bends are not acceptable. 13. Provide stainless steel coil casings and intermediate tube supports 14. The maximum height between drain pans should be 24". Provide coils that have drain pans that completely pass through the cooling coil finned surface area. 15. The coil air pressure drop should be lees than 0.8" wc. 16. The coil water pressure drop should be less than 15'. 17. Provide low pressure drop (2 PSID maximum) automatic control valves for the cooling coil duty. Desired valves are pressure independent characterized port ball valves - Belimo or equivalent, designed for outdoor duty. Provide with a sun/rain shield for the actuators. 18. Insulation system should be vapor tight and aluminum skinned. 14

15 Table 1. AHU blow thru configuration. Typical Design Near-Optimal Design Parameter Draw Thru 6 Row 550 FPM Blow Thru 8 Row 350 FPM Mixed Air Temperature DB 77 F 77 F Mixed Air Temperature WB 64.8 F 64.8 F Coil Entering DB 77 F 79.5 F Coil Entering WB 64.8 F 66 F Coil Leaving DB 52.5 F 55 F Coil Leaving WB 51.5 F 54 F Air Handling Unit Supply Air Temperature ( F) Chilled Water Supply Temperature ( F) Chilled Water Return Temperature ( F) Chilled Water Temperature Differential ( F) CHW TD % Change 169% Cooling Coil Water DP (ft H2O) CC DP % Change -21% Cooling Coil Air DP (InWC) CC DP % Change -49% Coil GPM Required Coil GPM % Change -65% Total BTUH BTUH % change (Iess over-dehumidification) -6% 15