International Energy Agency Energy Conservation in Buildings and Community Systems (ECBCS) Programme Annex 46, U.S. Army Installations Management Command (IMCOM), US Department of Energy, Ft. Lee and The PERTAN Group September 19, 20, 2009 Fort Lee, VA Presented by Scot M. Duncan, P.E. Vice President Retrofit Originality Incorporated 21382 Countryside Drive Lake Forest, CA 92630 949 370 8582 www.roi-engineering.com sduncan@roi-engineering.com
J.1 Use Variable Frequency Drives (VFD s) for the Primary Chilled Water pumps above 5 HP 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 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.
Primary Pump VFD Energy Curve Primary Pump Energy Use - Variable Speed vs. Constant Speed Power Draw % 100 80 60 40 20 Constant Speed Theoretical Curve Probable Curve 0 1 2 3 4 5 6 7 8 9 10 Speed %/10
J.2 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.
CDWP VFD Energy Savings 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.
Condenser Pump VFD Energy Curve Condenser Pump Energy Use Variable Speed vs. Constant Speed Power Draw% 100 80 60 40 20 Constant Speed Theoretical Curve Probable Curve 0 1 2 3 4 5 6 7 8 9 10 Speed %/10
J.3 Cooling Load Based Optimization Most Mechanical Engineers have never operated an HVAC system, and were never properly trained in the art and science of energy conservation and building controls. Most control system designers are not versed in energy conservation design strategies. Most operating engineers don t have PhD s, and don t want HVAC and control systems that require a PhD to understand.
Overview Merely having great equipment and a fantastic system design does not guarantee that a system will be energy efficient. Typical energy efficient control strategies leave a substantial amount of savings on the table. Control system strategies that cannot be easily understood by the operating staff will quickly be disabled, and the system will revert to manual operation, and savings will be lost.
Summary Poor control strategies can waste more energy than great equipment can save. The controls must adapt to changing end use loads and conditions and allow the operating staff the ability to tune the system to meet the needs of the various connected loads.
Sample VSE/LOBOS Design and Control Logic Results 6/30/07 Peak OSA Temperature = 103 F red line Average TOTAL chiller plant efficiency = Less than 0.50 kw/ton yellow line Chiller, Approx 0.35 kw/ton, blue line Cooling Tower, Approx 0.08 kw/ton, white line Chilled Water pump, Approx 0.04 kw/ton, pink line Condenser water pump, Approx 0.03 kw/ton, green line
AHU Summary coils that provide a 10 F to 13 F approach temperature.. 100 degrees outside 52.7 F CHW supply temperature Max 54.5 F supply air temp. on all AHU s At 100 F outside, we have a 1.8 degree approach temperature between the CHWS temp and the supply air temp leaving the coils. This allows very high CHW temperatures and thus very high system efficiencies. This compares to typical designs that use
Chiller Plant Snapshot Chiller @ 0.32 kw/ton Total chiller plant @ 0.45 kw/ton, including pumps, chillers and CTF s 100 F OSA
Complete day performance: tons and kw/ton 6-13-07 Blue = total plant (0.39 to 0.51 kw/ton), including all pumps, CH s and CT s Yellow = load in tons (380 to 530 tons)
Control to the Loads The system must respond to the needs of the loads, be it a single floor in the winter, or the entire facility in the heat of the summer. If the loads are not being properly cared for and met, the system is not doing its job. As soon as occupant complaints start coming in, the system will go to manual operation, if it is not easy for the Engineering Staff to tune and adjust it.
The Basics Don t Change To save substantial energy with any system, you have to incorporate central plant and airside optimization software. Central Plant Optimization routines must look at the site loads without knowing AHU load information, you cannot maximize energy savings, or respond to load changes properly. AHU Optimization routines are required to minimize energy use and promote occupant comfort! The System Must be Operator Friendly If the person that designed it cannot explain it in an understandable manner to the people that operate the facility, it is too complex.
Load Based Optimization on AHU 56 F at 0.49 INWC @ 10 th Floor AHU@0.24 kw/t & Total HVAC sys @ 0.64 kw/t Penthouse fan system serves all 10 floors with 0.49 static pressure. Old setpoint was 4 + 75% load on plant
Total Plant @ 0.33 kw/ton and Total HVAC System at 0.52 kw/ton 55% plant load
Total Plant Efficiency @ 0.38 kw/t Total HVAC System (including Supply and Return Fans) @ 0.51 kw/ton (VSE/LOBOS) 33% plant load
Don t just look at kw per ton! It can be extremely misleading! Low Temp Air Extreme Test - 39 F Air, 0.15 INWC static pressure 0.03 AHU kw/ton, 0.49 Total HVAC system kw/ton While this looks impressive from a kw/ton basis, the dehumidification load to provide 39 F air temperatures increased the tonnage dramatically, so overall energy use went up, not down.
J.3 Cooling Load Based Optimization, cont. 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.
Load Based Optimization System (LOBOS) 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.
Load Based Optimization System (LOBOS) 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.
Load Based Optimization System (LOBOS) 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. 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.
Load Based Optimization System (LOBOS) 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.
Load Based Optimization System (LOBOS) 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.
Load Based Optimization System (LOBOS) 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.
Load Based Optimization System (LOBOS) 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. This is shown in the following trend of supply air temperature vs. supply air temperature setpoint
LOBOS On, then turned off Red = Supply air temperature, Green = Supply air temp setpoint
Notes for previous page 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. Optimized chiller equipment staging can reduce the energy consumption penalty imposed by running too much constant speed equipment by over 50%.
Temp Scale kw/ton Scale Condenser water return temperature Condenser water supply temperature Chiller kw per ton of cooling Great with LOBOS, 36% worse W/O LOBOS Chilled water return temperature Chilled water supply temperature
Discussion of Previous Page This test was 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 the most 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.
Discussion, continued The first test, shown in the preceding page, 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.
Discussion, continued 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.
Discussion, continued As can be seen, the chiller efficiency was made dramatically worse, 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 by letting the temperature rise, 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.
Discussion, continued 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. Remember Bad Controls Can Waste More Energy Than Good Equipment Can Save.
Control Strategy Make it understandable. Make it consistent/repeatable from plant to plant. Make it tunable by the Operator (within limits) Make it self tuning.
J4. LOBOS Control Screens - Plant 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.
J4. LOBOS Control Screens - AHU 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.
J4. LOBOS Control Screens - AHU 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.
J4. LOBOS Control Screens Control system consistency from HVAC system to HVAC system should be a long term goal. If every tank, rifle or HUMVEE operated differently from the last one, how effective do you think our soldiers would be? HVAC systems are the same the human interface and behind the scenes control logic should all have the same look and feel, and the controls strategies should be the same: When the cooling loads are high, provide higher pressures and colder water and air to meet the loads. When the cooling loads are low, provide lower pressures and warmer water and air to save energy.
J.5 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.
J.5 Use High Efficiency Dehumidification Strategy. 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.
J.5 Use High Efficiency Dehumidification Strategy. 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. 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.
D.2.2.8 Inefficient Dehumidification Systems (Inefficiency) Supplying nearly saturated 55 F air into a humid space results in condensation in the spaces, which can lead to mold and mildew problems. Electric or hot water reheat systems are sometimes employed to decrease the RH of the supply air to the spaces to reduce the potential for condensation, but these systems are typically turned off in the field to reduce energy use. Inefficient air dehumidification results in significant repair and maintenance costs. The cost of mold remediation outweighs the total first cost of the HVAC system by an order of magnitude over the life of the HVAC system. Changing dehumidification/reheat system design strategy can reduce energy consumption and cooling and reheat loads, improve IAQ, and reduce the potential for mold growth in occupied spaces. These factors will reduce the level, frequency, and expense of remediation that seems to occur on a regular basis.
D.2.2.8 Inefficient Dehumidification Systems (Inefficiency) cont Air supply systems that must provide both cooling and dehumidification are typically designed in a way that wastes chiller and boiler plant energy. Using standard AHU design strategies that dehumidify the air and then reheat it to reduce RH can waste 50% or more of the chiller plant and boiler plant energy when compared to using AHU designs based on high-efficiency dehumidification systems (HEDS) design strategies for dedicated outside air systems (DOAS) or variable air volume (VAV) systems combined with a separate heating and cooling system
Moisture Control In order to reduce the potential for biological issues, the building envelope and HVAC system must be designed in concert to properly control moisture. Relative Humidity in the facility must be maintained within acceptable boundaries to promote occupant comfort and health, while reducing the potential for biological issues.
Dehumidification/Reheat Systems Many areas in the continental United States only have dehumidification problems in the summer. Other, more far flung locations require continual dehumidification and reheat to maintain comfort conditions and reduce biological issues.
Dehumidification Systems Problems Billions of dollars in damage occurs each year due to biological growth caused by high Relative Humidity in facilities. This is preventable, and can be energy efficient at the same time!
Dehumidification Systems Problems To reduce relative humidity in the spaces, the air must be dried out, typically by cooling it to 45 F to 55 F, then re-heating it to around 65 F typically by using electrical or hot water re-heat sources. In this manner, very cold, 100% saturated supply air does not enter the spaces. If cold, saturated air enters a humid room, biological issues will likely follow.
Dehumidification Systems Problems High RH can be caused by the desire to save energy by reducing the dehumidification (cooling) and re-heat load on the system. If you cool the air less, you save chiller energy, but you increase the RH in the space. If you dry the air out by cooling it to the correct level, but then do not re-heat the air (to save heating energy), the cold saturated air will encourage condensation everywhere it touches.
Dehumidification Systems Problems At one site that we visited, biological growth issues created the need to completely re-build one of the living facilities. The new HVAC system uses chilled water for dehumidification and electric strip heaters (at 1.0 kw per room!) for reheat to lower the relative humidity of the spaces. While the design of the system will work, the annual energy consumption of this one 250 room facility will increase by approximately 1,500,000 kwh per year. The strip reheat coils will soon be disconnected to try and comply with energy conservation goals, and the biological problems will return, creating a massive waste of funds, but coming out of a different, less obvious budget.
High Efficiency Dehumidification Systems (HEDS) We ve developed a high efficiency dehumidification system that can reduce energy consumption by 52% (high loads) to 72% (low loads), while improving chiller plant and boiler plant efficiency. Combining VSE design strategies with Load Based Optimization Controls, (LOBOS) and HEDS can reduce facility energy consumption, while reducing the RH to reduce the potential for biological damage to occur.
High Efficiency Dehumidification Systems (HEDS) 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.
High Efficiency Dehumidification Systems (HEDS) Variable Volume System Performance Comparison "Normal HVAC" vs. "HEDS" Entering Conditions Leaving Conditions % Design CFM DB WB DB Dewpoint 100% 78 65 65.1 55 75% 77 64.5 65.7 55 50% 76 64 67.2 55 25% 75 63 68.1 55 % 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% 460013 219240 52% 75% 340795 148330 56% 50% 227500 81250 64% 25% 108160 30400 72%
High Efficiency Dehumidification Systems (HEDS) DOAS Performance Comparison "Normal DOAS" vs. "HEDS-DOAS" Entering Conditions Leaving Conditions CHWS Temperature at Unit DB WB DB Dewpoint 45 F 98 82 65 45 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 1561813 1126150 28%
High Efficiency Dehumidification Systems (HEDS) High Efficiency Dehumidification System Energy Savings vs. Design Airflow @ 100% reheat to 65F/68F Typical for DOAS, Barracks, labs, manufacturing, underfloor air distribution systems, etc. High Efficiency Dehumidification System Energy Savings @ 100% Reheat to 65F/68F 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 1 2 3 4 Percent of Design Airflow Percent of Cooling and Re-heat Energy Saved
High Efficiency Dehumidification Systems (HEDS) High Efficiency Dehumidification System Energy Savings vs. Design Airflow @ 25% reheat to 65F/68F Typical for facilities that need minimal reheat. High Efficiency Dehumidification System Energy Savings @ 25% Reheat to 65F/68F 100% 90% 80% 70% 60% 50% 40% 30% 20% 10% 0% 1 2 3 4 Percent of Design Airflow Percentage of Cooling and Reheat Energy Saved
J.6 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, 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, a kw transducer on each chiller, 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.
J.7 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. Water temperatures of 120 F to 135 F can be obtained.
J.8 Develop an HVAC and Control System Design Criteria for New and Retrofit Projects. Weather Correct Standard Designs for the various areas of the country should be developed for Barracks, DFAC s, Offices etc. Regional differences must be accounted for in the envelope and HVAC designs. I Really, Really, Really want to help make this happen!
J.8 Develop an HVAC and Control System Design Criteria for New and Retrofit Projects. The design-build method seems to be delivering systems and designs that may provide an inexpensive first cost, but have higher than necessary lifecycle costs. Since we are still living with WWII era facilities, it is safe to say that lifecycle costs should be an important consideration.
J.8 Develop an HVAC and Control System Design Criteria for New and Retrofit Projects. A design standard for each different climate type 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.
J.8 Develop an HVAC and Control System Design Criteria for New and Retrofit Projects. 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.
J.9 AHU and Their Cooling Coils Retrofit Strategies Undersized cooling coils are a huge contributor to energy waste. If the cooling coils are undersized, as most of them are, fan energy is wasted, chiller energy is wasted, chilled water pump energy is wasted, condenser water pump energy is wasted and cooling tower fan energy is wasted. Additionally, undersized cooling coils can be a leading contributor to biological growth due to inadequate humidity control.
J.9 AHU and Their Cooling Coils Retrofit Strategies, cont 8.1. 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. 8.2. 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. 8.3. The maximum cooling coil face velocity should be 350 feet per minute for blow through and 300 feet per minute for draw through systems. 8.4. The desired fans are direct drive plug fans. 8.5. The cooling coils should be 8 rows deep. 8.6. The cooling coils should have 12 fins per inch. 8.7. The maximum leaving air temperature should be 55 F.
J.9 AHU and Their Cooling Coils Retrofit Strategies, cont 8.8 The minimum fin thickness is 0.008" 8.9. The minimum tube wall thickness is 0.028" 8.10. The minimum tube diameter is 5/8" 8.11. The coils shall be equipped with individually replaceable return bends of no less than 0.035" thickness 8.12. Hairpin return bends are not acceptable. 8.13. Provide stainless steel coil casings and intermediate tube supports 8.14. 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.
J.9 AHU and Their Cooling Coils Retrofit Strategies, cont 8.15. The coil air pressure drop should be less than 0.8" wc. 8.16 The coil water pressure drop should be less than 15'. 8.17 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. 8.18 Insulation system should be vapor tight and aluminum skinned.
Summary A recent, Peer Reviewed report in the ASHRAE Journal found that cooling coil face velocities in the 250 to 350 feet per minute range offer the best lifecycle cost, in contrast with the old school practice of sizing cooling coils at 500 to 550 feet per minute face velocity. We have been recommending low face velocity coils for years and have found it to be true.
Poor Cooling Coil Selections The typical Low Delta T syndrome was evident at each facility we visited. The Low Delta T syndrome has been explained in detail in numerous other publications, but suffice it to say that it leads to the waste of huge amounts of pumping and chiller energy, and the core cause of the Low DT Syndrome is cooling coils that are too small.
Poor Cooling Coil Selections Most facilities were running 40 F to 42 F chilled water temperatures to provide adequate dehumidification. One set of plants was running 38 F water for dehumidification. Small cooling coils require excessive CHW flow rates and very low CHW temperatures to try to meet their loads. High air velocities across the coils led to water carry-off into the AHU and ductwork
Poor Cooling Coil Selections Selecting cooling coils to provide 55 F supply air temperature in a blow-thru fan application, with 50 F chilled water supply and 65 F to 70 F chilled water return temperatures will cure many ills, and promote efficient system operation. 8 row, 12 fin per inch coils with a 300 to 350 feet per minute face velocity will typically meet these needs, while reducing water carry off from the coils and the biological remediation issues that result from water carry-off.
Poor Cooling Coil Selections The use of 50 F CHWS temperatures and a 15 F to 20 F temperature differential allows the chillers to be piped in series. The combination of using a 50 F CHWS temperature and a 15 F to 20 F TD will improve the peak day system efficiency by 25% to 30%, and the annual efficiency by an equal or greater amount.
Importance of Coil Selection and Fan Orientation 8 row 12 fin per inch 300 to 350 FPM cooling coils are desired for peak system efficiency in most cases. Blow through coil design to increase CHW system TD and reduce peak loads due to reduction in over-dehumidification. Blow thru coils can increase CHW system TD by 34% to 65%+ and decrease peak day cooling loads by 5% or more.
Subcooling the air due to fan heat added downstream of the coil occurs with this design Typical AHU Design High air and water pressure drops Typical coil size 4 to 6 row, 550 FPM face velocity Fixed, or poorly reset, setpoints for static and temperature. 1.5 to 2 and 55 F are typical AHU setpoints. High pressure drop 2- way or 3-way valves
No subcooling of air due to fan heat added upstream of the coil Optimized AHU Design CHW TD increased by over 165% (24 vs. 9 ) Desired Coil Size 8 row, 350 FPM face velocity Low air and water DP s Belt drive shown, we prefer direct drive fans Static and temperature setpoints continually reset based on the loads (LOBOS) Low pressure drop 2-way valves Allows chillers in series and substantial pumping savings
Strategy to Increase Chilled Water System Temperature differential Many facilities are plagued by low chilled water system temperature differentials. This is commonly referred to as the Low Delta T Syndrome This hurts energy performance, whether or not the system has TES installed. If the system is equipped with properly selected twoway throttling type cooling coil control valves, and the chilled water supply temperature and differential pressure are properly controlled, the most likely culprit for low system temperature differentials is undersized or worn-out cooling coils.
Cooling Coil Performance Comparisons The following pages compare blow thru and draw thru cooling coil selections. Normal design coils are compared to coils that reduce air and water pressure drop, increase chilled water system temperature differentials and reduce AHHU, chiller and pumping system energy consumption.
Draw Thru 6 R 550 FPM Coil vs. Blow Thru 8 R 350 FPM Coil
6R 550 FPM Draw thru vs Blow thru and 8R 550 FPM Draw thru vs Blow thru
6R 350 FPM Draw thru vs Blow thru and 8R 350 FPM Draw thru vs Blow thru
Draw Thru 6 R 550 FPM vs. Blow Thru 8 R 350 FPM for 100% OSA system
TES Optimized Cooling Coil 32 Degree TD 200% of Typical TES System Designs Doubles the Capacity of Most Installed TES Systems
30 Degree + TD s are Possible! We have many systems that operate with 26 F to 32 F chilled water system temperature differentials. It can be done, and it is not that big of a deal to make it happen. Big coils allow lots of leeway for energy conservation, and also allow loads to change without having to spend $$$ to upgrade the HVAC system.
Summary To reduce energy consumption, you need to improve heat transfer efficiency: Utilize large face area cooling and heating/reheat coils to maximize system temperature differentials and minimize air and water pressure losses. Large cooling coils reduce air pressure drops and allow the use of high chilled water supply temperatures, while still dehumidifying, raising chiller system efficiency. Large face area heating and/or reheat coils reduce air pressure drops and allow the use of low hot water supply temperatures. This can allow the effective use of low quality reclaimed heat, and/or highly efficient condensing boilers.
Use the Right Equipment A variable speed chiller used for heat recovery would be OK, if heat recovery mode was only enabled when there was a large need for heat, rather than continuously. Raising the head pressure for heat recovery duty on a VFD chiller wastes substantial energy. A small dedicated HR chiller or Water-Sourced Heat Pump and a HW storage tank is a better solution for heat recovery, in most cases.
Summary Energy Conservation = water conservation. If a cooling system is water cooled, saving energy will reduce the amount of heat that has to be rejected by evaporating water, so water savings will result. It may be possible to capture and reuse condensed moisture off of the cooling coils.
Put the VFD on the Biggest Chiller! VFD chiller performance is related to refrigerant lift and relative load. Refrigerant lift is related to chilled and condenser water temperatures and refrigerant approach temperatures. Refrigerant approach temperatures are related to relative load and heat transfer surface area. Relative load and heat transfer surface area are related to the size of the chiller. The bigger the chiller, the lower the relative load will be and the greater the heat transfer surface area will be. The lower the relative load and the greater the heat transfer surface area, the closer the approach temperatures can be. The closer the approach temperatures can be, the lower the lift can be. The lower the lift, the more efficient the chiller can be (within limits).
Fully Loaded VFD chiller @ 0.35 kw/ton VFD Chiller Efficiency is more related to lift than to load
2040 Main Street (VSE/LOBOS lite) Put the VFD on the biggest Chiller you have! 750 ton VFD chiller at 6.3% load (47 tons) total plant efficiency = 0.42 kw/ton. 250 ton CS chiller at 20% load (49 tons) total plant efficiency = 1.62 kw/ton.
WAKE UP! Time to go to the next presentation! Thank you for giving me the opportunity to help you help our troops, and help our Country. It is an honor to work with you. Scot M. Duncan, P.E.