HVAC systems commissioning in a manufacturing plant.
Building commissioning is a process to improve the building energy performance from its design phase, construction phase, and operation phase [ASHRAE 2007]. New building commissioning is driven more strongly by non-energy objectives such as thermal comfort, indoor air quality, and overall building performance with an emphasis on ensuring that the building functions according with the design intent, whereas the commissioning of existing building is more strongly driven by energy efficiency objectives by identifying, resolving the operating problems, optimizing system energy use [Mills et al, 2004; Liu et al. 2002]. As one of the most cost-effective ways to improve energy efficiency, commissioning plays an important role in achieving building energy saving goals. In this article, a specific case study with energy saving-oriented commissioning was conducted in a manufacturing plant to optimize the performance with the main methods of retrofitting and optimizing the system operation and controls.
The case study plant, originally built in 1959, had a comprehensive and self-sufficient system including chilled water system, processing water system, boiler system, compressed air system, and air conditioning system. Later, system retrofits were made gradually such as reconstruction of the original coal fired boiler into dual fuel (natural gas and oil) fired boiler with an automatic control system, installation of variable frequency drives (VFDs) for the chilled water pumps and so on so as to improve the system performance. However, for the whole plant, great energy saving potential still exists allowing for further increases in system efficiency. In 2008, a detailed system inspection was conducted and potential energy saving opportunities were identified and proposed. The retrofitting and most system optimizations were implemented in 2009. After that, a one-year follow-up was also provided for operator training and troubleshooting. Based on the whole inspection and implementation process, this article briefly introduces the existing system information, points out the energy saving opportunities, and presents the system retrofits and improved control strategies. The operational data before and after the system optimization were also analyzed and compared with a total annual energy savings presented.
The case study plant is located in Omaha, Nebraska. The central plant with a gross area of 71,803 sq. ft. (6,671sq. meters), consists of chiller system and boiler system. The air handling units serve office building and main plant with a total area of 1,212,940 sq. ft (112,685 sq. meters). The whole system works with a 24/7 schedule.
[FIGURE 1 OMITTED]
The chiller system supplies chilled water mainly for the air handling units (AHUs) for several buildings. The chilled water system, shown in Figure 1, was designed as a primary-secondary system consisting of five chillers in parallel: four 1200 ton (4,219 kW) electric centrifugal ones and a steam-driven one which was seldom used recently. Each chiller was equipped with a constant speed primary pump of 50 HP (36.8 kW). Five 200 HP (147.1 kW) variable speed secondary pumps in parallel delivered the chilled water to the AHUs. In recent years, four electric chillers were still in operation, which were covered in the work scope.
The boiler system included three 70,000 lb/hr (31,751kg/ hr) dual-fuel fired boilers with two of them still in operation. Each boiler was built with two burners operated in parallel, one constant speed forced draft (FD) fan equipped for each burner, and one induced draft (ID) fan with fluid drive at the boiler stack outlet. The feed water system consisted of two 50 HP (36.8 kW) constant speed pumps delivering feed water to each boiler drum.
The air conditioning systems were comprised of thirty-one single-zone constant air volume (CAV) AHUs serving the manufacturing area, three single duct variable air volume (VAV) AHUs, and one DX rooftop unit (RTU) serving the office area. For the thirty-one CAV units, there were three types of cooling coil configurations: type 1, cooling coil with cooling valve only; type 2, cooling coil with cooling valve and constant speed booster pump; type 3, cooling coil with cooling valve and variable speed booster pump, as seen in Figure 2. The single duct VAV units were equipped with type 1 cooling coils. The DX RTU, equipped with two reciprocating compressors, supplied constant air flow to the office room.
[FIGURE 2 OMITTED]
ENERGY SAVING OPPORTUNITIES
After a detailed inspection, the following energy saving opportunities were identified.
Chilled Water System
The existing chilled water system, as seen in Figure 1, was a constant primary flow and variable secondary flow system, which caused excessive chilled water to bypass the chillers or the building through the bypass line under off-design condition when the chiller design flow did not match with the building side flow, which resulted in significant pump power consumption. The blending of chilled water supply and return not only reduces chiller's COP, but may also degrade the AHUs' performances and cause building comfort issues. Therefore, a single loop system with bypass line fully closed was proposed as well as other related system retrofits and controls to eliminate these issues and improve the system performance.
Due to the reduction of production, the boiler operated under partial load for most of the time. It ran with low efficiency under lower boiler load with the flue gas oxygen level as high as 16% which indicated too much excess air. However, each boiler had two burners operating together in parallel which can actually work separately as two individual burners. In addition, when the constant speed FD fans ran, the related damper was closed most of the time which resulted in unnecessary fan power consumption. Similarly, the feed water pumps ran with constant speed all the time, whereas the boiler feed water valves were found to operate in low position range most of the time, which caused extra pump power consumption and degraded the valve lifespan. So, the boiler system efficiency could be improved by optimizing the burner operation sequence, reducing the excessive combustion air, and optimizing FD fan control and feed water pump control.
Air Conditioning System
The thirty-one CAV AHUs with constant speed motors consumed excessive fan power under partial load conditions, of which twenty-one were designed with two-position inlet guide vanes for the supply fans. VFD was proposed to be installed for each fan to improve the fan efficiency to overcome the drawbacks of inlet guide vanes as well. The three VAV systems, designed with constant static pressure set point, caused excessive fan head and fan power. Automatic static pressure reset was proposed for the improved control. For the CAV DX rooftop unit, the existing compressor staging control caused unstable space air temperature which can be further optimized. Meanwhile a VFD was proposed to be installed for the supply fan.
Based on these inspection results, retrofits and new control strategies were recommended and implemented on the site in 2009.
For the chilled water system, a series of tests were conducted before converting the system to a single loop system to verify its feasibility. The test results demonstrated that when fully closing the isolation valve on the bypass line and shutting down primary pumps, the secondary pumps had the capability to provide enough differential pressure for the remote AHU, since the cooling valve on the remote AHU still worked within functional range.
For air handling units, all the CAV systems were converted to VAV systems. Two Fan Airflow Stations (FASs) [Liu 2006] were installed for two VAV units to optimize the static pressure.
Table 1 summarizes the detailed retrofits for all the systems.
Table 1. Summary of System Retrofits System Retrofits Chilled water Install new control panels for chiller 1-3 system Remove the flow control valves and primary pumps Shut off the isolation valve on the bypass pipeline Boiler system Install two VFDs for FD fans of the two boilers in operation Install two VFDs for the feed water pumps CAV AHUs Lock the inlet guide vane fully open, shut off the reheat valve and fully open the bypass damper permanently. Install VFDs for all supply fans and return fans. VAV units Install Fan Airflow Stations on two VAV units DX RTU Install VFD on supply fan Replace the existing two-position actuator with a new modulating actuator for the outside air damper.
SYSTEM CONTROL OPTIMIZATION
Chilled Water System
Existing Control: The chiller start/stop was determined by the operators based on their experiences.
Operation of the primary pump was based on the operation of related chiller. When a chiller was to start, the associated primary pump was started first. It ran at constant speed and would not stop until the chiller was stopped.
The secondary pump speed was modulated to maintain the differential pressure of remote coil at set point of 1.5 psi (13.1kPa). All pumps online ran at the same speed. The start/stop of a lag pump was based on 90% and 40% principle. When the online pump speed was higher than 90% of design speed, a lag pump was started; when the online pump speed was lower than 40%, a lag pump was stopped. When a lag pump was started, it ran at the same speed as the lead pump of 90%, which could cause pump cavitation at the moment it was started.
The system chilled water supply temperature was fixed at 44[degrees]F (6.7[degrees]C) all the time.
Improved Control: In the single loop system without bypass pipeline and primary pumps, the chilled water flow through chillers was variable and always matched with the building-side flow. With the chiller control panel installed, the chiller can communicate with the system controller. The sequence of chiller staging was developed. When the load ratios of all chillers were higher than 80%, a lag chiller was started; when the load ratios of all chillers were lower than 40%, one chiller was shut down, which was based on the principle that chiller has a higher efficiency when running between 40% and 80% load ratios [Liu et al. 2002].
The chilled water supply temperature set point (CHWST StPt) was reset based on the system chilled water flow rate, as seen in Figure 3. If more than one chiller were online, or only one chiller was online and the system chilled water flow was higher than 60% of chiller design flow, the set point was set at 44[degrees]F (6.7[degrees]C). If only one chiller was online and the chilled water flow was lower than 60%, this set point was reset from 44[degrees]F (6.7[degrees]C) to 48[degrees]F (8.8[degrees]C) to maintain the system chilled water flow rate above the chiller allowable low limit flow recommended by the chiller manufacturer.
[FIGURE 3 OMITTED]
For the secondary pump control, the best pump efficiency method was applied to find the critical speed when calling for or shutting down a pump. For this site, it was found that the critical speed had less influence on the pump power consumption. Therefore, 90% and 40% speed limit was reserved. The only distinction was that when a lag pump was called, it would speed up gradually until reaching the same speed as the lead pump, which solved the pump cavitations issue.
Existing Control: The steam pressure set point of the boiler system was set at 110 psi.
When a boiler was running, two burners ran together at the same firing rate with a total minimum rate of 120therms/ hr (12,661MW). The combustion airflow of each burner was maintained at its set point by modulating the FD damper with the FD fan running at the constant speed. The Air/fuel ratio data were the same as that when the system was built years ago.
The feed water system line pressure was maintained at a fixed set point of 120 psi (827.4 kPa) with feed water pump running at constant speed all the time.
Optimized Control: The system steam pressure set point was reduced to 50 psi based on the requirements of production and equipment.
With the existing control and new steam pressure set point, when the boiler heat demand was less than 120 therms/hr (3,516 kW), the operation of two burners could cause the system steam pressure higher than the set point during summer which indicated energy waste. Therefore the single/double burner mode was built. When the system heat demand was lower than 120 therms/hr (3,516 kW), one burner was shut down and the system ran in single burner mode. When the system heat demand was higher than 120 therms/hr (3,516 kW), the offline burner was fired. With single burner mode built, the boiler can operate with the minimum firing rate of 60 therms/hr (1,758 kW).
With VFD installed for FD fans, the burner combustion airflow was controlled by modulating the FD fan speed with the FD fan damper full open all the time, which delivered the burner with required airflow but consumed less fan power.
Among the factors that influenced the boiler efficiency, excess air was an important one. Practically, for complete combustion, more air must be supplied than theoretically required [David 2002]. However, too much excess air degraded the boiler efficiency by heating excess cold air to stack temperature [Energy efficiency handbook]. Using oxygen level in flue gas as an indicator for excess air, usually, 3% oxygen level in flue gas means 3% efficiency drops [Boiler Efficiency Improvement guide]. Considering the control and system changes, the old boiler air-fuel curve when the system was built need to be calibrated. A test was conducted to adjust the curves for both single burner and double burner modes. After the adjustment of the air/fuel ratio, the new oxygen level under various firing rates before and after the test were given in Figure 4, which demonstrates the reduction of excess air and the improvement of the boiler efficiency.
[FIGURE 4 OMITTED]
However, because the boiler code had a strict requirement on the amount of the lowest combustion air of a boiler, although a burner was shut down in single burner mode, the associated FD fan still needed to run to supply excessive combustion airflow so as to maintain the total airflow in compliance with the boiler code requirement, under which condition the efficiency of the boiler was the best that can be achieved.
With VFD installed in the feed water system, the system line pressure set point was maintained by modulating the feed water pump speed with the line pressure set point reset from 70 psi (482.6 kPa) to 120 psi (827.4 kPa) based on the system heat demand and feed water valve position. Since the boiler drum level was controlled by the feed water valve, as long as the feed water valve operated in the functional range, the boiler drum level was in good control. The trending data before and after the optimization in Figure 5 show that the feed water valve operated at an average opening of 78% for optimized control compared with 20% for the existing control, which indicates better valve performance and less pump power consumption.
[FIGURE 5 OMITTED]
Air Conditioning Systems
Existing Control: For the thirty-one AHUs, the supply fans ran with constant speed. The air temperature was controlled using the methods listed in Table 2. For all the systems, the economizer was not used. For the three VAV units, the system static pressure was maintained at a constant set point by modulating the VFD speed.
Table 2. The Space and Supply Air Temperature Control with the Existing Control Systems Space air Temperature (Tspa) Supply Air Temperature (Tsa) Single zone units Controlled by modulating the Not (with type 1 and cooling valve controlled type 2 cooling coil) Single zone units Controlled by modulating the Not (with type 3 cooling booster pump speed controlled coil) DX rooftop unit The compressors were staged to Not maintain Tspa set point at controlled 74[degrees]F (23.3[degrees]C). When Tspa > 75.5[degrees]F (24.2[degrees]C), the first compressor was turned on. If Tspa could not be maintained within 5 minutes, the second compressor was turned on. Single duct VAV Controlled by modulating the Controlled by units damper and reheat coil at the modulating the terminal box cooling valve without reset
Optimized Control: The Temperature-based economizer was used for all the single zone AHUs and single duct VAV systems, with the following sequences: 1) The economizer was activated when outside air temperature was bellow 65[degrees]F (18.3[degrees]C), and deactivated when the outside air temperature was over 68[degrees]F (20[degrees]C). 2) In economizer mode, the outside air damper and return air damper were modulated together to maintain the mixed air temperature set point which was two degrees lower than the supply air temperature set point. 3) When the economizer was off, the outside air damper was set at the minimum position.
The space air temperature and supply air temperature were controlled differently for different systems, as described in Table 3.
Table 3. The Space and Supply Air Temperature Control with the Improved Control Systems Space Air Temperature Supply Air Temperature (Tsa) (Tspa) Single zone Controlled by Controlled at 55[degrees]F units (with modulating the fan (12.8[degrees]C) by modulating type 1 and speed with the the cooling valve. When fan speed type 2 minimum speed of 20% was at the minimum, Tsa set point cooling would be reset from 55[degrees]F coil) (12.8[degrees]C) to 60[degrees]F (15.6[degrees]C) based on the Tspa. Single zone Controlled at 55[degrees]F units (with (12.8[degrees]C) by modulating type 3 the booster pump speed. When fan cooling speed was at the minimum, Tsa set coil) point would be reset from 55[degrees]F (12.8[degrees]C) to 60[degrees]F (15.6[degrees]C) based on the Tspa. DX rooftop Controlled by When Tsa > 65[degrees]F unit modulating the fan (18.3[degrees]C) or supply fan speed with the speed > 80%, the first minimum speed of 50% compressor was turned on; when Tsa < 53[degrees]F (11.7[degrees]C), the first compressor was turned off; when the first compressor was on, if Tsa > 60[degrees]F (15.6[degrees]C) for 2 minutes, the second compressor was turned on; when both two compressors were on, if the Tsa < 53[degrees]F (11.7[degrees]C), the second compressor was turned off.
Figure 6 and Figure 7 compares the results of two control methods used for DX unit. Better supply air temperature control was achieved.
[FIGURE 6 OMITTED]
[FIGURE 7 OMITTED]
For the three single duct VAV systems, the supply air temperature was controlled by the cooling valve and its set point can be reset based on the outside air temperature. The existing control of space temperature was unchanged. The static pressure was maintained by modulating the supply fan speed with the set point reset based on the supply airflow calculated from the Fan Airflow Station, as shown in 8.
[FIGURE 8 OMITTED]
After implementing the proposed control strategy on the chilled water system, boiler system, and air conditioning systems, the space thermal comfort was well maintained, overall energy efficiency and reliability was greatly improved, and fan power consumption was significantly reduced as seen in Figure 9 and Figure 10 which compare the electricity and gas consumption under different average daily temperatures before and after commissioning. Table 4 lists the consumption and savings for electricity and gas. The total annual electricity and gas cost were reduced by $1,069,000 and $201,859 respectively. However, the depression of economy had some impacts on the energy usage in this plant by reduced occupancy and machines usage, and so on. Since there was no available data log to calculate this portion, 60% of energy savings was assumed due to the economy impact based on the comparison of the numbers of employees and operating machines. With these taken into consideration, the energy savings due to the commissioning were 15% and 7% for electricity and gas consumption respectively, with a total amount of cost about $508,558. The total cost including retrofits and services are about $501,120. Therefore, the payback is about one year.
[FIGURE 9 OMITTED]
[FIGURE 10 OMITTED]
Table 4. Energy Consumption and Savings Before and After Retrofitting Pre Post Sub-Savings Electricity Consumption 47,677,486 29,477,281 18,200,205 (kWh) Cost ($) 2,030,499 1,735,721 1,069,537 Gas consumption 185,939 151,150 34,789 (DTH) (GJ) (196,165) (159,463) (36,702) Gas cost ($) 1,597,872 877,032 201,859 Total savings ($) - - 1,271,396 Adjusted total savings ($) - - 508,558
This paper presents a detailed commissioning process for the HVAC system of an existing manufacturing plant. Commissioning covers the retrofits and the optimization of the operation and system control for the chilled water system, central heating system, and all air handling units based on the building owner requirements. Significant annual energy savings were achieved. However, considering that the plant experienced an economic decline in recent years, not all the energy savings were attributed to the result of commissioning. 60% of total savings was attributed to the economic decline based on the reduction of employment and machine usage, so that a total amount of $508,558 savings was finally achieved, about 15% reduction for electricity use and 7% reduction for gas consumption. The payback is one year, which demonstrates the cost-effectiveness of the commissioning services.
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Liu G., 2006. Development and applications of fan airflow station and pump water flow station in heating, ventilating and air-conditioning (HVAC) systems. PhD thesis. University of Nebraska-Lincoln.
Liu M., D.E.Claridge, and D. Turner, 2002. Continuous Commissioning Guidebook: Maximize Building Energy Efficiency and Comfort, Federal Energy Management Program.
Student Member ASHRAE
Student Member ASHRAE
Mingsheng Liu, PhD, PE
Jinrong Wang, PE
Yunhua Li and Bei Zhang are doctoral students and research assistants in the Department of Architectural Engineering, University of Nebraska--Lincoln, Omaha, NE. Mingsheng Liu is president and CTO of Bes-Tech Inc., Omaha, NE. Lixia Wu is a director of engineering at Bes-Tech Inc., Philadelphia, PA. Jinrong Wang is the manager and Tom Lewis is a senior technical analysis engineer of commercial and industrial solutions at Omaha Public Power District, Omaha, NE.
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|Author:||Li, Yunhua; Zhang, Bei; Liu, Mingsheng; Wu, Lixia; Wang, Jinrong; Lewis, Tom|
|Date:||Jul 1, 2012|
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