Air condition systems in modern, glass enclosed buildings are essential for business operations as well as human comfort. While every effort is made at the design stage to put in the most optimum air conditioning capacity, during the operational phase of building, most building owners and occupiers see the set points drift from the design values and internal ambient temperatures not being maintained as per the original design or as per the expectations of the occupants (too hot or too cold!)

The HVAC team has the difficult task of balancing between the cost of operations of the HVAC system (one of the highest costs in an operational building) and the human comfort. The typical approach of the maintenance team is to run more capacity to meet the change in requirements, which tend to increase cost of operations. The case study presented in this article describes how the energy management team of a large portfolio of buildings did what engineers do best – study the design, identify gaps and then implement cost effective solutions to improve efficiency as well as reduce cost.

Background Information of HVAC System

The HVAC system for five large multi tenanted building complexes consists of water cooled centrifugal chillers with VFD control. The portfolio covers approximately 10 million square ft. of office space in the NCR and east regions. A total of 59 chillers are installed across the portfolio with capacities of the chillers ranging from 450TR to 800TR. The chillers are distributed in 19 banks in the 5 complexes. The chilled water circuit is primary-variable secondary catering to AHU’s located in different floors. The chilled water circuit is primary-variable secondary catering to AHU’s located in different floors.

Operational Challenge Faced by Maintenance Team

Although the HVAC systems for each building complex have capacity control systems with BMS integration of the air handling units, the complaints from occupants of inadequate cooling leading to higher ambient temperatures were increasing, resulting in tenant dissatisfaction. The building maintenance team’s initial approach to address the issue was to increase the chiller banks in operations to reduce the AHU chilled water temperature. However, this increase the cost of operations considerably and also did not seem to be improving the operations to a great extent.

Root Cause Analysis

The task of finding a more cost effective and long lasting solution was entrusted to the energy management team of maintenance department. The data for one building was initially analyzed using both the system generated BMS data as well as manual data collection to corroborate the design inputs. The key observations deduced from the detailed review of system data were

  • Temperature of the chilled water flowing to the first AHU is on the higher side.
    • Temperature difference between chilled water outlet & first AHU inlet is around 3°C.
    • Mixing of chilled water happening through the de-coupler line from primary suction header to secondary suction header because of head difference in between the headers.

The main reason for the relatively higher chilled water temperature reaching the AHU’s was identified as the mixing of the chilled water supply and return streams. This was on account of incorrect balancing in the headers leading to warmer supply water going into the supply line. The schematic of the chilled water supply line arrangement is shown in figure 1.

Addressing the Problem

The maintenance team and the energy management reviewed the current arrangements and identified the following ways to prevent mixing of primary & secondary chilled water:

  • Motorized de-coupler valve has been installed in all the plant rooms to segregate primary & secondary chilled water circuits.
    • The valve has been closed maintaining the desired chilled water flow through the chillers.
    • Butterfly valves present in the standby chillers are completely closed in order to avoid mixing of return & supply chilled water.
    • During winter season the motorized de-coupler valve will be adjusted based on frequency of VFD present in the secondary pumps.
    • At a later stage the motorized de-coupler valve will be integrated with the BMS system & secondary pump VFD so that operation of the same gets controlled automatically.

The changes made the system are shown in figure 2 in bold lines.

Cost Benefit Analysis

The energy consumption of the chillers was closely monitored for the next three months to assess the impact of the change. The chart below shows the trend of energy savings achieved in 1st quarter of 2017-18

The return of investment for the change worked out to be less than a month. Considering an average unit cost as Rs.10/kWh (includes Grid & DG cost), the calculations for the payback are as follows:

Direct Benefits Achieved:
• Lesser running hours of chillers.
• Lesser running hours of primary, secondary & condenser water pumps.
• Lesser running hours of cooling tower.

Intangible Benefits Achieved:
• Tenant satisfaction
• Reduced maintenance cost of equipment (chiller, pumps, cooling tower) because of lesser running hours.
• Lesser man hour wastage because of lesser HVAC complains from tenants.

Conclusion

Energy is a major component of the opex costs in a building. With the increasing cost pressures on occupiers to keep costs low, the maintenance teams have to find innovative ways to optimize the use of energy and thus, reduce costs. The team was faced with increasing costs as well as increased occupant discomfort due to the higher chilled water temperatures at the AHU inlet. By studying the system in detail, the energy management as well as maintenance teams jointly worked out a solution that was cost effective and easy to implement. This resulted in considerable savings to the landlord as well as the occupiers in addition to achieving the result of satisfied occupants of the building.

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