Building Description

The facility is a 54-bed heart hospital with four operating theaters; Cath and diagnostic labs, ICU, CCU, and an around the clock emergency room serviced by air and ground transportation. The facility is approximately 160,000 square feet multi-story (basement and two floors above grade) building. The heating, ventilating and air conditioning (HVAC) system consists of:

• Constant flow chilled water/hot water systems including 2 parallel piped water cooled water chillers, approximately 320 tons each.
• 8 central station air-handling units, all of which equipped for variable volume (VAV) using either variable frequency drives or inlet guide vanes, all having both supply and return fans and 100% outside air economizer capabilities.
• Approximately 240 constant and variable volume boxes, most with reheat coils.
• Complete direct digitally controlled Energy Management System (EMS).

Project Problems/Issues

• Inability to satisfy all of the facility’s temperature, humidity requirements, including ICU areas due to original HVAC system design and installation
• Space pressure problems, i.e. pressures between sterile areas inverse from required
• Questionable design practices
• Risk management and regulatory agency compliance issues due to the above
• Excessive utility costs

Project Overview

Energy-Environment-Economics provided a detailed study and report of the HVAC System’s existing conditions and operations including design and DDC system deficiencies, potential energy conservation opportunities with budgeted project costs and economic analysis, and maintenance related operational and efficiency analysis. The result of this study became an approximately $1,800,000 HVAC system redesign and modifications for the facility, including construction document preparation, commissioning and Optimissioning^SM of the entire facility.

Solutions Provided

The modifications included replacement of numerous in-duct fire/smoke dampers (FSD) and air handling unit supply air discharge ducts to reduce the air distribution system’s total pressure drop. The analysis determined that the total system pressure drop was significantly higher than the design intent, and as a result the air handling units did not have the capability to provide the airflow required to all zones, especially those distant from the air handling units. The higher than design system pressures were also causing serious duct leakage, up to 25% on some systems, exacerbating the comfort issues. The following graph illustrates the design and calculated (actual) conditions:

This lack of capacity, even at 100% fan speeds, resulted in an inability to maintain temperature/humidity requirements as well as poor ventilation control and indoor air quality. Also, with these conditions there was no way to capitalize on the energy management capabilities (VAV) of the air handling units. To complicate matters, large space pressure differences were existing between sterile areas due to return air fan control schemes and the original outside air system design criteria.

The original design for outside air included a large, passive (not forced air) outside air “plenum” to which individual outside air ducts attached to all air handling units. Preliminary testing showed that this “sharing” of the main outside air duct was resulting in some air handling units actually introducing return air from other air handling units via the outside air system. In other words, among other noted problems, sterile (surgery, etc.) areas were introducing air from less sterile areas. The entire outside air deliver system was egregiously deficient.

Part of the HVAC system redesign required that the outside air ducts to each air handling unit be replaced with smaller ducts and were equipped with outside air flow measuring stations. The smaller sizes provided the minimum velocities necessary to make the stations work correctly and to permit outside air control and monitoring. Also, Energy-Environment-Economics designed and implemented innovative outside air/return air/mixed air damper control algorithms, combined with a revised supply air fan/return air fan tracking algorithm, that solved the outside air delivery issues.

Through exhaustive field work, the engineering study also identified select areas where space was available to increase the size and lower the subsequent pressure drops of the FSD’s that had especially high pressure drops and would have the most effect on shifting the system curve back to design as shown above. Affected areas were systematically taken off-line during low census and off-hour times, and these FSD’s were replaced and the system brought back on-line in the morning. Accurate construction documents were mandatory, as was scheduling of all related shutdown activities. During this process, the ductwork was also re-sealed to minimize the extreme leakage.

System testing during analysis also revealed large pressure drops at the supply air discharge ducts at each air handling unit, caused by system effect pressure losses due to duct configurations. New designs were developed, and these changes were made simultaneously during the system shutdowns. Finally, to maximize the system shift, existing inlet guide vanes were removed and replaced with variable frequency drives. The net result of the air distribution system modifications was to shift the system curve within design, allowing sufficient air at required temperatures to be supplied to all zones. The outside air delivery system corrections, combined with the modifications to the EMS, solved the IAQ and space pressure control issues. All corrections to the air distribution system collectively solved the very serious problems, and were also very successful in reducing the energy consumption of the system.

System performance and efficiency was vastly improved through the proper application of variable air technologies and converting the constant volume chilled water system to variable primary. Inefficient equipment such as 3-way control valves and fan inlet guide vanes were replaced with pressure-independent 2-way control valves and variable frequency drives respectively. The installation of properly sized cooling towers and plate frame heat exchanger provided non-existent redundancy and reduced operating costs. All new devices - variable frequency drives, outside air flow measuring and control devices, et. al. – were integrated into the Energy Management System (EMS) sequences of operations.

The sequences of operations were completely rewritten for maximum performance and efficiency during the OptimissioningSM process, including reset schedules for the air handling units. EMS communication with the chillers was established, enabling reset schedules to be established and more reliable control over central plant operations. Space pressure sensors and control algorithms were incorporated, as were air handling unit outside air control algorithms. Differential pressure sensors were installed into the revised chilled water system, and all facets of the new system’s controls requirements were designed, implemented and optimized. All critical EMS setpoints, including economizer changeover, chilled water supply, supply air temperature, and air handling unit static were painstakingly established for maximum energy efficiency. All VAV/FPVAV boxes were calibrated during the HVAC system wide Test & Balance process.

Results

These system improvements were completed with no interruption in services, as affected areas were systematically taken off-line during low census periods and returned to operation the following morning(s). the new control algorithms for the entire system(s), coupled with the installation of new and/or relocated controls devices, allowed us to properly monitor, control and most importantly satisfy all space temperature and pressure and outside air flow requirements throughout the entire facility at all times and any conditions. The project resulted in eliminating all deficiencies associated with the HVAC system and reduced energy costs by approximately $100,000/year, but most importantly all compliance and risk management issues were addressed and corrected. Click here to read more.