The goal of HVAC in a hospital is to provide adequate ventilation and occupant comfort as it relates to temperature and humidity. To save energy the goal should be to move and temper the air no more than necessary. My recommendations here will be focused on reducing the volume of air that is moved and figuring out ways to temper the air as little as possible.
Correctly size equipment – It is often the case that engineers will oversize equipment during new construction to ensure occupant comfort during initial occupancy. During a retro-commissioning effort, it may be possible to identify equipment that can be removed and replaced with smaller, more efficient equipment. If the return on investment does not warrant an immediate replacement, then the equipment should be identified for replacement with smaller equipment during routine maintenance or major repairs later in the building’s life cycle.
*Variable Frequency Drives - The most obvious place for savings is in the application of variable frequency drives. The cost of variable frequency drives has become so low that the application of a VFD to very small devices will often result in a very short time to recover the investment. In most cases where it is possible to slow down an electric motor, it should be done. Changes to a sequence of operation may be necessary, but are often worthwhile. The cube law is often used to show that a reduction in speed of 50% results in an 87% reduction in energy consumption (%Power = % Speed3 ).
*Eliminate 100% Outside Air Systems Wherever Practicable - Common areas that require 100% outside air are operating rooms (ORs) and labs. If 100% outside air AHUs are being used for differing space types that don’t require outside air, modulating outside air dampers and return air systems should be installed to reduce outside air. If a space is mixed use, it may be worthwhile to isolate it into separate spaces. Construction after initial occupancy can result in areas that are over-ventilated because first costs would have been higher to change the ventilation system to something more efficient.
*Demand Based Ventilation - The goal of demand based ventilation is to reduce the cost of ventilating a given space by reducing the number of air changes per hour (ACH) to the minimum level that will maintain the safety and comfort of the occupants. Typically the ventilation for an area in a building is designed to provide adequate ventilation in some hypothetical worst case scenario, or based on some assumptions about use and occupancy levels. Methods of demand based ventilation include controlling the level of CO2 or multi-parameter demand control where the levels of many contaminants are controlled.
*Reduce Outside Airflow Rates to ASHRAE 62.1-2010 - Most buildings are bringing in more outside air than they are required to per ASHRAE 62.1-2010. Outside airflow rates should be reduced to the minimum allowed by ASHRAE for each space. Consider installing CO2 sensors in return air ductwork and maintaining interior CO2 levels at <700 ppm above outside air CO2 levels during occupied hours. Also consider implementing an outside air damper reset schedule based on time of day and occupancy patterns within the building. During unoccupied hours and during morning warm-up cycles, all outside air dampers should be 100% closed. Even though much of a hospital is occupied 24 hours per day, there are probably opportunities to incorporate occupancy schedules in office space, auditoriums, and conference rooms. A careful review of the use of the space should be undertaken to make sure occupancy schedules are used wherever possible and are tightly controlled around actual usage times.
Multi-parameter demand-controlled ventilation system - This method of ventilation senses a variety of airborne contaminants and increases the ventilation rate to dilute and purge the affected spaces. The system utilizes multiple sample points or sensors that are placed in various areas of the hospital to measure contaminants in real time. Air samples are collected from each area every few minutes and analyzed locally or transported to a centralized sensor array for analysis. The goal is to eliminate over ventilation and the costs associated with running the ventilation equipment more than is necessary.
Fan Walls –A fan wall is a rectangular array of four to 300+ fans, all discharging into a single plenum. In a traditional system using large direct drive motors, it is common to see motor loads at 1/4th to 1/8th of their nameplate rating for a significant portion of their operating hours, resulting in reduced power factors and lower fan and motor efficiencies. To more closely match site conditions, a fan wall array can turn one or more fans off to allow the remaining fans and motors to be loaded at or near peak efficiency.
Recovery Devices – Energy recovery ventilation (ERV) recovers the energy contained in air that is exhausted from a space and using it to precondition the incoming outdoor ventilation air. During warmer seasons it pre-cools and dehumidifies while humidifying and pre-heating in the cooler seasons.
Pressure Independent Variable Air Volume - For non-critical spaces with lower ventilation requirements (such as medical office floors), consider switching to variable air volume systems and install variable frequency drives on all motors.
Displacement Ventilation - Consider using a displacement ventilation system rather than traditional overhead ventilation. Displacement ventilation is a room air distribution method where conditioned air is supplied at floor level and extracted above the occupied zone, usually at ceiling height. Such systems have the potential to improve both energy efficiency and infection control. In my opinion the jury is still out on energy efficiency, but infection control has made this attractive to hospitals. Your client may not realize enough of a benefit to apply this technology, but it is worth some consideration.
Displacement ventilation systems introduce cool air at low velocity, improving ventilation effectiveness within the occupant zone. As a room becomes warmer through use, the air and its contaminants rise, generally resulting in better air quality than exists in traditionally ventilated areas. Additional benefits of displacement ventilation follow. The idea is that energy is saved by reducing the number of air changes per hour and air is supplied at a warmer temperature.
Recent findings of the Healthcare Ventilation Research Collaborative (based on actual measurements) indicate that displacement ventilation with 4 ACH provides the same or better air quality for patient rooms than mixing ventilation at 6 ACH. http://www.reuters.com/article/2011/05/31/idUS221085+31-May-2011+BW20110531
Small Modular Boilers - Evaluate the opportunity to install multiple small boilers to meet the heating load of a given area. It is more efficient to operate smaller boilers when the heating load is 25% to 50% of the design capacity than it is to use one large boiler to meet a partial load. This is not always possible in hospitals if a large portion of the load on the heating system is from process hot water and steam use. If you find that the load on the heating system is not closely related to the outside air temperature, this may not be an option.
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