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Improving Commercial HVAC Efficiency

Posted by Nick Chiarelli on Aug 7, 2018 8:36:37 AM
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5 Systems to Consider for Improved Commercial HVAC Efficiency

Commercial HVAC UnitsAs a building owner, one of the greatest expenses is the cost of heating and cooling a building. While it can be easy to simply accept this as a necessity, it should be known that there are ways to improve commercial HVAC efficiency and reduce the cost of utilities, leaving money to be spent elsewhere in the building.

The upfront cost of a system upgrade can often be offset by the dollars saved using less electricity, gas, oil, propane, or other energy source. This payback period can be evaluated during the initial conceptual phases of the project by most mechanical engineering firms through energy modelling computer software. This allows you to evaluate the feasibility of possible upgrades before jumping into any commitments.

Here we'll explore 5 systems that have the potential to drastically reduce utility costs in existing buildings and should be considered when any new building is being designed.

Air-Side Economizer

There are many hours of the year when cooling is required in a building and the outside air temperature is actually cold enough to provide a sufficient amount of cooling capacity. Instead of using an energy demanding compressor to cool the air in an air handling system, an air-side economizer system can be used to pull air from the outdoors into the occupied space to provide “free cooling.” This is still using fan energy, so it is not completely free, but it is drastically less energy than operating a compressor.

This system utilizes a control sequence which will sense when the outdoor air temperature is cooler than the return air temperature and will open the outdoor air intake damper to allow more outdoor air into the supply ductwork, thus providing natural cooling to the occupied space.

If the air handling unit is located indoors, then the challenge is to install a larger outdoor air intake duct from the AHU to the location where outdoor air is being drawn in. In systems without air-side economizer cycles, this outdoor air intake duct is sized for the minimum outdoor airflow rate required to meet code-dictated ventilation rates. This is usually around 25% of the total airflow of the air handling unit. In other words, the duct will have to be about four times the cross sectional area in order to allow 100% of the air handling unit’s total airflow to come from the outdoor air intake location. If the air handling unit is located outside, this is not an issue, as the air handling unit simply pulls outdoor air directly from the environment in which it is located.

Energy Recovery

There are various ways to recover wasted energy that is spent by an exhaust system. The most common system utilizes an energy recovery wheel located inside an air handling unit. Half of the wheel is located within the exhaust air stream, and the other half of the wheel is located in the outdoor air stream. The wheel rotates in order to transfer energy from the exhaust air stream into the outdoor air intake stream.

Commercial HVAC Ventilation

In commercial buildings, a large portion of the energy consumption is due to heating and cooling the outdoor ventilation air that is required by code. This air can be very cold in the winter in northern climates and very warm in the summer. The return air stream temperature is typically maintained at approximately 75°F which has the ability to heat up winter air and cool down hot summer air before it enters the heating or cooling coils where costly energy must be used to change the temperature of the air.

The energy recovery potential and effectiveness of this system increases with higher temperature differentials between the outdoor air and return air temperatures.

In the winter, the energy recovery wheel is heated by the 75°F return air. As the wheel rotates, the warm sections of the wheel come in contact with cold outdoor air which gets heated up before the outdoor air comes in contact with the heating coil. These wheels typically transfer about 70% of the potential energy between the air streams, which is a very significant amount, especially when it’s below zero degrees outside.

Energy recovery wheels are not the correct application for all buildings, but they are appropriate for most commercial applications where there are no hazardous odors in the exhaust air stream. This is because the energy recovery wheels are slightly porous to aid in the energy transfer process. Their porous design allows about 1% of the exhaust air stream to enter the supply air stream. This small amount is not typically a concern, but applicable codes should be referenced for individual circumstances.

Heat pipe recovery loops provide a similar energy transfer result and do not allow any exhaust air into the supply air stream. These are used for labs and other types of occupancies where exhaust streams may be hazardous, but the infrastructure required is more complex, which increases the cost and payback period.

Filter Pressure Sensors

The design of commercial HVAC systems should always be focused on reducing the amount of energy required to operate the system. Reducing fan speeds by providing variable frequency drives, or by providing larger ductwork to decrease the pressure required to force air through the system are two examples. One item that falls more on the building maintenance side is to regularly change air filters in air handling units as they get dirty. If filters are serving their purpose, they’re getting dirty by trapping airborne particles before they enter occupied spaces. This process clogs the filters and makes it more difficult for air to flow through their respective filter media. The end result is that the fan system has to work harder to provide the required airflow, which increases the electrical demand, costing more money to operate.

Electronic pressure sensors installed on each side of the filter will read the pressure drop across the filter and send data to the building controls system to alert the owner when the pressure drop increases above a set differential pressure limit. This set point limit should be evaluated for each specific application.

Tip:  If a building does not have a building controls system, a simple manometer type pressure sensor and gauge can be used and frequently checked by building maintenance staff to determine when it is time for filters to be replaced or cleaned.

Condensing Boilers

If a building has old cast iron hot water heating boilers that are nearing the end of their useful life, then switching them out with condensing type hot water boilers can be a good way to lower the natural gas or fuel oil bill. These condensing boilers have efficiency ratings around 90%-95% when operated as they are intended. Cast iron boilers have efficiency ratings around 80%-85%.

Cast Iron Boiler Valve

Condensing boilers must be operated correctly in order to see the most savings. This means operating at a lower hot water loop temperature. This lower hot water temperature produces a lower exhaust stack gas temperature which actually produces condensation during the heating process. Condensation is formed when the exhaust gases come in contact with the low-temperature hot water return piping which should be designed to be below the exhaust gas dew point. Typical hot water return temperatures are designed between 100°F to 130°F which is known to be below the exhaust gas dew point in most cases. When the exhaust gas water vapor condenses, this process recovers the latent heat of vaporization and is what sets these types of boilers apart when it comes to efficiency.

One item to consider is that due to the condensation formed inside these units, there is potential for corrosion of the boiler internal components. Due to this risk of corrosion, the boilers must be made of a non-corrosive material such as stainless steel, which increases the cost of the equipment.

Another consideration before switching out a high temperature cast iron boiler hot water loop (operating around 200°F supply temperature with a low temperature hot water loop operating around 140°F) is that this lower water temperature does not provide as much heating capacity at equivalent water flow rates. This means that all of the terminal equipment such as baseboard radiators, variable air volume units, fan coil units, etc. will all have to be evaluated for their potential heating capacity with a lower hot water supply temperature.

Magnetic Bearing Chillers

New technology in chillers has also produced higher efficiency equipment for building owners to consider. This new technology consists of a magnetic bearing compressor, which utilizes a magnetic field to produce lift and allow the compressor to spin without any friction instead of using bearings found in a traditional centrifugal chiller. The energy savings are most noticeable at lower operating speeds, which is likely where most chillers operate for the majority of their life. Because there is no friction, the system requires no oil as a lubricant, which also saves cost due to less maintenance and also due to the lack of decreasing efficiency ratings over time caused by the presence of oil in the chiller.

When considering this type of system, the cost is substantially higher than any conventional chillers. For new buildings and for replacing failed equipment, the magnetic bearing chillers can make for a quick payback. On the other hand, if a building has chillers with significant life remaining, a retrofit doesn’t always equate to an attractive payback period due to the high initial cost of the magnetic bearing chillers.

Higher efficiencies are not the only benefit of magnetic bearing chillers. The sound produced by these machines is incredibly low compared to conventional chillers. If the location of a chiller has the potential to be a nuisance due to loud compressors, then a magnetic bearing chiller might be an easy solution.

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Topics: commercial hvac efficiency