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Demystifying HVAC Based Dehumidification

Demystifying HVAC based dehumidification

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Selecting the right heating, ventilation and air conditioning solution is critical to patient well-being in healthcare facilities. How HVAC-based dehumidification for new construction and retrofit applications can help reduce the risk of hospital-acquired infections and improve indoor air quality.

Hospital administrations are taking a closer look at HVAC - based dehumidifiers to control indoor humidity levels. Their goal is to bolster their overall tight against airborne hospital-acquired infections (HAI) and increase indoor air quality' (IAQ) for patients, employees and visitors.

Studies have shown that relative humidity levels above 60 per cent in healthcare facilities can promote airborne biological contaminants, such as tuberculosis, influenza and other respiratory ailments.

The Center for Health Design (CHD) in Concord, California, analyzed 120 independent studies before concluding that clinical outcomes improve when patients receive quality-centered healthcare in facilities where the temperature, humidity and IAQ are effectively managed. The CHD also claims airborne pathogens cause more than 30 per cent of HAIs.

As a result, retrofitting dehumidifiers into existing units or incorporating them into newly installed HVAC systems has become a growing trend. For example, an active desiccant dehumidification unit won this year's IAQ category at the Air Conditioning, Heating and Refrigeration Expo's AHR Innovation Awards. The unit is designed to provide surgery centres with ASHRAE Standard 170-mandated relative humidity levels as low as 20 per cent.

Outdoor air induction and return air are major contributors to humidity as they travel through the HVAC system. Consequendy, choosing the best sized and suited commercial dehumidifier for a particular project is critical to achieving the desired results.

Codes based on ASHRAE Standard 62.1 require HVA C systems to introduce as much as 15 to 20 per cent outdoor air, depending on their original engineering design. In summer months, this creates a tremendous humidity load that conventional air conditioning systems can't handle without help from dehumidifiers. It's also an energy drain, because air conditioning systems work harder to condense moisture out of supply and return air streams.

SELECTING A DEHUMIDIFIER FOR NEW CONSTRUCTION

New construction is the best scenario in which to incorporate dehumidifiers, because the project is designed from scratch. The trend in healthcare facilities is toward energy recovery' ventilators (ERV), which typically use desiccant-based enthalpy wheels to adsorb moisture and transfer energy. These machines can be installed separately from the HVAC system(s) or bolted onto the HVAC unit; however, both methods precondition the outdoor air and return air for the HVAC system.

In the case of the Montreal-based Centre Hopitalier de l'Universite de Montreal (CHUM), 47 ERVs will dehumidify outdoor air and recover energy from return air for what's considered the largest hospital addition in North America. It will measure more than 2.5-million square feet when it's completed later in this decade. Engineers have specified molecular sieve enthalpy wheel technology to reduce moisture in the 100-per cent outdoor air that supplies the building's 2.8-million-cubic-feet-per-minute requirement.

The ERV design carefully avoids cross-contaminating the outdoor air with indoor contaminants such as biological and volatile organic compounds when recovering energy. Keeping indoor humidity at below 60 per cent relative humidity will minimiz e airborne microbial procreation.

Enthalpy wheels are ideal for heat recovery in hospitals. However, the wheel's desiccant media choice — typically silica gel or molecular sieve material — is critical for preventing the incoming outdoor air from residual contamination from the indoor air that is returned for exhaust or energy recovery.

Silica gel has a propensity for cross-contamination. Its pores can adsorb and harbour contaminants from the indoor air that are returned through the wheel for energy recovery and then exhausted. The contaminant build-up also decreases latent heat transfer.

Molecular sieve materials, conversely, aren't susceptible to contaminant adsorption. Their smaller pore size only allows the transfer of the water vapour from the exhaust to supply air streams.

During winter, when outdoor air is uncomfortably below 30-per-cent relative humidity, humidity collected from the exhaust air can be added to the supply air to raise indoor relative humidity* to more comfortable levels.

Because of HAI concerns, it's critical not to contaminate incoming outdoor air with outgoing indoor air contaminants while extracting moisture from the enthalpy wheel during humidification.

To further minimize cross-contamination, desiccant surfaces can be specified with anti-microbial coatings to help prevent the media from harbouring airborne biological contaminants.

SELECTING A DEHUMIDIFIER FOR RETROFIT APPLICATIONS

Facility managers have three choices when considering dehumidification as a retrofit. The first is an ERV that is separate and supplies conditioned air to an existing rooftop HVAC system. The second is an ERV that is bolted onto the existing rooftop HVAC system. And the third is an ERV that has built-in heating and cooling coils supplied by a building's central plant and operates as a self-contained HVAC system.

Selecting a dehumidifier can be a complex decision. However, most manufacturers offer free software that their manufacturer's representatives use to specify the best dehumidifier for a particular project. Inputting the region automatically factors in the project's specific geographical annual weather/ humidity conditions.

Then, these web browser-based programs produce input/ output conditions, Air Conditioning, Heating and Refrigeration Institute (AHRI) efficiency requirements, load calculations on desiccant wheel(s), static pressures and losses based on inputs, annual operational costs and many other parameters.

Additionally, some software can compare results to a competitive product's efficiency using published non-partisan AHRI data, such as Guideline V "Calculating the Efficiency of Energy Recovery Ventilators and Its Effect on Efficiency and Sizing of Building HVAC Systems," which is intended for service contractors, engineers and building owners.

CHOOSING ROI VS. RER

An HVAC system's static pressure is critical to fen energy consumed and consequential operational costs. All ERV wheels are not equal in tenns of the static pressure they produce; however, many facility managers choose ERV project bids based on the cheapest cost.

AHRI's Guideline V illustrates that it's not retum-on-investment (ROI) of first costs that's important, but rather ROI based on energy saving of the equipment efficiency, which is commonly called recovery-efficiency-ratio (RER).

Studies have shown that relative humidity levels above 60 per cent in healthcare facilities can promote airborne biological contaminants.

Accounting purely for high efficiency and capital cost in wheel replacement, without regard to static pressure, which requires fans to use more energy, may in fact defeat attempts by the building owner to maximize long-term energy savings and ROI.

Instead, review wheel replacement options for dieir RER, which considers the efficiency' and the static pressure of a desiccant wheel replacement. Failing to calculate the RER could result in tens of thousands of dollars in lost energy savings over the course of the ERV wheel's lifecycle.

The Centers for Disease Control and Prevention estimate that 70 per cent of HAI cases are preventable. Adding the proper dehumidifier for the application might make the difference between life and death. Not to mention, it can also improve IAQ for patients, staff and visitors and cut down on operational costs, especially when RER is factored into the selection.