School and hospital construction projects have two things in common: construction timelines and indoor air quality issues are critical.
School building projects usually have to be completed before the start of a new academic year as students and teachers will expect classrooms to be ready. Hospitals are also expected to open on a specified date as doctors and patients have treatments and procedures scheduled.
Also, building owners fear the growth of mould and/or proliferation of bacteria and viruses. Schools want students healthy and awake and hospitals want patients to recover quickly. Good indoor air quality (IAQ) plays a large part of accomplishing both goals.
This article will focus on how desiccant dehumidifiers can help remedy these concerns in both the construction and occupancy phase of a project as well as provide points for achieving Leadership in Engineering and Environment Design
Dry argument
A desiccant dehumidifier is a product that has the ability pull large amounts of moisture out of an air stream, (see figure 1).
The advantage of desiccant technology is that it can reduce the dew point (vapour pressure) of air well below what is typical of a chilled water coil or a packaged DX system. The process uses a rotating wheel that is coated with a desiccant material. As the process air stream passes through the wheel, the desiccant will adsorb large amounts of moisture, dropping the air’s dew point and vapour pressure, while adding sensible heat at the same time.
As the wheel becomes laden with moisture, the wheel rotates into another airstream that is heated in order to release the moisture or reactivate the desiccant prior to rotating back into the process air stream.
For this article, the process air stream is equal to the outdoor air we are bringing into the building. By supplying air into a building that has a lower dew point than the surrounding environment, the air can keep the building dry.
Schools and hospitals have critical construction timelines. Contractors may be able to work fast, but they are limited by the materials they use and more specifically, how long it takes those materials to cure.
Concrete takes time to dry, wall ‘mud’ takes time to set up and paint takes time to dry. Other exterior forces are also out of the contractor’s control. Rain can get sheetrock wet or flood the construction site and damage materials such as wood flooring. Even something as common as a string of high humidity days can slow the drying process down.
Any contractor will agree that the faster concrete cures, or paint dries or fire suppressant coatings set, the quicker they can continue the building process. Valuable time is lost due to normal drying cycles based on ambient temperature and humidity.
By employing a desiccant dehumidifier during construction, drying times are reduced significantly. Air is provided to a space with a lower vapour pressure than the surrounding air; the water that is inside concrete, or fire proofing or sheetrock, will quickly escape out of those products allowing them to dry in less time.
For example, experience has shown that indoor concrete under normal ambient conditions can take 37 days to dry. But with a properly sized desiccant unit blowing dry air across the slab, it can dry in as little as 14 days. Drywall compounds normally require two to three days to cure, but with a desiccant dehumidifier, it can be ready to sand in as little as 24 hours regardless of the ambient temperature and humidity.
By using a desiccant dehumidifier, the drying of a building – a process in itself that does not add value, but requires time – can be shortened substantially, saving the contractor money and keeping the project on schedule.
Keeping mould at bay
Another advantage of keeping the building dry during construction is that it will prevent moist materials from becoming harbourers of mould and mildew.
Everyone has seen the wet job site with standing water inside the building prior to the sheetrock and other materials being installed. If this water comes into contact with the sheetrock, or is trapped behind a wall, it can provide the moisture needed for mould to begin growing in places that will be hard to find.
By keeping a construction space dry, below 60 per cent outside air (OA), mould will not be able to grow. This is the first step toward good IAQ during occupancy.
These techniques can be employed by the contractor on his own or required per the plans and specifications by the architect or engineer. However, employing desiccant technology during the occupancy phase begins with the HVAC engineer and his overall design concept.
Post-construction occupancy
Design professionals are faced with ever-increasing challenges when designing and operating educational and medical facilities.
ASHRAE Standard 62.1 prescribes the amount of OA that must be introduced into the building to ensure good IAQ. Over the past 15 years, many different designs have been implemented, some successfully and some less so. In 2007, ASHRAE released the newest version of HVAC applications that describes the most successful approach and should be considered the best design practice for processing ventilation air.
During construction
The HVAC system chosen may be a centralised system, such as a chiller plant, with ductwork distributing air to each space through fan coil units, VAV boxes or a chilled beam system. The design may also call for a decentralised system where each space has its own cooling/heating device such as a single zone water source heat pump (WSHP). ASHRAE 2007 HVAC application handbook 6.7 explains: “although most centralised and decentralised systems are very effective at handling the space sensible cooling and heating load, they are less effective (or ineffective) at handling ventilation air and the latent loads. As a result, a dedicated outdoor air system (DOAS) should be used.”
Further on, ASHRAE states that DOAS should be used to also control the space moisture levels. “It is preferable, however, to introduce the outdoor air at a lower humidity ratio than the desired space humidity ratio, to allow the zone HVAC unit to handle only the space sensible load.”
This system divides out the moisture component from the room’s air conditioning system to conquer the many issues associated with building moisture control, energy efficiency, IAQ and others. This method is a change from traditional engineering.
Historically, engineers wanting to use a DOAS would plan to use a system that would deliver OA at a neutral condition: 24°C and 12.8°C dew point. This delivered condition is commonly associated with maintaining 24°C and 50 per cent RH. A simple DOAS can accomplish these conditions during the cooling season by processing OA through a cold coil down to a 12.8°C saturated condition and then reheating the air back up to 24°C, ensuring the air is delivered at 12.8°C dew point.
However, introducing OA at a 12.8°C dew point will not hold the space at 50 per cent RH. A space will generate its own additional moisture from the occupants, space processes (such as in a laboratory) and infiltration. When carpets are mopped or steamed clean on a regular basis that will introduce a large amount of additional moisture to the space. All of this additional moisture must be removed from the space in order to control RH within the recommended 40 to 60 per cent range. Without this moisture control, structural damage and occupant health risks can happen.
The best design practice uses a DOAS to manage the latent load and the AC system to handle the sensible load only. This method not only increases the HVAC system efficiency, but will provide superior IAQ and moisture control.
Using a desiccant dehumidifier as the DOAS will allow the engineer to design a unit that will ‘over-dry’ the outside air past the required space dew point set point and allow the building to control the moisture and thus the 50 per cent RH. This will make certain the proper space moisture content is maintained and will prevent mould growth as well as bacteria and virus proliferation, ensuring good IAQ.
Desiccants help gain the LEED
In America there are several areas where using desiccants can help earn LEED points for a project.
In construction, by using desiccants to keep the building and its materials dry, the equipment can assist in meeting SMACNA IAQ guidelines for occupied buildings under construction.
Desiccants can also be used to bring in outdoor air to flush the building prior to occupancy. The important reason to use desiccants is that it will also continue to keep the building dry while all the volatile organic compounds from construction are removed.
Once a building is occupied, desiccants that use recovered energy as the reactivation source can lead to large energy savings for the dew point delivered. It can also deliver additional ventilation air for the same energy as a standard cool/reheat unit delivering the minimum ventilation air.
Using desiccants that follow ASHRAE’S recommendations for the DOAS can provide control of the space relative humidity required to comply with this design.
Desiccant technology is viewed as a viable method to generate air lower than a 7.2°C dew point. In fact, for applications requiring dew points below 1.6°C, desiccant dehumidification is the most reliable and modern approach used.
When low dew point air is used to help speed up the internal drying process of a building under construction or as part of the overall HVAC concept to keep the occupied space within proper moisture tolerances, yeveryone wins – contractors, engineers, owners, students, teachers, doctors and patients.