Striking a Balance
HVAC engineers provide key pointers in evaluating desiccant dehumidification vs. reheat to optimally meet a facility's needs. CSE: What are the advantages and disadvantages of desiccant dehumidification? PALM: Traditionally, a mechanical refrigeration application requires cooling the supply air well below its dew point to remove the humidity and then adding back sensible heat or reheat to atta...
HVAC engineers provide key pointers in evaluating desiccant dehumidification vs. reheat to optimally meet a facility's needs.
CSE: What are the advantages and disadvantages of desiccant dehumidification?
PALM : Traditionally, a mechanical refrigeration application requires cooling the supply air well below its dew point to remove the humidity and then adding back sensible heat or reheat to attain the final supply air temperature required to satisfy the room sensible cooling load.
Desiccant dehumidification removes the moisture in the supply airstream by the chemical and physical process of adsorption and is largely based on vapor pressure differential.
KHATRI : Desiccant dehumidifiers are most effective when the supply air temperature to a space is below 40°F dew point. Using desiccant dehumidification at these temperatures avoids condensate freeze-up problems at the cooling coil.
MOFFITT : On the other hand, a disadvantage is the high temperature required to regenerate the desiccant, essentially removing the water vapor that the desiccant has adsorbed from the air. The coefficient of performance (COP) of this dehumidification process is inefficient, typically less than 0.6. And unless there is on-site power generation, this regeneration heat often comes from natural gas, which is costly. If used in a commercial application, another disadvantage is that the air produced is warm, adding cooling load to the space. These systems dry out the air and heat it up, but then it has to be cooled back down.
PALM : Dry desiccants, I think, are a better option over fairly limited liquid desiccants, in that rotary-wheel-type heat exchangers can be employed, providing the ability to readily transfer both heat and moisture between two airstreams and provide capital cost savings resulting from smaller heating and cooling plant requirements and the associated operating cost savings.
Still, heat energy is required to regenerate the desiccant. Depending on the required room relative humidity, the source of the rotary wheel regeneration energy may be a general building exhaust airstream or may require a heat source such as a gas burner to warm a dedicated regeneration airstream, which is often raw outdoor air. Overall, with desiccant dehumidification, there is a substantial reduction in heat energy required, but there is also often a reduction in the cooling energy required to bring supply air to the same specified air condition. This is done by preheating or precooling the mixed airstream with what would otherwise be wasted energy from an exhaust airstream through the rotary energy recovery/dehumidification wheel.
Of course, the disadvantage of this scheme is the potential impact to the air handling unit's overall size and weight. Units that use energy wheels require two airstreams—either parallel or series—increasing unit size and weight and thereby affecting the structural steel design.
CSE: Have there been any notable advances in desiccant technology that make this approach more viable?
MOFFITT : As Mr. Palm outlined, the most notable advancements involve combining desiccants with standard vapor compression equipment. A desiccant rotor is used to enhance the dehumidification capability of a standard DX or chilled water coil. The cooling coil removes most of the water from the air via condensation and then a desiccant rotor boosts the latent performance of the unit by removing more moisture via adsorption. Most of the dehumidification work is accomplished by the vapor compression equipment vs. the desiccant. This results in a more energy-efficient desiccant dehumidifier. Since the rotor is not removing as much water vapor, low-grade heat can be used to regenerate the desiccant. This allows for recovered heat to be used, such as condenser heat. Return air from the conditioned space can also be used to regenerate the desiccant, and return air can be utilized by using the rotor in series with the cooling coil; here, all the dehumidification is accomplished by the cooling coil, and the desiccant enhances its performance.
Removing the need for new energy for regeneration heat has greatly increased the likelihood of desiccants being used in commercial HVAC systems. This new class of desiccant-enhanced dehumidifiers has found a way to combine the two technologies to achieve an efficient way to dehumidify vs. trying to use one technology instead of the other.
PALM : There have also been advances in the area of contamination. Contamination of the supply airstream can occur through the energy transfer process of the rotary wheel passing through two different airstreams. This has become less of a concern. Current energy wheel technology uses either 3- or 4-angstrom molecular sieves that limit the potential for larger molecules to transfer from the exhaust airstream to the supply airstream while allowing the sensible and latent energies of the airstreams to transfer. In addition, pressure differential relationships maintained between exhaust and supply air paths along with planned purge air, limits the potential for supply air contamination. Wheels are tailored for sensible and latent capacity ratios, rotational speeds are varied and wheels are used in series to wring almost every BTU possible from the exhaust airstream and minimize the amount of recooling or reheating need.
CSE: So are there any reasons an engineer would still use reheat for purposes of dehumidification?
KHATRI : Cooling-based systems with reheat have a lower first cost and are more effective when the supply air dew point required is more than 40°F. But these systems also have higher power requirements.
MOFFITT : At the same time, cooling and reheating the air can be a simple and economical way to dehumidify a building, especially when the reheat energy is recovered. The primary advantage is that any heat that may be needed at part-load conditions can be recovered from the condenser. This can be done either directly via hot refrigerant gas before it enters the condenser on air-cooled systems, or from the warm condenser water from water-cooled systems. The main disadvantage is the ability to achieve lower supply air dew-point temperatures. The cooling system may have difficulties getting below 45°F if DX, below 42°F if chilled water and below 38°F if chilled glycol. However, most comfort applications require supply air dew-point temperatures between 50°F and 60°F to keep the space relative humidity under control. This is well within the abilities of cool-reheat systems.
PALM : But these types of systems, while a lower capital cost, are typically associated with higher operating costs. Today most jurisdictions have an energy code that refers to ASHRAE 90.1, discouraging simultaneous heating and cooling of the same airstream, unless required to meet certain process requirements or occupancies necessitating humidity or space pressure control such as hospitals or laboratories.
The main disadvantage of mechanical cooling and reheat for dehumidification is that the cooling coil is the road block to low supply air dew-point temperature, which is the main parameter determining room relative humidity.
CSE: How does one best determine which option is optimal for a given situation?
MOFFITT : First, determine the supply-air dew-point temperature required to dehumidify the space. Is it within the range that refrigeration equipment can provide? Next, determine what heat sources are available. Is there on-site power generation or an industrial process with waste heat? Is there condenser water available to recover heat? Can I recover heat from hot refrigerant gas, if air cooled? Is there return air coming back to the air handler? What is the sensible load in the space? Do I also need cooling or heating when dehumidification is needed?
If a system uses a gas-fired heater or electric heat to regenerate a desiccant, it will most likely use more energy than a cool-reheat system, even if the reheat energy is not recovered. If comparable systems use similar heat, often a full system life-cycle analysis is needed to see which technology wins out, keeping in check what happens to the system cooling and heating loads.
PALM : Don't forget that confirming compliance with the energy code is the first step, followed by a life-cycle cost analysis, which is generally the best approach for analyzing options for HVAC systems.
In addition, mechanical cooling with reheat is limited because of the problem of frosting of moisture in the airstream on the cold surfaces of the cooling coils. Using an operating room as an example, a supply airstream with a dew-point temperature of 50
A non-heat regenerated or “passive” wheel desiccant system with a supplemental cooling coil can easily achieve supply air dew-point temperatures approaching 45supply air dew-point temperatures of 35ºF or lower, resulting in a 40% relative humidity at a room temperature of 60ºF.
Desiccant dehumidification allows one to more closely match the capacity to the load, resulting in less energy use than the reheat approach—like riding with one foot on the gas and the other on the brake.
CSE: Do certain building types and climates lend themselves to one technology over the other?
PALM : Building occupancies that require low supply-air temperatures or high ventilation rates benefit from energy recovery wheel applications—auditoriums, casinos, theaters, conference centers, restaurants and educational buildings—anything with high population densities. As a matter of fact, 90.1 indicates that systems greater than 5,000 cfm with 70% or more outside air are required to use exhaust air energy recovery with an effectiveness of at least 50%.
KHATRI : As mentioned, building and spaces requiring low dew-point levels—such as industrial, pharmaceutical and cold storage facilities—favor desiccant dehumidification. It's also used for people comfort where moderate, but dry air is preferred.
MOFFITT : Climate is key. How the space is used will determine how many hours a year the humidity will need to be controlled and to what level.
Farhan Khatri, P.E., LEED AP
Arnold and O'Sheridan
Ronald Moffitt, P.E.
Principal Applications Engineer
Trane Commercial Systems
Johannes Palm, P.E.
Senior Mechanical Engineer
Albert Kahn Assocs.
A High IAQ
As knowledge of the built environment continues to compound, issues of good indoor air quality and proper humidity levels are more on the minds of building owners.
“A lot of this [awareness] has been driven from experiences with mold remediation where an owner has had mold in a building from bad building envelope and system design,” relates Ronald Moffitt, P.E., principal applications engineer, Trane Commercial Systems, Lexington, Ky. “These instances have become more pronounced in recent years. Several reasons may be because buildings are built quicker, tighter and with less internal sensible loads. This means once water gets in, it's harder to get it out, especially if there is no humidity control,” he explains.
Farhan Khatri, P.E., LEED AP, sustainability strategist, Arnold and O'Sheridan, Madison, Wis. adds that a greater focus on employee comfort has also heightened awareness with regard to these issues.
“Owners are concerned that their employees are not happy even when the space thermostat set point is satisfied, because the relative humidity levels are too high or low. Consequently, complaints about chills, stuffy noses and allergies are driving more clients to consider humidity control in commercial/corporate market, as owners want to protect their occupants and buildings from these conditions,” he claims.
In addition, the consultant plays a key role in raising awareness, according to Moffitt.
And once the decision is made to place a greater focus on controlling humidity, owners can tap into a greater array of technological tools which are now available on the market.