Due to the recent implementation of air quality standards within the manufacturing industry, more facility managers are being required to address indoor air quality (IAQ) issues within their plants. It has become essential for them to find ways to introduce 100 percent outdoor air to promote the health and safety of personnel or to eliminate the buildup of exhaust byproducts from various manufacturing processes.
However, many unintended side effects occur when higher volumes of outdoor air are introduced into a facility. Most often, humidity will rise and cause equipment or entire production lines to malfunction. While employees may benefit from fresh outdoor air, high humidity makes them less comfortable and, as a result, they have lower output or production.
To avoid these types of problems, companies need to assess their situation before introducing outdoor air into their facilities. Many managers are surprised at the amount of moisture that must ultimately be removed to prevent problems in their operations. The best approach is to calculate a facility’s outdoor air requirements and then size the appropriate dehumidification equipment to serve the need. In most cases, this step solves or prevents indoor air problems without even having to change the main air handler’s capacity or functionality.
Calculating Energy Removal RequirementsIn the example that I will discuss, the air entering the dehumidification system is 100 percent outdoor air. Proper system size is selected by calculating the amount of energy that must be removed from the entering air at the maximum design condition to achieve a desired leaving air dewpoint (LADP). The most direct calculation to determine LADP is known as the total enthalpy method. It is based on the enthalpy difference (BTU/lb) between the maximum design condition and the specified leaving air condition, multiplied by the airflow.
Rate of Energy Removal Required (BTU/hr) = ΔH x airflow x 4.5
ΔH is the enthalpy difference in BTU/lb,
airflow, expressed in cfm (ft3/min), is the specified outdoor air volume, and
the value 4.5, as measured in (min/hr x lb/ft3), is a conversion factor of 60 min/hr divided by 13.5 ft3/lb (of air).
Because the weight of air varies with temperature, further accuracy could be gained by using the precise weights for the two different temperatures involved, but this approximation is nearly always sufficient for sizing purposes. The enthalpy difference is calculated by taking the enthalpy value (BTU/lb) at the entering wet bulb temperature and subtracting the enthalpy value at the design dewpoint.
As an example, suppose you are sizing a pretreatment dehumidifier for a building in St. Louis, with required outdoor air introduction of 2,000 cfm. The wet bulb temperature design value is 78°F (see ASHRAE chart). At saturation, 78°F wet bulb equals 78°F dewpoint, the associated enthalpy value is 41.5 BTU/lb (table 1). If the air handler expects air at 72°F and 55 percent relative humidity, or 55 dewpoint, you can look up a corresponding enthalpy from table 1, which shows enthalpy would be 23.2 BTU/lb. In this example, the dehumidifier will need sufficient capacity to remove energy at the following rate:
Rate of Energy Removal Required (BTU/hr) = (41.5 - 23.2) x 2,000 x 4.5
Rate of Energy Removal Required (BTU/hr) = 164,700 BTU/hr
This energy removal rate then is compared to the capacities for various dehumidification systems to help determine the best system for the application.
Note that the total enthalpy method simplifies the sizing discussion by focusing on total energy removal (combined latent and sensible) rather than on a moisture load (often presented in lb/hr) to be handled by the dehumidifier. Instead of trying to develop a moisture load from dewpoint and wet bulb values, the values are used directly to arrive at the required dehumidifier capacity.
The ASHRAE guidelines (see table) state the design condition simply as a peak wet bulb temperature. Associated with that temperature is a wet bulb line on the psychrometric chart. Sizing for the enthalpy difference between the peak wet bulb and the leaving air dewpoint will ensure that the dehumidifier can handle the wide range of dry bulb temperature/relative humidity combinations that fall along or beneath the wet bulb line (figure 1). A dehumidifier sized to remove the necessary energy to meet a 78°F wet bulb requirement for St. Louis, for example, also will handle 85°F up to 70 percent relative humidity or 90°F up to 60 percent relative humidity. If the dehumidifier was tested at different points along the wet bulb line, the amounts of latent vs. sensible heat removed would change significantly, but the total heat removed would not.
Dehumidifier Selection and PerformanceWith 100 percent outdoor air dehumidifiers, it is important to understand how to select the correct system for the application as well as to understand how the dehumidifier will perform under the varying full and part load conditions it will encounter.
The correct dehumidifier is selected by specifying the following criteria:
- Volume of air required.
- Maximum design condition (db/wb).
- Leaving air dewpoint required.
- Desired leaving air temperature.
The total energy removal required and, therefore, the dehumidification capacity needed are directly proportional to airflow. Conversely, for the same airflow, a lower leaving air dewpoint can be achieved by moving to a dehumidification system with greater capacity.
For example, compare the performance of two dehumidifiers with entering air at 78°F wet bulb, a 2,000 cfm airflow requirement to meet ASHRAE 62, and a required leaving air dewpoint of 55°F or lower to match the original design conditions for an existing air handler. (See table 2 for the capacities.) At an airflow rate of 2,000 cfm, the smaller unit can only produce a leaving air dewpoint of 60°F, which will not meet the 55°F requirement. The larger unit, at the 2,000 cfm airflow, can produce a leaving air dewpoint of 55°F and would be acceptable for this application.
A convenient way to portray the performance of a dehumidification system over the wide range of ambient conditions is to plot a graph with “entering air wet bulb temperatures” on the x-axis and “leaving air dewpoint capabilities” on the y-axis (figure 2). Given the entering wet bulb temperature and airflow, the leaving air dewpoint can be read off the chart to show the resulting leaving air condition at part load conditions.
Proper dehumidification system sizing can be accomplished by calculating the amount of total (latent and sensible) heat to be removed per hour from the additional outdoor air, based on ASHRAE wet bulb temperature design values. As a convenience, some manufacturers provide graphs from which the leaving air dewpoint can be obtained for a given entering wet bulb temperature and air flow requirement. Some even have software to do the calculations.
Without pretreatment, increased outdoor air brought into an air handler solves one indoor air problem only to cause others. By pretreating outdoor air with a partial or variable partial reheat dehumidification system, all the benefits of a healthy, productive environment for building occupants and equipment can be realized without introducing excessive moisture or improper temperatures.