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The following methods have been used successfully to calculate vapor loads, replacing the extensive calculations and laboratory tests that might otherwise be required when a designer considers a new space humidity problem or application.
Actual data from moisture loads entering a space through walls, floors, and ceiling are available for various moisture loads and classes of construction.
For standard types of construction, Bry-Air has determined values for calculating the moisture load entering a space at controlled humidity levels. Usually these calculations are relatively easy. The following tables are aids for load calculations.
Outside humidity levels shown in Table I are deliberately higher than data for design specifications. This compensates for days when the design wet-bulb temperatures are reached and the design dry-bulb temperatures are lower than expected (thus creating higher total humidity). Use the area design wet-bulb and the specific humidity figures shown here to accurately rate the moisture control situation. Further information on design can be found in the ASHRAE Fundamentals Handbook, “Weather Data and Design Considerations”.
Space moisture load is a combination of permeation and infiltration and both will be encountered in determining the load. Permeation is a straight line function of the difference in interior and exterior vapor pressures (determined by gr/lb). As shown in Table III, infiltration, represented in air changes per hour is not straight line because of the two factors involved:
Each pound of air entering the space will impose a moisture load determined by the difference in interior and exterior moisture content. Since the vapor pressure differs as the moisture content, the vapor will move at a higher velocity than the air. The combination of the two factors, results in the space moisture load increasing at an ever increasing rate as the difference between the interior and exterior moisture contents increase.
In view of the above, the F1 factor is used to adjust for the increased vapor velocity. Therefore, the combination of the F1 and F2 factors represent the space moisture load anticipated from both permeation and infiltration.
If the product of F3 x F4 is less than 0.5, use 0.5. If the room is completely vapor proofed, with continuous vapor barrier under the floor (or of allmetal, welded material) the factor may be reduced to 0.3.
Calculate the Permeating Load through a Structure
To determine the grains of moisture penetrating the construction into a controlled space, use the following calculation.
V
C x ∆G x F1 x F2 x F3 x F4 = Grs/hr. (To determine grains/minute divide answer by 60).
= Amount of vapor able to permeate the closed space through construction and vapor barriers.
V = volume of controlled space in question – ft.3
C=14 = constant used to translate ft.3 to pounds. This constant is used regardless of the density of the air.
G = difference between the grs/lb of outside air and the grs/lb desired in the controlled space.
F1 = moisture difference factor (Multiplier from Table II).
F2 = Permeation factor (Multiplier from Table III).
F3 = Construction factor – Table IV { See note with Table IV
F4 = Barrier factor – Table IV
The above equation can be used to solve a typical example as follows:
Problem- Find the amount of moisture that will permeate the room defined below.
Sample Calculation – Space to be controlled:
Room with 12″ masonry walls.
Two coats of aluminum paint as vapor barrier.
Volume of room – 22,000 ft.3
Outside Design: 95°F db 77°F wb (Table I Shows 130 gr/lb)
Required – To hold in room – 40 gr/lb
V
C x ∆G x F1 x F2 x F3 x F4 = Grains per hour.
V = 22,000
C = 14
G = 130 – 40 = 90 Problem stipulates 40 gr/lb in the room;therefore, 130 – 40 = 90.
F1 = 2.29 From Table II (Factor for a moisture difference of 90 gr/lb).
F2 = 0.58 From Table III Locate 22,000 on bottom line. Travel up and readcurve at 0.58.
F3 = 1.0 From Table IV
F4 = .75 From Table IV – (Factor for 2 coats of paint)
22,000/14 x 90 x 2.29 x 0.58 x 1.0 x 0.75 = 140,884 grs/hr
140,884/60 = 2348 grs/min
Moisture through Intermittent Openings
When openings as service or personnel doors are opened periodically, moisture-laden air can enter the conditioned space. Also, vapor is constantly seeking drier space and will seep around and through doors, even when they are closed.
Obviously the first precaution is to assure that openings are adequately vapor-sealed. Then the drying equipment must deal with the moisture load that comes into a controlled space when the door is open. Assuming that the door is open only for short periods, calculate the moisture load as follows:
Ohr x A
C x G x F1 = Grains
Ohr = number of times each hour the door is opened. (If unknown, assume personnel doors to be opened 2 times/hr for every occupant.)
A = area of the door opening in square feet.
C = 7 = constant
G = difference in specific humidity in grs/lb between controlled space and the adjacent space.
See Table I for outside wb to determine adjacent specific humidity.
F1 = factor from Table II for moisture difference.
Examples:
Door area – 3′ x 7′
Door opens – 6 times each hour
Moisture difference – 90 grs/lb
Solution:
Ohr x A
C x G x F1 = grains per hour of additional load
6 x 21
7 x 90 x 2.29 = 3710 grains per hour added to controlled space.
Note that if the door is open for longer periods, use the calculation scheme below.
Moisture Through Fixed Openings (Conveyors, Open Windows, Etc.)
Calculate the amount of moisture that travels through a fixed opening from a wet space to a drier space as follows:
A x 300
C x D x G x F1 = grains per hour of additional load
A = Area of fixed opening in square feet
300 = Experimental constant – velocity of vapor, ft/hr at 35 gr difference
C = 14 = constant factor to translate ft.3 to pounds
D = feet = depth of opening
G = grains = difference in grs/lb between wet space and drier space
F1 = Moisture Difference Factor from Table II.
Example:
Conveyor opening
Depth of opening
Moisture difference 2 sq ft
1.5 sq ft
90 grains
Solution:
A x 300
C x D x G x F1 = grains per hour of additional load
2 x 300
14 x 1.5 x 90 x 2.29 = 5889 grains per hour
Moisture Originating In the Controlled Space
Moisture or vapor originating in the controlled space comes from any of several sources, depending on the intended use for the space. Three basic sources of moisture are:
Population load, including people and animals
Product load, brought in by the product
Process load.
Population load. People working in an area add moisture to the air because of breathing and the evaporation of perspiration. When animals occupy the controlled space, moisture release is contributed by their excrement.
How much moisture do people or animals add to a controlled space? Such factors as the level of activity and the ambient temperature, atmospheric pressure, and humidity are well documented. (See Appendix 3, page 40)
For animals, weigh the amount of water consumed during a given period and assume that much water will be eliminated.
Product load. Any material manufactured in a controlled area can bring moisture with it and then release the moisture into the work area. Material brought into a warehouse tends to become drier; it gives up moisture over a period of time and loads the drying equipment accordingly.
All materials should be suspect. For example, most metals bring very little moisture, but nonmetals can carry surprisingly large amounts of water. The material’s supplier should have information on its moisture carrying characteristics.
If such data are unavailable, a simple test should prevent an unexpected and substantial moisture load problem. Place a sample of the material in a small, dry container or place some material in a tall hopper and blow air over it to dry it. Measure the moisture loss over an appropriate time interval to determine its dwell time, or how fast it gives up moisture. In some cases, a small pilot plant can be used to acquire definite data.
Process load. The manufacturing process itself may expel moisture into the atmosphere of a controlled space. Open tanks or trays of liquid will add to the moisture load.
Other contributors include open steam exhausts, unvented combustion cycles, and aging or curing cycles.
Ventilating Air-Vapor Load (Vapor Brought In With Outside Air)
Ventilating or make-up air from the outside contains moisture that must be removed. Some designers add this moisture load to the total calculated internal load to determine the required capacity of the drying equipment.
However, Bry-Air recommends this air not be considered part of the internal load. Rather, it should be considered at its point of entry.
If this added, or make-up air from outside mixes with the return air and all go through the dehumidifier, then it is not added to the internal moisture load. But if only part of this outside/return air mixture passes through the dehumidifier, then the part bypassing the dehumidifier must be added to the internal load of the room.
The added air is only part of the total air used in controlling space humidity. Since it rarely gets into the controlled space without first going through the dehumidifier, consider it at its point of entry-at the dehumidifier.
Among the normal elements of an air drying system, the air entering the drying equipment is a mixture of return air from the controlled space and make-up air from the outside. The temperature and moisture content of this mixture depend on its two contributing air streams.
Calculating the Various Moisture Loads
Moisture-laden air enters through the process inlet and moves through the desiccant media. The desiccant adsorbs the water vapor and the dehumidified air is then delivered through the process outlet directly into the controlled space or air stream. Then, as the desiccant media rotates into the reactivation air stream, the hot air entering through the reactivation inlet drives off the moisture and exhausts it into the atmosphere. After reactivation the hot, dry desiccant rotates back into the process air stream where a small portion of the process air cools the desiccant so that it can begin the adsorption process allover again.
Construction of a Controlled Space
To prepare any space for humidity control, certain precautions are necessary, regardless of the type of air drying equipment or the method used to do the drying.
Satisfactory moisture control better known as customer satisfaction depends on the many variables.
The Nature of Water Vapor
Consider two closed rooms, adjacent to one another. If the partial pressure of the water vapor in room #1 is greater than the partial pressure of the water vapor in room #2, then the water vapor will travel through the wall into room #2 regardless of the composition of the wall.
Let’s take the hypothetical example a step further. If the absolute humidity of the air in room #1 is greater than that of the air in room #2, then the water vapor pressure will be higher in room #1. Therefore, when drying room #2, the problem of new water coming through the wall from room #1 must be considered.
A vapor barrier can slow down the passage of vapor from wet to drier areas, but it cannot keep water out; it can only slow the rate of penetration.
The choice of vapor barrier is based on the degree of dryness required in the controlled space, the efficiency of the equipment being used for drying, and the cost of construction.
Commercial vapor barriers moisture resistant construction material, paints, and other coatings offer a variety of design alternatives. Manufacturers of vapor barrier materials can supply specific information on their products.
In addition to the vapor barrier, certain aspects of construction must be given careful attention.
Construction Considerations
Several techniques control the permeation of water vapor:
1. Any vapor barrier must be continuous, without breaks or tears.
2. All lap joining must be tightly closed (this is particularly critical when mechanical or caulked joints are used).
3. Insulation between vapor barriers can be a potential problem: if construction occurs in humid weather, water can be “sealed in” between the two vapor barriers. Sealed-in vapor will travel into the controlled space and impose an extra drying load on the drying equipment. This extra load lasts only until the insulation dries out, but meanwhile, humidity control is difficult.
If a heat source is present (even heat from the sun), serious damage can be caused by the expanding trapped vapor. There have been cases when so-called “non-permeable” materials have split open at a joint because of vapor pressure. Examples include a floor or tiled wall that has literally lifted from its mounting surface because the surface was wet during application.
4. Final inside vapor barriers should be applied only after the enclosed area has been dried. Drying equipment should be used to withdraw as much moisture as possible before the final barrier is applied. Of course, without a barrier in place equipment cannot dry the air to design specifications, but a significant amount of moisture can and should be removed before all the vapor barrier material is in place. (Although this strategy runs counter to most industrial planning suggestions, the concept of drying the structure before applying the final vapor barrier is a precaution that is often overlooked and can help prevent customer dissatisfaction.)
5. All doors, service or personnel, should be weather-stripped or air-locked through vestibules if the desired dryness warrants it. Any crack or opening around a door will admit vapor.
6. When conveyor openings or similar elements are used, a drop curtain, shroud, or tunnel can restrain the movement of water vapor.
Desiccant Dehumidification vs. Mechanical Refrigeration
Both desiccant dehumidifiers and mechanical refrigeration systems can remove moisture from the air, so the question is which type is best suited for a given application? There really are no simple answers to this question but there are several generally accepted guidelines which most dehumidifier manufacturers follow:
- Both desiccant-based and refrigeration-based dehumidification systems work most efficiently when used together. The advantages of each compensate for the limitations of the other.
- Refrigeration-based dehumidification systems are more economical than desiccants at high temperatures and high moisture levels. In general, mechanical refrigeration systems are seldom used for applications below 45% RH. For example, in order to maintain an outlet condition of 40% RH it would be necessary to bring the coil temperature down to 30º F, which results in the formation of ice on the coil and a reduction in moisture removal capacity. Efforts to prevent this (defrost cycles, tandem coils, brine solutions etc.) can be very cumbersome and expensive.
- Desiccant-based systems are more economical than refrigeration systems at lower temperatures and lower moisture levels. Typically, a desiccant dehumidification system is utilized for applications below 45% RH down to 1% RH. Thus, in many applications, a DX or chilled water pre- cooling coil is mounted directly at the dehumidifier inlet. This design allows for removal of much of the initial heat and moisture prior to entering the dehumidifier where the moisture is reduced even further.
- The difference in the costs of electrical power and thermal energy (i.e. natural gas or steam) will determine the ideal mix of desiccant to refrigeration-based dehumidification in a given application. If thermal energy is cheap and power costs are high, a desiccant based system will be most economical to remove the bulk of the moisture from the air. If power is inexpensive and thermal energy for reactivation is costly, a refrigeration based system is the most efficient choice.
In general, Bry-Air representatives should be looking for applications which require humidity control at levels below 45% RH. Each month we send you an Application Update which discusses various applications which require humidity control below 45% RH.
The most common applications requiring this 45% RH level or below are: Pharmaceutical, Food and Candy, Chemical Laboratories. Automotive, Military, and Marine Storage.
Most applications requiring 50% RH or higher are probably not worth expending a whole lot of effort on because they can usually be achieved through mechanical refrigeration. In some cases, however, the use of a desiccant system can reduce operating costs of the existing refrigeration system. For example, when treating ventilation air in building HVAC systems, the dehumidification of the fresh air with the desiccant system decreases the installed cost of the cooling system, and eliminates deep coils with high air and liquid-side pressure drops. This saves considerable fan and pump energy as well.
Learn more today – contact us to request more information on Bry-Air solutions for your industrial desiccant dehumidification needs.
How To Produce Dry Air
Because the amount of water that can be contained in air is a function of the temperature and pressure on that air, our next step is to look at ways to remove moisture by changing the temperature or pressure.
Using Compression to Dry Air
As air is compressed, the dew point or temperature at which water will condense is raised. Therefore, to get dry air we need to find a way to cool the compressed air. But costs can be prohibitive because equipment, space, and auxiliary equipment are necessary for the process. However, if compressed air is already used in the primary operation and only very small amounts of dry air are needed for humidity control, compression may be a feasible route to dry air.
When air at extremely high pressure (over 200 lbs/sq in) is needed, small quantities of high pressure air may be used to maintain small enclosures at the required moisture level. It is also possible to use small amounts of the high pressure air with a smaller air facility to control moisture on a limited scale.
Using Reduced Temperatures to Dry Air
Lowering air temperature decreases the air’s ability to hold moisture. Thus, the air can be made drier by cooling it. However cooling air just to dry it is usually not practical. An exception might be when cool air is needed anyhow, that air’s dryness satisfies the needed moisture conditions, and enough conditioned air is available. Normally, this method is reserved for applications where outdoor air is being dried to levels only slightly lower than the incoming ambient – that is, the system air.
To remove large amounts of water by cooling the air, over-cooling and subsequent reheating are required. But such procedures typically have problems with operation and maintenance, as well as cycle and control; the method is unsuitable for producing large quantities of dry air. Another limitation to this technique is the freezing point of water. When air is dried via refrigeration, the cooling surfaces of the coils may reach sub-freezing temperatures. This causes ice to form, which, in turn, reduces the efficiency of the cooling system. So anti-icing devices or dual systems and defrost cycles may be required.
To prevent such cooling coil icing, a brine spray is commonly used. The brine must be reconstituted periodically or continuously. This requires additional equipment, maintenance and operating costs. Although this strategy is workable and often satisfactory, the complexities associated with cycling and controlling are detracting factors.
A special case involves a brine spray that can pick up moisture from the air at normal temperatures. This brine must be cooled and regenerated or reconcentrated either continuously or periodically. To deliver air at very low moisture, such a system is necessarily complex. For example, the brine must be mechanically refrigerated, and at all levels of drying, cooling must be used during the moisture absorbing cycle and after the regenerating or reconstituting cycles.
Using Desiccants to Dry Air
The most simple, straightforward way to obtain dry air is to use desiccants—that is, adsorbents or materials that have a natural affinity for water. A desiccant is able to take up the additional moisture given up by the air without changing its size or shape. So an air stream can pass through a desiccant and become significantly drier without elaborate cooling, compression, cooling water, or other complex systems or controls. After the drying task is complete, the desiccant is regenerated via heat. Then the desiccant is ready to dry more air.
A Bry-Air Dehumidifier utilizes only a relatively small amount of desiccant at any one time and constantly regenerates it as part of a continuous cycle. This simple device is manufactured in two designs and many sizes, from very small to very large to meet various dry air requirements.
An added feature of the Bry-Air Dehumidifier is its ability to function equally well at extremely low to very high levels of humidity with no regeneration problems and no changes in cycle control. Its versatility in performing in any type of application is unique among most methods of drying air.
Product Drying Dehumidifier Engineering Information
Product drying applications include two general types: bulk drying and continuous drying. In bulk drying the material is loaded into a compartment and the entire load is dried as a batch. With continuous drying the wet material is continuously fed into a drying chamber and material continuously leaves the chamber, dried to the desired moisture level.
Drying potentials can be increased in two ways:
- Raising the product temperature by exposing it to heated air
- Physically removing moisture from the surrounding air
The quantity of air needed for proper drying will vary widely with either type of drying system. But the drying characteristics and the approach to the problem are similar.
The Bry-Air Dehumidifier performs no miracles extracting moisture from the product into the surrounding air. But by maintaining the air at a lower moisture level, the Dehumidifier can increase the drying potential and the drying rate. More important, it can remove the variable of weather as a factor in a drying operation.
Heating is less expensive than drying, so the obvious question is: Where do Bry-Air Dehumidifiers apply?
In most drying processes, the released moisture goes into the air and must be physically removed or diluted with outside air. However, without a desiccant dehumidifier, the lowest possible moisture level in the chamber will equal that of the outside make-up air. But in practical terms, the air in the chamber will generally be somewhat higher than that of the outside air.
When heat is used alone, the drying potential is limited by the specific humidity of the outside air plus the safe temperature to which the product can be raised. Generally, a proper drying potential can be established with heat and outside air if the temperature can be raised to 140°F or above. If the temperature cannot be raised over 120°F, then a Bry-Air Dehumidifier is the best solution. For temperatures in the 120° to 140°F range, the decision depends on the product characteristics and the desired degree of dryness.
Drying operations involve the removal of free moisture, hygroscopic moisture, or a combination of both. Free moisture is water held on the surface or between molecules of a substance. Free moisture occurs when actual liquid water is used to mix or wash the product prior to drying. Hygroscopic moisture is held within the material’s cells. Hygroscopic moisture will take up or dispel water in relation to the relative humidity of the air mixture to which it is exposed. When in equilibrium with air at 100% RH, the material will be hygroscopically saturated. Any hygroscopic material containing free moisture must be hygroscopically saturated.
The removal of free water is a surface evaporation function. The surface water temperature should be assumed to be the wet-bulb temperature of the surrounding air mixture. Note that air velocity is critical to the drying speed.
The removal of hygroscopic moisture depends on the relative humidity difference between that of the products’ equilibrium condition and that of the surrounding air. Velocity of the air over the product has little or no bearing on the drying speed.
The figure below shows a typical drying curve. The sudden change in drying rate (at the critical point) denotes where the initial drying via removal of free moisture ends and hygroscopic drying takes over. In other words, the product has lost its free moisture, but is still hygroscopically saturated.
Each material has a different physical form that determines how it holds or gives up moisture. Since many of the newer materials lack published data on their drying rates, selecting appropriate air drying equipment must be done experimentally. The net effective drying surface and the hygroscopic properties cannot be determined in any other way.
Most drying problems are really a request for improving the speed or quality of an existing drying operation. For example, before today’s advanced dehumidifying equipment was available, candy manufacturers could make their product only in winter. In summer, attempts to manufacture candy might often end with a moldy product. Now, to meet production demands, the use of cooling equipment and a desiccant dehumidifier can imitate winter conditions all year.
Solving a drying problem usually involves a rather simple analysis of the drying cycle. If the analysis (that is, the test run) can occur during weather conditions that consistently give the desired drying result, the problem is simplified. Regardless, any test run will show the product’s characteristics and give clues for solving the problem.
The test run should be made under actual production operation to secure information in either of the two following categories.
Bulk type drying system
Several trays in different locations in the compartment should be weighed and identified before being placed in the drying cabinet. They should be weighed at the start and at predetermined intervals (usually hourly), subtracting the tray weight, and quickly returning the tray to its original position after weighing. At the same time a wet- and dry-bulb reading (average throughout the cabinet) and air velocity reading over the product should be taken. Continue these procedures until the product is satisfactorily dried; weight should be noted at this point. The purpose is to establish a totally dry weight. Temperature should be high enough to keep the RH in the surrounding air at 5% or less.
Continuous type drying system
Here one must remove material samples at the start, finish, and at regular intervals along the drying tunnel. Such test points should be accurately marked and related to the drying time. Each sample should be weighed as soon as removed, then thoroughly dried at elevated temperature and reweighed. The dry-bulb temperature, wet-bulb temperature, and air velocity over the product should be determined at each point of product supply as well as at the start and end of the drying tunnel.
From this information the weight readings can be converted into percent of moisture and plotted against drying time. Moisture content should be expressed as a percentage of the product’s bone dry weight, not as a percent weight of the test sample. If both free and hygroscopic water are removed from the sample, a characteristic curve will resemble that shown in Figure 1.
Sizing the Desiccant Dehumidifier
Bulk type drying. On the characteristic curve, indicate the wet-bulb and dew-point temperatures equivalent to the reading taken during the test up to the critical point. From the critical point to the curve’s end, show the dry-bulb temperature and the RH. The hygroscopic drying phase should be considered in making the first analysis (that is, the drying curve).
Some hygroscopic moisture (near the product’s surface) is removed at the critical point, so make two assumptions:
- The product is hygroscopically saturated at this point
- The product is substantially in equilibrium with the final RH at the end of the test (when it reaches the desired moisture content).
Thus, the average drying potential for this part of the test is:
If our test took 12 hours and we want it to be complete in 8 hours, or two-thirds the amount of time, then the hygroscopic portion of the test, which took 9 hours, needs to be completed in 6 hours. Further, the product’s moisture level at the critical point minus the moisture remaining after complete drying equals the total weight of water to be removed in 6 hours. This amount can be converted into grains per minute.
To accomplish this dryness faster, the drying potential must be increased proportionately to the rate of test time vs. the desired time. But the product’s average moisture level will be unchanged. Therefore, the average RH for drying is found by: Average product RH – Required RH potential.
The average product RH combined with the average dry-bulb temperature dictates the specific humidity that must be maintained and defines the operating conditions for the Desiccant Dehumidifier.
The drying temperature should be as high as practical (usually 10°F below the maximum allowable product temperature); 95°F entering air is the highest recommended level. Thus, if temperatures greater than 95°F are needed in the drying chamber, the recirculating air should be cooled to 95°F or below. Here the cost of the cooling coil, booster fan, and water used will be offset by the gain in moisture removal capacity. (Reduced ratings for inlet temperatures up to 115°F can be calculated. See your Bry-Air representative for details.)
The Bry-Air Dehumidifier will handle a mixture of recirculated air (at the average specific humidity already determined) and a minimum of 5% outside air. This establishes the level at which the dehumidifier must operate. From the Typical Performance Curves chart, the leaving moisture is determined. The difference between the grs/lb. moisture level maintained in the dehumidifier and the same parameter in the air leaving the dehumidifier is the pick-up factor. This figure divided into the average required moisture removal (in grs/min determines the dehumidifier size in lb/min air capacity.
This unit capacity must be checked against the “free moisture” requirement this way:
Knowing the desired drying temperature helps pinpoint the inlet where the necessary moisture removal will occur. For example, if 42 gr/lb. must be removed (the drying temperature is 95°F), then follow the 95°F curve to the point where the difference between the inlet and outlet moisture is 42 gr/lb. Here the result is nearly 60 gr/lb. that is where the leaving moisture is approximately 18 gr/lb.
To be safe, use a condition approximately 5 gr/lb. above that shown on the curve and allow for 5%outside air. Then the needed dew-point and wet-bulb temperature values can be established. Determine the vapor pressure equivalent for the temperatures using the difference between these items establishes the drying potential. Then determine the average vapor pressure difference for the test run from the same table using the test dew-point and wet bulb readings. The ratio of vapor pressure difference with the Bry-Air Desiccant Dehumidifier over that measured during the test should equal or be greater than the ratio of the drying time (test vs. desired).
Since air velocity also affects free moisture evaporation, drying can be somewhat controlled by changing air velocity to as high a level as possible without disturbing the product. Use a by-pass or fans within the chamber to increase the total circulation in the drying air circuit above the Bry-Air Dehumidifier’s capacity.
For the test run, establish a velocity factor:
(1 + test velocity in ft/min) 230*
* An established constant.
Also establish a velocity factor for the actual design:
(1 + actual velocity in ft/min) 230
At any given vapor pressure difference, the evaporation will vary directly according to the above factors.
Continuous drying. Since continuous drying systems characteristically have open ends, they usually require a great deal of additional outside or make-up air to compensate for all the openings. For efficiency, keep such openings as small as practically possible. A minimum leakage equivalent to a 200 fpm velocity through the area should be positively introduced into the system.
The typical flow pattern for a continuous drying operation, shown below, has a separate circulating system for free moisture removal; the dehumidifier discharge is directed through the hygroscopic moisture phase. This configuration takes advantage of rapid circulation in the first space without carrying the wetter air into the final drying space. Note the separate circulating system for the free moisture removal stage dehumidifier discharge is directed through the hygroscopic moisture phase. This arrangement allows rapid circulation in the first space without carrying over moisture into the final drying space.
Use the bulk drying method to establish the vapor pressure difference to allow drying to proceed satisfactorily in the free moisture stage. Keep the velocity and temperature as high as practical. Design specifications will help establish the total circulation. The temperature, plus necessary vapor pressure difference, will establish the specific humidity (in gr/lb.) that must be maintained.
Express the total product moisture removal in gr/min and add the moisture load introduced by make-up air. This latter load derives from the difference in specific humidity between the maximum design outdoor level and that maintained in the compartment multiplied by the quantity of outdoor air (in lb./min). The proportionate quantity of recirculated and outside air also determines the specific humidity of the mixture-which typifies air entering the Dehumidifier.
If the pre-cooling Bry-Air Desiccant Dehumidifier is used, the air temperature leaving the coil determines the Dehumidifier operating level. Refer to the Typical Performance Curves chart to calculate the moisture level leaving the Dehumidifier and determine the removal per pound value:
Total moisture removal load. (grs/min) Moisture removed by Dehumidifier (grs/lb)
Equals Dehumidifier size Ib/min air capacity
The next step is to check the performance of the Dehumidifier in the hygroscopic drying section, measured in grs/min.
Dehumidifier performance (lb/min) 2 + Moisture level of air leaving dehumidifier Equals average moisture content of air in this portion of the drying chamber
Use the curve in the Typical Performance Curves chart to determine the temperature of air leaving the dehumidifier. The departing moisture will have a cooling effect; to find the average temperature subtract 0.625°F for each grs/lb. pickup.
Now that moisture content and temperature are known, the average RH needed is easily determined from a psychrometric chart. Compare this figure with the necessary RH to insure proper drying within the bulk type dehumidifier. This comparison will reveal whether or not the dehumidifier has the capacity to produce the desired drying rate.
Maintaining drying temperature. As already noted, water evaporation is a cooling process. Approximately 1052 Btu are needed to evaporate one pound of water. In other words, 6.65 gr/lb. represent 1 Btu or 1 grs/lb. represents 0.625°F.
As a product is dried, it releases moisture. Without proper control, this moisture can cool down the environment and result in an equilibrium condition where the drying practically ceases. So to maintain drying temperatures, heat must be supplied in an amount represented by the evaporation rate. Also, heat can be lost by conduction through cabinet walls. Thus it may be necessary to control the product to drying temperature (heat or cool it), and heat the make-up air to maintain the optimal drying temperature.
The process of adsorption is an exchange of heat in a like amount in the reverse direction. Thus, air heats as it passes through the desiccant dehumidifier. Approximately 30% additional heat builds up in the desiccant from the previous reactivation period, so the dehumidifier supplies all the required heat for evaporation and an additional 30% for other purposes. In some instances that additional heat is required; in other cases, cooling may be needed.
Dehumidifier Capacity Control
Several methods provide dehumidifier control.
On/off control of the dehumidifier.
Humidistat or dew-point control monitoring of space or return air is a method used where continuous process air is not needed. Often the dehumidifier is installed as an independent unit and is not tied into the make-up or outside air circulation system.
On/off control of reactivation heaters and blower.
This control method applies to continuous process air flow situations. However, the process air will have more variation in humidity than with other control methods.
Modulation of reactivation inlet temperature.
This strategy yields reduced energy consumption and supplies the minimum energy needed to maintain the process condition.
Modulation of reactivation inlet temperature and air volume.
By modulating the reactivation air volume and temperature at specific values, the reactivation capability is increased and can be used over a wide range of operating conditions. This method also compensates for reductions in adsorption capacity.
Process face and bypass damper control.
Here the moisture control of leaving air is due to varying the volume of air that bypasses the dehumidifier. However, a constant supply air volume must be maintained. This is the best scenario for tight humidity control.
Conclusion
Information in this manual was prepared to help customers choose the most effective and efficient dehumidifiers. Please contact Bry-Air’s dehumidifier experts for additional assistance and for more detailed information about physical characteristics and performance data relating to Bry-Air Dehumidifiers.
Sizing The Dehumidifier
When deciding what size dehumidifier to use, remember that controlled space requirements sometimes exceed the anticipated design peak load. Unusual and unforeseen humidity loads – such as from abnormal weather conditions or the introduction of high-moisture content raw materials – can burden the drying equipment. Here we present a number of issues that must be considered in approaching and solving specific drying problems. Six typical humidity control examples are presented:
- Food and drug manufacturing, specifically raw materials and processing equipment
- Storage or equipment areas (Standby warehouse)
- Product drying
- Controlled humidity and temperature areas
- Specific purposes for dry air production
- Prevention of condensation (Water treatment plant)
The Uses of Dry Air
In many manufacturing processes, humidity control is necessary to complete a particular process successfully. Because failure of a process can be directly tied to humidity level control, it is vital to know:
- What equipment is available
- How to choose appropriately sized equipment
- How to effectively use the equipment to control moisture in the process area.
Since dry air may be desired for many applications and specific problems encountered may be as complex as the atmosphere itself, three important steps are the focus of this library: how to select, size, and apply the correct Bry-Air dehumidifier. Consider the following typical situations as examples to assist in guiding us to your solution.
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