PART FIVE – SIZING THE DEHUMIDIFIER

When deciding what size Dehumidifier to use, remember that controlled space requirements sometimes exceed the anticipated design peak load. Unusual and unforseen 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 example are presented:

• Food and drug manufacturing, specifically raw materials and processing equipment (Production of hard candy)
• 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)
  Note: Dehumidifier performance used in these examples can be found in Appendix 4.

EXAMPLE 1: PRODUCTION OF HARD CANDY
During the candy and cough drop production, the material is in a plastic state. It must flow and be shaped by stamping machines. If the presence of excess moisture causes the material to become sticky, it will not flow freely and it will adhere to the stamping machine.

To eliminate this material and equipment problem, dry the surrounding air.

Physical Facts

1. Area to be conditioned - 60' x 42' x 16'
2. Outside Design Condition - 95°F db*; 75°F wb*
3. Controlled Space requirement** - 75°F db;35 percent RH
4. Physical Openings - 1 door(6' x 7'); opened 6 times/hr
5. Number of people working in area - 10
6. Construction - 8" masonry
7. Make-up air specified by owner - 350cfm
   

* db = dry bulb value; wb = wet bulb value
** See Appendix 5.

Problem

To determine the size of dehumidifier necessary to maintain the desired controlled space conditions.

Assumptions

1. The door is adequately weather stripped and is of standard construction.
2. Ten workers in the area maintain a moderate pace; each requires ventilating air.
3. The interior of the control space is constructed with two coats of vapor barrier paint.
4. There are no other openings in and out of the controlled space.
5. All physical cracks are sealed.
6. A vapor barrier is provided in or under the concrete floor.

Space Moisture Loads to be Computed
1. Permeation load
2. Load through the door
3. Population load

PERMEATION LOAD
V x G x F1 x F2 x F3 x F4 = Grains per hour C
V	=	60 x 42 x 16 = 40,320 ft.3
C	=	14 (Specific volume of dry air @ 95°F)
G	=	75 grs/lb, outside design wet bulb of 75°F gives 121 gr/lb from Table I. Controlled space requirement of 75°F db, 35% rh yields 46 grains per pound from a standard Psychrometric chart. Therefore, 121-46=75 grs/lb.
F1	=	1.94	From Table II	– Factor for moisture difference of 75 gr/lb - interpolated
F2	=	.5	From Table III
F3	=	1.0	From Table IV	– Factor for 8" masonry
F4	=	.75	From Table IV	– Factor for 2 coats of paint
40,320 x 75 x 1.94 x .5 x 1.0 x .75 = 157,140 grs/hour     14

DOOR LOAD
Ohr x A x G x F1 = grs/hr          C
Ohr	=	6
A	=	6 x 7 = 42 sq ft
C	=	7
G	=	75 grs/lb
F1	=	1.94
6 x 42 x 75 x 1.94 = 5,238 gr/hr        7

POPULATION LOAD

At a db of 75°F and working at a moderate rate, a person will expel 2,540 grains each hour. (See Appendix 3)

Therefore ten people will add 10 x 2,540 = 25,400 grains each hour

The drying system and load requirement are shown in the schematic below:

Note that 350-cubic feet per minute (cfm) outside air is based on a requirement of 30 cfm for each of the 10 workers is introduced at the dehumidifier. The effect of this air on the ultimate dehumidifier size will be handled below. Proceed with the following calculation:

X = C x ( gr/hr) / (S-G)
                60
     
Where :  X           = Delivery air fate from dryer to space in cfm
               gr/hr      = Total moisture load in grain per hour in the space

            C=14 = constant
            S= 46 = Grs/lb moisture requirement of controlled space. In the absence of a ventilation requirement this would be the inlet
            condition at the dryer.
            G= Grs/lb of air leaving dryer. Refer to Chart 1, Appendix 4, Enter curve at 46 grain "Inlet moisture condition." Intersect 75°
            Inlet air temp curve at 14 gr/lb
            14 x (187,778) / (46 -14)=1369 cfm
                          60

From the above calculation the space moisture load is 187,778 gr/min. 1369 cfm air at 14 grs/lb will maintain the space design conditions.

At this stage in the procedure, it is necessary to resort to the method of approximation to select the correct dryer.

In addition to handling the space load the dryer must handle the moisture load contributed by the 350 cfm outside air requirement. So use a 2000 cfm Bry-Air Dryer (MVB-20C or FVB 4000).

If the dryer has a delivery rate of 2000 cfm, and if 350 cfm of outside air is to be introduced, there remains 1650 cfm of air from the conditioned space. Tabulate this air mixture

  350 cfm x 121 gr/lb   =    42,350
1650 cfm x 46   gr/lb   =    75,900
2000 cfm                          118,250

Then 118,250 = 59.1 grs/lb
             2000

Refer again to Chart 1, Appendix 4, it shows that air entering the dryer at 59.1 grs/lb would leave the dryer at approximately 23 grs/lb. (NOTE: Interpolate between the 75° and 85° curves since the air is a mixture of 75°F and 95°F = 79° F.)

Total moisture pickup X (S - G) x 60 - Total Moisture Pickup
                                        C

2000 x (46-23) x 60 = 197,143 grs/hr total removal capacity
  14

The following work sheet is a demonstration of what the calculations will look like.

In the above calculations, moisture gain or air leakage in the process ductwork was not considered. If, however, the process and return ductwork did contribute to the moisture load, the total duct volume would be an additional space. Then the permeation calculation on page 9 would be used: V = duct volume; C = 14; F1 from Table II, with moisture difference G measured from inside process air duct to surround ambient; F3 for tight, good commercial ductwork = 0.6. Add the resultant moisture gain to the room total load. A nominal allowance for process air lost due to duct leakage = 5 percent.

Recommendation

Selecting an MVB-20C or FVB-4000 at 2000 CFM is the best choice for the hard candy manufacturing example. While it may seem to be an oversized selection, consider that all desiccants in all manufacturers' desiccant dryers will age, will possibly become physically and chemically contaminated with dirt, dust, or chemicals, and will gradually lose their effectiveness. Fortunately, with the MVB-20C or FVB-4000, higher levels of moisture in the leaving air upto 24 grs/lb dry air could be tolerated prior to a desiccant change. So what appears to be an oversized selection would actually allow much longer use of a desiccant charge or rotor and provide the economies of longer use.

BRY-AIR DEHUMIDIFIER CALCULATION SHEET
PROJECT: EXAMPLE 1 - PRODUCTION OF HARD CANDY

Calculation Sheet

Example II: STANDBY WAREHOUSE

Moisture damage in a standby or storage warehouse can be avoided by surrounding the machinery, equipment, or material with dry air.

Physical Facts

1. Area to be conditioned - 210' x 176' x 45' = 1,663,200 cubic feet
2. Outside design condition - 95°F db; 77°F wb
3. Controlled space requirement* - 85°F db; 40 percent rh
4. No physical openings nor appreciable amount of door openings or closing specified
5. No people working in the area
6. Construction - 8" masonry.

* See Appendix 5

Problem

To determine the size of the dehumidifier required to maintain standby conditions.

Assumptions

1. All physical cracks are seated and the floor properly vapor-proofed.
2. If the room is completely vapor-proofed, use Table 4 on page 8.
3. Two coats of vapor barrier paint have been applied externally for metal clad construction.

• Outside walls are usually easier to access than inside walls for paint application.
• Coating the outside walls discourages water permeation into the wall and thus minimizes water accumulation in the wall structure itself.

Space Moisture Loads to be Computed

• Permeation load
• Moisture Load

The Permeation Load is the only moisture load involved in this example.

V   x G x F1 x F2 x F3 x F4 = grains per hours
C

Where:
V	=	210 x 176 x 45= 1,663,200 cubic feet
C	=	14 = Constant
G	=	58 grs/lb, outside design wet bulb of 77°F gives 130 gr/lb from Table I page 8. Controlled space requirement of 85°F db, 40% rh yields 72 grs/lb from a standard Psychrometric chart. Therefore, 130-72=58.
F1	=	1.54 from Table II	-Factor for moisture difference of 63grs/lb
F2	=	0.24 from Table III	-extrapolated as straight line for a volume of 1,663,200 cubic feet
F3	=	1.0 from Table IV	-Factor for 8" masonry
F4	=	.75 from Table IV	-Factor for 2 coats of paint
1,663,200 x 58 x 1.54 x 0.24 x 1.0 x 0.75 = 1,910,019 grs/lb.            14

Refer to schematic below, which shows the load requirements and drying system.

Therefore 11,427 cfm of air (33 gr/lb) from the dehumidifier is needed to maintain a grain level of 72 gr/lb.

In a building of this size and shape , air distribution ducts are practical for effectively spreading the air so it can return to a common point and re-enter the dehumidifier.

Recommendation

Use one Bry-Air FLB-25000 or MVB-125E Dehumidifier at 12,500 CFM in this standby warehouse example with a fan sized to handle the necessary static pressure of the duct system.

BRY- AIR DEHUMIDIFIER CALCULATION SHEET
PROJECT: EXAMPLE II - STANDBY WAREHOUSE

Calculation Sheet

Example III: Product Drying

Here we have a room used to remove water vapor from such products as cattle feed mixes, nylon or rayon cord for tires, raw plastic material, granular chemicals, raw paper stock, cardboard stock for coatings, or other similar products.

In this example, the room is used for drying cattle feed mixes, which are contained on drying carts that stand in the room until the specified level of dryness is attained.

Space condition requirements and product movement rate are determined by the manufacturer.

Physical Facts

1. Drying room size - 40' x 65' x 16'
2. Outside design condition - 93°F; db; 73°F wb
3. Controlled space requirement - 95°F; 15 percent rh (36 gr/lb)
4. One double door; (a) 6' x 7' (b) Opens at 2 times/hr
5. There are no other openings
6. There are no workers in room except to bring mix in and out
7. Product movement rate - 1500 lb/hr (i.e. carts with trays of mix are moved into the drying room at the rate of 1500 lb/hr)
8. Product enters room at 8 percent moisture and leaves at 4 percent moisture
9. Drying room wall construction - 8" masonry
10. 350 cfm outside air required by manufacturer

1. All physical cracks are sealed.
2. The double door is weather stripped.
3. Two coats of vapor barrier paint have been applied to the wall and ceiling construction of the drying room; the floor is suitably protected against vapor permeation.

Moisture Loads to be Computed

1. Product load
2. Permeation load
3. Door load

Product Load:
Since the product will lose 4 percent moisture (by weight) and there are 1500 pounds of product each hour:

1500 lb/hr x (8 % - 4 %) = 60 lb/hr water removal
Since one pound of water equals 7000 grains, then:
60 x 7000 = 420,000 gr/hr product load
Note that the time needed to reduce the material to a 4 percent moisture level would have to be given or experimentally determined. This would then decide the amount of material to process and the size of the drying chamber needed.

PERMEATION LOAD
V x G x F1 x F2 x F3 x F4 C			= Grains per hour
V	=	40' x 65' x 16'	= 41,600 cu ft.
C	=	14 constant
G	=	77 gr/lb, outside design wb of 73°F gives 113 gr/lb. from Table-1. Drying room space requirement of 95°F, 15% rh yields 36 gr/lb from the Psychrometric Chart.
F1	=	1.99 From Table II	- Factor for moisture difference of 84 grains.
F2	=	0.50 From Table III	- Permeation factor
F3	=	1.0 From Table IV	- Factor for 8" masonry.
F4	=	0.75 From Table IV	- Factor for 2 coats paint.
41,600 x 77 x 1.99 x 0.50 x 1.0 x 0.75    14			= grs/hr = 170,742 grs/hr

DOOR LOAD
Ohr	=	A x G x F1 C	= grs/hr
Ohr	=	2
A	=	6' x 7'	= 42 sq. ft.
C	=	7 (constant)
G	=	77 grs/lb
F1	=	1.99 From Table II - Factor for moisture difference of 84 grains.
2 x 42 x 77 x 1.99 = grs/hr.        7                                = 1839 grains

Total Moisture Load

420,000	gr/hr	Product Load
170,742	gr/hr	Permeation Load
1,839	gr/hr	Door Load
592,581	gr/hr	Total Moisture Load

The 350 cfm outside air requirement will be considered at a later stage in the calculation.

Proceed with calculation as follows:
   

            X = C x (gr/hr) / (S-G)
                             60
 

Where	X	=	cfm - air rate from dryer
	C	=	14 (constant)
	S	=	36 = grs/lb drying room controlled space requirement. In the absence of an outside air requirement this would be the inlet condition at dryer.
	G	=	15 gr/lb - equals condition of air leaving dryer. Refer to Chart 1, appendix 4. enter curve at 36 - intersect 95°F curve at 15 gr/lb
	X	=	14 x (592,321) / (36 - 15)
			60
	X	=	6584 cfm

Recommendation

The Bry-Air MVB-75E or FLB 12500 Dehumidifier, rated at 7500 cfm, should be adequate. However, the first step should be to determine if this Dehumidifier has enough capacity to handle the 350 cfm outside air in addition to the moisture load in the drying room.

If the dryer has a delivery rate of 7500 cfm and 350 cfm of outside air is to be introduced, there remains 7150 cfm from the conditioned space. Tabulate this air mixture as follows:

350 cfm x 113 gr/lb = 39,550

7150 cfm x 36 gr/lb 257,400
7500                       296,950 

296,950 = 39.6 gr/lb
   7500

Reference to Chart 1, Appendix 4, shows that air entering the dryer at 39.5 gr/lb would leave at 17 gr/lb

7500 x (39.6 -17) x 60 = 726,285 gr/hr
  14

The computed moisture load is 592,581 gr/hr. Therefore, the MVB-75E or FLB 12500 is adequate to handle the moisture load.

BRY- AIR DEHUMIDIFIER CALCULATION SHEET
PROJECT: EXAMPLE III - STANDBY WAREHOUSE

Calculation Sheet

Example IV: CONTROLLED HUMIDITY AND TEMPERATURE AREAS

Many air conditioned manufacturing areas often have a required air flow to handle a sensible load in that space. This air quantity requirement and the accompanying dehumidifier size are usually greater` than those needed to handle a latent load.

By designing, a system for the sensible load situation and then determining the appropriate dehumidifier to handle the moisture load, the desired conditions for the space can be maintained.

Physical Facts
1. Area to be conditioned - 62.5' x 55' x 14'
2. Outside design conditions - 95°F db; 77°F wb
3. Controlled space requirement - 55°F db; 30 percent rh; 20 gr/lb
4. Door - 1 (6' x 8'), 6 openings/hr; 1 (3' x7'), 4 openings/hr
5. Other (fixed) openings - 2.8 sq. ft., w/tunnel 10' deep
6. Number of people working in area - 10
7. Air required for sensible temperature control - 24,715 cfm, 42°F
8. Construction - Block walls; drywall ceiling with vapor proofing; concrete floor on grade
9. Make-up air required - 2400 cfm
10. Air available for make-up - 50°F db/49°F wb; 50 gr/lb.

Problem
To determine the size of the dehumidifier needed in a controlled humidity and temperature area.

Moisture Load to be Computed
1. Permeation
2. Load through doors
3. Load through fixed openings
4. Population load

PERMEATION LOAD

V x G x F1 x F2 x F3 x F4 = grains per hour C
V	=	62.5' x 55' x 14' = 48,125 cu. ft.
C	=	14 constant
G	=	110 (Ambient 130 gr/lb - room 20 gr/lb)
F1	=	2.76 from Table II	- Factor for moisture difference for 110 gr/lb
F2	=	0.48 from Table III	- for 48,125 cu.ft.
F3	=	1.0 from Table IV	- Frame masonry & frame construction
F4	=	0.9 for vapor proof paint on walls & ceiling, untreated concrete floor
48,125 x 110 x 2.76 x 0.48 x 1.0 x 0.9 = 450,846 gr/hr             14

DOOR LOAD
Ohr x A x G x F1 = grs/hr              C
Ohr	=	6 openings/hr.
A	=	6' x 8' = 48 sq. ft.
C	=	7 = constant
Gr	=	110 gr/lb
F1	=	2.76
6 x 48 x 110 x 2.76 = 12,491 gr/hr            7

Ohr	=	4 openings/hr.
A	=	3' x 7' = 21 sq. ft.
C	=	7 = constant
Gr	=	110 gr/lb
F1	=	2.76
4 x 21 x 110 x 2.76 = 3,643 gr/hr            7

FIXED OPENINGS

A x 300 x G x F1 = grs/hr        C x D
A	=	area, 2.8 sq. ft.
300	=	Constant (vel. of vapor)
C	=	14 Constant
D	=	Depth of tunnel = 10'
G	=	110 gr/lb
F1	=	2.76
2.8 x 300 x 110 x 2.76 = 1,822 gr/hr  14 x 10

POPULATION LOAD

At a db of 55°F and working at a "light physical exertion" - 1100 gr/hr/person
10 people x 1100 gr = 11,000 gr/hr

TOTAL ROOM MOISTURE LOAD

The total room latent moisture load is 479,802 gr/hr, which is added into the calculation below to find the entering grain condition needed for the space.

Total cfm x (S - G) x 60 = Room load (gr/hr)
     14 
Total cfm = 24,715 cfm
14 = constant
S = 20 gr/lb. (condition of controlled space)
G = Unknown grain level needed entering space
60 = min/hr

24,715 x (20 - G) x 60 = 479,802 gr/hr.
   14 
G = 15.4 gr/lb

Thus the air to the room must be 15.4 gr/lb and the air mixture (return from the room plus the dehumidifier discharge) entering the main system fan should be 15 gr/lb. to allow for possible leakage into the system duct work. Here one must resort to trial and error techniques to select the dehumidifier size.

cfm x (S - G) x 60 = X
 14 
cfm - 7500 cfm - dehumidifier capacity (trial)
14 = constant
S = 20 gr/lb. condition in the controlled space
G = 4 gr/lb. air leaving dehumidifier (Chart 1, Appendix 4) with entering air 53°F, 30 gr/lb.

7500 x (20-4) x 60 = 514,285 gr/hr
  14

Note that the make-up air of 2400 cfm must mix with 5100 cfm of return air before entering the dehumidifier.

RECOMMENDATION

The MVB 75E or FLB-12500 Dehumidifier will satisfy the room load conditions when mixed with the remaining 17,215 cfm of return air and delivered into the conditioned space the conditioned space.

BRY- AIR DEHUMIDIFIER CALCULATION SHEET
PROJECT: EXAMPLE IV - CONTROLLED HUMIDITY AND TEMPERATURE AREAS

Calculation Sheet

Example V: PRODUCTION OF DRY AIR FOR SPECIFIC PURPOSE

Many applications require a specific quantity of outside air to be delivered at a given moisture content and temperature. This requirement may be a need to make up air exhausted from a space or to supply air for a process such as a drying oven.

PHYSICAL FACTS

1. Maximum allowable moisture content - 17 gr/lb of dry air
2. Maximum allowable temperature - 115°F
3. Quantity of air required - 6,000 cfm
4. Maximum condition of outside air - 95°F, 130 gr/lb

From Chart 1, Appendix 4, it is obvious that 130 grain air cannot be reduced to 17 grains in a single pass through a dehumidifier, without other conditioning. Examination of Chart 1 shows that to produce 17 grains air leaving the dehumidifier, the inlet condition should be 64 grains or less at 60°F or less.

This is accomplished as shown above by installing a cooling coil upstream of the dehumidifier to reduce the temperature and moisture content of the outside air.

Example VI: WATER TREATMENT PLANTS

In most water pumping stations, filteration plants, and waste water control plants, control of humidity in the pipe galleries, pump rooms, and control rooms is of prime importance. By reducing the dew-point temperature of the air below the temperature of the piping and walls, sweating and condensation can be eliminated. By circulating warm, dry air through the areas, water accumulation is avoided, maintenance for electrical controls, motors, and instruments is reduced, and paint lasts longer on the pipes, valves, and flanges.

A standard rule-of-thumb is used to approximate this type of application load:

Volume of space to be conditioned = CFM dehumidifier
                          25

(For each 25,000 cu.ft. space, supply 1,000 cfm of dry air.)

The use of an after-cooling coil for the dry air discharge from the dehumidifier can be omitted in most installations since the warm, dry air (low rh) will help heat the space during cool or winter conditions. Heat should not build up to an objectionable level because the large piping and wall areas are at the same temperature as the water in the system. Warm air also has the advantage of reducing the rh and increasing the air's capacity to carry away moisture.

ZERO LEAK SYSTEM FOR A LOW HUMIDITY SPACE

In a system where the ductwork and components are outside the controlled space, dry air leaving the system will induce the flow of humid air into the system. If the humid air is not dehumidified, each cfm will induce a load absorbing the capacity provided by 5 to 10 cfm of the dehumidified air. If all air that enters the system must pass through the dehumidifier, the additional load will be reduced by 50 percent or more. Typical air handling units (AHUs) are not built to be vapor tight. Standard sheet metal type ductwork has lapped seams that allow leakage. However, excellent silicone-based sealants are available; for applications requiring very dry spaces, the seams must be welded shut.

Having a "zero leak" condition means that all air leaving the controlled space is under positive pressure (to minimize infilteration), and all the entering air passes through the dehumidifier. The cost of dehumidification is high when moisture levels must be low. In these situations, even a small leak can double operating costs. Since ducts are a once-only expense, attention to ductwork is vital. The other approach to this situation-a higher capacity dehumidifier- means added costs, year after year.

It is unusual to find air handling units constructed for zero leak performance. Leaks are anticipated at removable panels, bearing flanges, drain pans, and through condensate drains with dry traps. Such units require additional sealing and check valves or positive water seals on the condensate lines. Cooling devices, especially the coils and fan, and the filter box need to be handled properly when cleaned, and they, too, must be tight. Obviously, there are many factors that ban restrict the zero leak principle.

One way to maximize the chances for a zero leak system is with air treatment equipment and dehumidifiers designed to fit and operate together effectively. Buying directly from the equipment manufacturer and not mixing sources of various equipment components focuses the accountability for moisture tightness.

Bry-Air constructs custom dehumidifiers and low dew-point AHUs for maximum efficiency. This extra care in design and construction means zero leak performance. Proper operating balance compensates for this situation.

When end users, consulting engineers, or other "specifiers" require a system to be substantially air tight, they means there is no leakage at any joint.

Assuring zero leak begins in the fabrication and construction stages. We recommend flanged or gasketed connections with welded seams and joints. As a minimum, all seams and joints should be caulked. Removable panels and access doors should be completely enclosed. (Coils will leak where return bends enter tube sheets.)

Prior to insulting, the system should be tested and deficiencies corrected. The best method is to use an open flame, if permissible. This requires candles or other sources of flame. In other situations, smoke devices or soap suds could be used.

When all these precautions in fabrication, construction, and testing are complete, the system will perform as designed. Periodic inspection during operation will allow leaks to be located and corrected.