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Without Causing Condensation on

R O O M M O I S T U R E R O O M M O I S T U R E

are arbitrary limits which have been established by a combination of theory and field experience. These limits ap-ply where the room dry-bulb temperature is 65 F db or over.

4s a rule of thumb, the air is reduced in tempera-ture approximately 8.3 F for every grain of moistempera-ture per cubic foot added. This value is often used as a check on the final room temperature as read

from the chart.

Example illustrates an evaporative cooling ap-plication with supplemental spray heads used in the space.

Example Cooling-With Auxiliary Sprays

Given:

An industrial application

Location Columbia, South Carolina Summer design 95 F 75 F

1. Leaving conditions from spray chamber 2. Room dry-bulb temperature

3. Supply air quantity with auxiliary sprays 4. Supply air quantity without auxiliary sprays

8. 1-139

EVAP COOLING

RISE WITHOUT AUXILIARY SPRAYS

db TEMP

7 7 F d b 89.2 Fdb db

db

56 -EVAPORATIVE COOLING, AUXILIARY

Solution:

=

= 95 (95 75) = 77 F db

is the same as the in an evaporative cooling process, Fig. 56.

2. Room dry-bulb temperature is evaluated by determining the moisture content of the space.

W = + 19 = 128 + 19 = 147

The 19 is the moisture added to the space by the auxiliary spray heads.

T h e is the point on the psychrometric chart where t h e i n t e r s e c t s t h e d e s i g n r e l a t i v e h u m i d i t y

line, Fig. 56.

t = 89.2 F

it can be assumed that the atomized water from the spray heads absorbs part of the room sensible heat and turns into water vapor at the final room wet-bulb temperature. The intersection of this

temperature with the moisture content of the air leaving the evaporative cooler is the theoretical dry-bulb equivalent temperature if the auxiliary sprays were not operating. The difference between this theoretical

bulb equivalent temperature and the temperature of the spray chamber, is used to determine the supply air quantity.

(from spray chamber) = 77 F.

The theoretical dry-bulb temp is 100.75 Fig.

Temp rise 23.75 F

= -- 1.08 XRSH X 23.75 =

t e m p 82,000 cfm

4. If no auxiliary sprays were to used, the room design would be where the RSHF line intersects the room relative humidity. From Fig. 56, the room dry-bulb is read

84.7

The supply air quantity required to maintain the room relative humidity is determined from the following equation:

R S H

= = 1.08 (84.7 77)

= 253,000 cfm

This air quantity is over three times the air quantity required when auxiliary sprays are used in the space.

However, it should be noted that, by reducing the air quantity, the room dry-bulb temperature increased from 84.7 F to 89.2 F.

Heating and Humidification -With Sprays

A heating and humidifying application is one in which heat and moisture are simultaneously added to the air, line Fig. 54. This may be required during the intermediate and winter seasons or dur-ing partial loads where both the dry-bulb tempera- ture and relative humidity are to be maintained.

Heating and humidification may be accomplished by either of the following methods:

1. Add heat to the spray water before it is sprayed into the air stream.

2. Preheat the air with a steam or hot water coil and then evaporatively cool it in the spray chamber.

Spray water is heated, by a steam to water inter-changer or by direct injection of steam into the water system. Since the supply air quantity and the spray water quantity have been determined from the summer design conditions, the only other re-quirement is to determine the amount of heat to be added to the spray water or to the preheater.

For applications requiring humidification, the room latent load is usually not calculated and the room sensible heat factor is assumed to be 1 .O.

Example 8 illustrates the psychrometric calcula-tions for a heating and humidifying application when the spray water is heated. It should be noted that this type of application occurs only when the quantity of outdoor air required is large in relation to the total air quantity.

Example 8 Heating and Humidification With Heated Spray Water Given:

industrial application Location Richmond, Virginia

l-140 ^{P A R T} L O A D E S T I M A T I N G

Winter F

Inside design rh

Ventilation 50,000 (see explanation above) Supply air 85,000

Design room heat loss Btu/hr Spray saturation efficiency

RSHF (winter conditions) 1.0 Make-up water F

Find:

1. Supply air conditions to the space

2. Entering and leaving spray water temperature 3. Heat added to spray water to select water heater, Solution:

= design room heat loss

1.08 x +

= 1.08 X 85,000 F d h

To determine the wet-bulb temperature, plot the RSHF line on the psychrometric chart and read the wet-bulb at the point where crosses this line (Fig. 57). Supply air wet-bull, to the space = 65.8 F wb.

2 . T o d e t e r m i n e t h e e n t e r i n g a n d l e a v i n g s p r a y w a t e r temperature, calculate the entering and leaving air

condi-tions at the spray

edb =

(15 50,000) (72 35,000) 85,000

To determine wet-bull) temperature of the air entering the spray chamber, plot the mixture line of outdoor and return room air on the psychrometric chart, and read

the temperature where crosses the mixture line, Fig.

t = 32.4 F

The air leaving the spray must have the same moisture content as the air in the room.

W = = 41

Since the spray has a saturation efficiency of t h e m o i s t u r e c o n t e n t o f c o m p l e t e l y s a t u r a t e d a i r is calculated as follows:

= Sat +

41 17

+ 17 42.3

The heating and humidification process line is plotted on the psychrometric chart between the moisture content of saturated air (42.3 and the entering conditions to the spray chamber (38.5 F db and 32.4 F wb), Fig. 57.

The leaving conditions are read from the psychrometric chart where the room moisture content line (41 ^l intersects the heating and humidification process line, Fig.

t = 43.6 F db = 43.4 F

The temperature of the leaving spray water is approxi-m a t e l y e q u a l t o t h e w e t - b u l b t e approxi-m p e r a t u r e o f t h e a i r leaving the spray chamber.

t = 43.4 F

NOTE: Numbers in parentheses at right edge of column refer to equations heginning on page .

REHEAT

Fdb 38.5 Fdb 99.2 Fdb

42.3 41

1 7

FIG. 57 HEATING AND HUMIDIFICATION, WITH HEATING SPRAY WATER

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