Dehumidification of Buildings During Winter Construction

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Technical Note (National Research Council of Canada. Division of Building Research), 1961-03-15

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. , _ " __ .J

DIVISION OF BUILDING RESEARCH

NATIONAL RESEARCH COUNCIL OF CANADA

'fEClHIN JICAlL

NOTlE

No.

326

NOT FOR PUBLICATION

PREPARED BY

N. B. Hutcheon CHECKED BY

FOR INTERNAL USE

APPROVED BY

DATE 15 March 1961

PREPARED FOR G_enera1 D' t .b t· ｾｳ ｲｾ u ｾｯｮ

SUBJECT

Dehumidification of Buildings during Winter Construction

An inquiry about the most economical means of removing

moisture from bUildings under construction in winter prompted a study which is now reported. It was asked, in particular, whether small iehumidifiers of the types now offered for domestic use would remove construction moisture more economically than the ventilation method which requires the heating of outside air. The results of the study are interesting, indicating that moisture may be· removed at a lower cost for energy under certain conditions by dehumidifiers of the mechanical refrigeration type.

It is first necessary to establish the temperature and

moisture level to be maintained in a room in which plastering and other moisture - contributing operations may be carried on. It is assumed at the outset that the space will be heated, and a temperature of 70°F has been selected. The moisture level will be limited in large part by windows. Window condensation will automatically limit moisture levels even if no positive steps are taken to remove moisture in any other way. The limiting humidities for no condensation on windows under various conditions are given in Canadian Building Digest

No.

40

•

2

-Table I

Building maintained at 70°F Outdoor Temperature Limiting RoHo forsinile Windows

o Wind ＴＱｾ ＲＷｾ Limiting R.H. for DouhleVfindows No Wind ＶＱｾ

49:.t

The psychrometrio data for the various conditions used in

the study are as given in Table 110 Enthalpy (heat content) is

expressed in Btu per lb. of dry airo Humidity ratio (moisture content) is expressed in lb. per lb. of dry airo

Table II OOF 20°F 40°F 70°F 70°F 70°F 70°F Saturated Saturated Saturated 27%

41%

ＴＹｾ 61:.t Enthal

a

Y Btu/lb0 oa. 084 701 1502 2104 2400 2502 2703 Humidity Batio lb./lho d.ao

000079

00022 00052 00042

00064

00076 00098

Cost of Moisture Removal by Ventilation

The moisture removed for each pound of dry air brought in at outside conditions and leaVing at room oonditions may be found

from the differences in humidity ratio from Table 110 The corresponding price which must be paid in terms of energy expended for heating this air may be found from the differences in Enthalpy between outdoor and indoor conditions, also to be found from Table 110 The ratio of heat removed to moisture removed gives the heat reqUired for eaoh pound of moisture removed by ventilationo This cost in energy may then be converted to oents per pound of water removed using.an average figure of $1 50 per million Btu, as shown in Table 1110

3 -Table III

Outdoor Condition Indoor Condition Heat per pound

of moisture removed Cost perJrund of mois e Removed 27%. 49% 41% 61% 6050 3600 4000 2660 0,,91 cents 054 060 040

Sample Calculation, Table III

0° to 70°, 27% Heat per pound of dry air = 2104-0,,84 = 2006 Btu

Moisture per pound of dry air

=

.0042-.00079

=

.0034 lb.

Heat per pound of mOisture removed

ｾ ＰｾｧＳｘ

= 6050 Btu.

Cost of 6050 Btu = 6050 x 150 = 0.91 cents

10

6

e

Removal of Moisture by Mechanical Refrigeration

There are small dehumidifiers available which employ a

compression refrigeration cycle, and have the essential components of a mechanical refrigeration system, namely a motor driven compressor,

evaporator and condenser" Cooling of air passing over the evaporator

extracts heat, and if the temperature is low enough, water is condensed from the air, and drains to a reservoir to be removed later as liquid. Meanwhile the heat extracted by the evaporator is rejected to the room air through the condenser, together with the energy equivalent of the mechanical work done on the refrigerant by the compressor"

The performance of such a system may be predicted as before, by comparing the quantities of heat and moisture removed per pound of

dry air processedo However, there are some complications" The first

involves prediction of the condition of the air upon leaving the

evaporator. This will depend upon many factors, but a reasonable

esti-mate for present purposes may be made. It will be assumed that air

leaves the evaporator in a saturated condition at 40°F, for which condition the Enthalpy and Humidity Ratio may be found from Table II to be 15.2 and.0052.

The second problem involves estimating the power input

required to operate the dehumidifier. This will be assumed to be

1 kilowatt per ton of capacity, or 1 kwh (3412 Btu) per 12000 Btu

removed from the evaporator. An assumption must also be made as to the

4

-A further complication arises since the refrigeration cycle dehumidifier returns to the space not only the heat removed in cooling the air but also the total heat equivalent of the electrical input plus

the latent heat removed from the condensed moistureo The air sensible

heat may be disregarded since it is returned to the space to make up

the deficit created by its removalo But the electrical input and the

heat extracted in condensing the water now become available for space

heating, or since dehumidification is being considered, for the heating

of ventilating air which will in turn make possible the removal of an

additional amount of water by ventilationo

The calculation of dehumidifier performance may now be made as follows:

Btu per Ibo moisture in evaporator

=

₀₀₀₄₆12.1

2630

Btu equivalent of electrical input =

12000

x

Latent heat in 1 Ibo water

=

1000 Btu approx.

Additional heat available for ventilation = 1000 + 748

Additional moisture removed by ventilation employing 1748 Btu (see Table III) with outdoor air at 20°F

=

ｾｬｧｧ

= .66 Ibo

Total moisture removed

=

1 + 066

=

1066 Ibo

Energy 2upplied = 748 Btu or

ｾ

= 022 kwh for 1.66 Ibo

Cost per lbo moisture removed

=

o2io661

=

0013 centso

Room at 70°F, ＶＱｾ roho (double glazing, 20°F outside)

Heat removed in evaporator

=

2703 - 15.2

=

12.1 Btu per Ibo

dry air

Moisture removed in evaporator

=

00098 - 00052

=

00046 lb. per

Ibo dry air

= 2630

3412

=

748 Btu/lbo

water

Similar calculations for the four conditions being considered, i.e. 0° and 20°F outside, double and single windows, provide the values given in Table IVo

Table IV

Electrical Power Requirements per Pound of Water

Removed by Refrigeration Cycle Dehumidifier

Outdoor Condition Indoor Condition Energy Requirements per Ibowater

Btu

_-

Kwh

-450 .13 1180 035 740 022 Cents 0013 035 022

5

-It may be noted that no values can be assigned for the 70°F ＲＷｾ internal condition corresponding to single windows at OOF outside since the dewpoint of this inside condition is about 35°Fe Since the dehumidifier is assumed to operate at 40°F it is unable to remove any water by condensatione The device would then serve no pUrpose except to act as a very expensive electric heater, providing the electrical input for use in heating ventilating aire

It should also be noted that the dehumidifier itself will not be directly affected by outside temperatures, since it responds only to the indoor conditione However, the indoor condition is assumed to vary with outdoor temperature, and in addition the effectiveness of the additional heat made available for moisture removal through venti-lation depends on the condition of the outside airo Thus the values given in Table IV are dependent upon outside air conditiono

mlemical Dehumidification Methods

-.- Chemical dehumidification involves removal of water by adsorption on a suitable chemical exposed to the airo The chemical, when loaded with moisture, may be regenerated by heating or may be

discarded 0 The latter case, employing commercial grade calcium chloride will be consideredo

Commercial grade calcium chloride is not completely anhydrous but has the form CaC1₂02H₂0o It will adsorb an additional four molecules of water at 70°F with a relative humidity of ＳＲｾ reho or highero That is, one pound of commercial grade calcium chloride will take up about

t

Ibe of watero The price of commercial grade material in quantity is about 107 cents per poundo The cost of material is therefore about 3e4 cents ｦｯｾ ..each pound of water taken upo However, as in the case of the refrigeration cycle dehumidifier the latent heat in the moisture removed is returned and is available to warm ventilating airo Making an allowance for this as before, the cost comes to about 205 cents per Ibo of water removed o

Calcium chloride might be regenerated by heating to drive off the watere This would be a messy job, and the. heat requirement might be as much as 3000 Btu or more per pound of water driven off, unless the regenerating process were carried on efficientlyo This heat could not readily be recovered for further use but would have to be wasted in order to get rid of the watero'

There are other chemical dehumidifiers specially designed for heat regeneration, employing sorbing materials such as alumina and

silica gelo These are rather cumbersome for the present requirement and may be expected to have a relatively high heat requirement for regeneration. It is not evident that these need be considered further at this time.

6

-Discussion

The use of sorbent-type dehumidifiers may be rejected. They will either be cumbersome, or troublesome in the present application, if regenerative, and in any event are likely to be relatively costly to operate. The use of calcium chloride, to be discarded when loaded with moisture, is too costly. The reloading of containers would be a messy job, and any spilled chemical would take on water until it

reached a liquid state, and would be exceedingly difficult to remove. Subfloors and other parts of the building could become contaminated as a result and a number of difficulties created.

The refrigeration cycle dehumidifier has possibilities. A comparison of Table III with Table IV shows that it may involve an energy cost of only one half that of the ventilation method. It will be noted that both these methods benefit from the highest possible

level of room moisture, and both increase in energy cost as the relative humidity to be maintained in the space decreases. Relative humidities will have to be reduced to the levels shown for the corresponding

temperatures. Unless this is done the windows will dehumidify by themselves and no benefit will be obtained from other dehumidifying methods. Only in interior rooms or closets without windows will it be possible to consider higher relative humidities than those shown.

It may be noted that the conditions selected for examination are representative of moderate mid-winter weather. A full comparison of the two methods would require an examination of probable performance under late fall and early spring conditions. Although higher humidities can be carried in warmer weather without window condensation, it is

unlikely that humidities higher than about 60% would provide the desired drying conditions within a bUilding.

Energy costs prOVide only part of the story. Capital as well as labour and maintenance costs must also be conSidered. In many

buildings the ｲ･ｧｵｬｾｲ heating system may already be in operation and only the energy costs need be considered. But ｨ･｡ｴｩｮｾ can be provided in the form of oil heaters at equipment costs of say '100.00 for

50,000 Btu per hour capacity. This capacity would be sufficient to remove up to 19 pounds of water per hour by ventilation, at

10

0_F

t 61%

inside and +20oF outside. Such a heater might require attention at the rate of

i

man hours per day for refuelling and other attention.

Refrigeration cycle dehumidifiers will on the other hand be much more expensive in first cost. A 1 ton refrigerating capacity

(12,000 Btu per hour) will cost $350.00 or more. Such a unit could, at the conditions considered for the oil heater above and for other conditions as assumed in this Note, provide for:the removal of 1.6 pounds of water per hour. It would require a 1 kilowatt electrical connection and some attention by way of removal of water. Units of

7

-ｾ hp or

*

hp capacity, plugged into normal outlets might be sufficient to provide dehumidification ｾｯｲ individual rooms such as offices,

bedrooms, dwelling rooms and so on, but the total investment in equipment at $100.00 per room could be quite high. The maintenance costs on such equipment could also be high. Heat transfer surfaces would readily be plugged by dust.

It is necessary, in order for the dehumidifier method to compete with the ventilation method that electrical power costs be low. A power cost of 2 cents per kwh would make them as costly in

terms of energy as the ventilation method. It is also necessary that they ｢ｾ designed and operated to produce the lowest practical dewpoint in the air leaving the evaporator. A value of 400 _{F has been assumed}

in the calculations and comparisons made here. There is a practical lower limit here, since if the evaporator operates below 32°F, a defrost problem is created, with its attendant complications.

Conclusions

It has been shown that refrigeration cycle ､･ｨｵｭｩ､ｩｾｩ･ｲｳ

operating at 400_{F dewpoint of air leaving evaporators and at power}

costs less than 2 cents per kwh can compete with the ventilation method of moisture removal so far as energy costs for operation are concerned. However, when capital and maintenance costs are considered, it is

unlikely that such electric dehumidifiers can take the place of the ventilation method ｾｯｲ moisture removal. There may be special cases, with interior rooms having no windows and not normally provided with means of heating where small dehumidifiers of

*

to

t

hp in size may be convenient and reasonably economical.