Document DD2B3rLmRq50mG082Nn6dqLb5

436 ____________ ;Chapter 23________________________________ 1945 Guide COMPARISON OF METHODS OF DEHUmSStCATION 7 ~ Almost all summer comfort air conditioning, as well as much industrial and commercial air conditioning, requires both, of the functions of sensible and latent heat removal from air. Each of the methods of cooling and dehumidification has as its objective either the removal of sensible orlatent heat, or both simultaneously. Choice of method and medium therefore depends solely on whether (a) method and medium are physi cally able to accomplish the desired result with practical equipment, and (6) method and medium are justifiable economically. Dehumidification ________________________ ^ and dehumidification requirements at maximum design conditions, there are many periods when practical operating conditions will result in an unsatisfactory balance of sensible and latent loads - unless there is a simultaneous use of cooling and reheating. In the normal range the choice between direct or chemical dehumidi fication, mechanical refrigeration, natural cold water, or ice, must be justified by the initial investment of available equipment; the availability and cost of prime energy sources; the charges to be allocated to space. occupied, labor of operation and maintenance; and the degree of control required. When required dew-point temperatures are below 45 F, direct or chemical methods deserve special consideration, as they do in applica tions where the task is primarily one of moisture removal with no necessity ' for sensible temperature reduction. ~ It is evident that it is-not possible to set forth definite rules governing I i i I i i rr -75" INLET AIR TEMPERATURE I I I I I II I / 020 30 40 SD 60 TO 80 90 DO liO 120130140 ENTERING MOISTURE CONDITIONS-GRAINS/LB Fig. 8. Silica Gel Dehumidifier Performance Data Steam Activation Type Referring particularly to the problem of moisture removal, it may be stated that dehumidificatiqn either by using chilled water, brine or direct expansion refrigerant, or by using solid or liquid drying agents, is equally practical from the viewpoint of engineering performance for the vast majority of comfort and for a great many industrial applications requiring dew-point temperatures above 45 F. For this reason, in the normal operating range, consideration is usually based upon practical factors which affect the over-all economics of the situation or upon the desire or necessity of attaining independent control of either temperature or humidity. In this connection, it should be remembered that though a satisfactory balance may exist between cooling Fig. 9. Activated Alumina Dehumidifier Performance Data Heavy Duty Type the choicfe^if the method of dehumidification. It is only possible to state certain general conditions. Direct or chemical methods tend to be favorable where: 1. Low temperature water is available (below 65 F), to meet sensible cooling re quirements. 2. Gas or steam is available at costs competitive with electricity. 3. Abnormally high internal latent heat load is encountered. 4. Abnormally low dqw-points are required (40 F or lower). 5. Required dry-bulb temperature is high or control of temperature .is unimportant in comparison to maintenance of proper humidity. 6. In low temperature dryers where complementary heat exchange can be utilized. (In such cases the sensible heat of the dry air is reduced by the evaporation of moisture from the product being dried.) Of the factors enumerated, Item 1, coupled with Item 2, tends to have the greatest influence since these factors affect the applicability to normal range requirements. However, each of the items listed has a direct influence upon economic considerations.' Direct or chemical methods tend to be unfavorable where: 1. Normal comfort dew-point temperatures are required with a predominantly sensible heat load and where mechanical refrigeration is required for sensible heat removal.