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plant may be able to run at a higher efficiency by

delivering its chilled water to the radiant panel at

a higher supply temperature (i.e., 55 to 60°F) than

would be required for a water to air heat exchanger

(~45°F). Higher supply temperatures increase a

radiant cooling system’s efficiency by increasing

the effective operating range for water-side

economizing and by permitting a higher evaporative

temperature for the chiller.

Latent cooling

Controlling indoor humidity reduces the chance

of undesired condensation on building surfaces,

such as floors. Equipment options to satisfy latent

loads and simultaneously control the dewpoint of a

space include:

• Mechanical systems with refrigerant (direct

expansion) or chilled water

• Desiccants - liquid or solid sorption

Mechanical dehumidifiers blow moisture-laden air

over coils that contain chilled refrigerant or water

at a temperature below the air’s dewpoint. The

moisture in the air condenses on the coil, resulting

in an airstream that has lower moisture content and

is at a lower drybulb temperature. If the drybulb

temperature of this lower-humidity air is below

design conditions, it may be reheated with waste

heat from a refrigeration vapor compression cycle.

Figure 3-7 Rotating horizontal desiccant.

Evaporating refrigerant can cool liquid rather than

air. The liquid is then used to cool air. The design

can cool air close to 32°F without freezing the

condensate, and has the advantage of equalizing

the load on the compressor and condenser when

many different air streams must be cooled by a

single refrigeration system.

22. Doebber, I., M. Moore, M. Deru. 2010. “Radiant Slab Cooling for Retail.” ASHRAE Journal 52(12):28-37.

23. Harriman III, Lewis G. 1989. The Dehumidification Handbook (Second Edition) p. 24

24. Harriman III, Lewis G. 1989. The Dehumidification Handbook (Second Edition) p. 26

25. ASHRAE Handbook – HVAC Systems and Equipment. 2008.

Desiccants provide dehumidification through liquid

or solid sorption. Liquid sorption conditioners use

a working solution that is typically composed of

lithium chloride or glycol, which has a very high

affinity for water vapor. In solid sorption, humid air

passes through an “active” desiccant. The lower

vapor pressure of the active desiccant results in

transfer of vapor from the air to the desiccant. The

saturated desiccant is then heated to release the

moisture into a separate air stream (the reactivation

air) that has a lower vapor pressure than the

desiccant.

A typical solid-sorption application is a

rotating horizontal bed (Figure 3-8).

Compressor raises

the pressure and

temperature of the

refrigerant gas

Refrigerant con-

densed back to a

liquid, releasing its

heat to the air passing

through the condens-

er coil.

Refrigerant expands inside the coil,

removing heat from the air passing

though the system.

Refrigerant Expansion Valve

Refrigerant CondenserCompressor

Liquid Refrigerant Storage

Refrigerant Expansion Valve

Compressor Refrigerant Condenser

Liquid Refrigerant Storage

Chilled water

Refrigerant - water heat exchanger

Water chilled by the refrigeration system

is circulated through the cooling coil that

chills and dehumidies the air.

Figure 3-6 Dehumidification with refrigerant.

Refrigeration systems transfer heat from one

airstream to another very efficiently, cooling one

and heating the other. This is the basis for most

cooling-based dehumidification systems.