Federal

Technology

Alert

A publication series

designed to speed the

adoption of energyefficient and renewable

technologies in the 

Federal sector Desiccant dehumidification technology provides a method of drying air

before it enters a conditioned space.

When combined with conventional 

cooling systems, desiccant dehumidification systems are a cost-effective

means of supplying cool, dry, filtered

air. These combined systems are called

“hybrid” systems. This Federal Technology Alert discusses how to evaluate

the cost-effectiveness of hybrid twowheel desiccant-based cooling systems,

summarizes desiccant dehumidification

process, reviews field applications, and

examines energy savings and other 

benefits. 

Energy-Saving Mechanism

Desiccant systems save energy 

by using low-grade thermal sources 

to remove moisture from the air and to

eliminate the overcooling and reheat

step necessary in certain conventional

Two-Wheel Desiccant 

Dehumidification System 

Technology for Dehumidification and Improving Indoor Air Quality

The U.S. Department of Energy

requests that no alterations be

made without permission in any

reproduction of this document.

vapor compression cooling systems.

Desiccant systems often permit reductions in the size of the conventional

cooling system; this saves energy and

decreases electrical demand. In some

applications of hybrid desiccant-based

systems, the vapor compression system

can be replaced with less expensive

direct or indirect evaporative cooling

systems. Hybrid systems can provide

year-round comfort. The direct or 

indirect heaters used for reactivation 

can supply comfort heating, and the 

heat wheel can be used to recover 

energy all year. 

Technology Selection

The Federal Technology Alert series

targets technologies that appear to have

the greatest untapped federal-sector

potential. Desiccant dehumidification

technology is one of the many energysaving technologies to emerge in the 
last 20 years for commercial buildings

applications. The hybrid two-wheel 

desiccant system has the potential 

for federal-sector energy savings and 

has demonstrated satisfactory field 

performance and reliability in specific

applications.

Potential

The technology has been shown 

to be technically valid and economically

attractive in many applications. This

Technology Alert reports on the collective experience of two-wheel desiccant

system users and evaluators, and also

provides guidance to future applications.

Application

Hybrid desiccant-based cooling 

systems can be used in any building

application. However, high initial costs

typically limit the use of this technology.

The systems are extremely effective, and

their use is well established in conditioning storage areas, ice arenas, hospital

operating rooms, and supermarkets.

Site-specific conditions and differing

application requirements must be 

understood before use of hybrid systems

in a building can be justified on economic grounds. A detailed analysis is

generally required to evaluate the costeffectiveness of a hybrid system with a

conventional cooling system. Typically,

hybrid systems should be considered if

the following conditions are met:

• low indoor humidity (below 50°F

dew-point)

• high latent load fraction (greater

than 25%)

• high fresh air intake (greater 

than 20%)

• high summer-time electric demand

and energy costs, and low summertime gas costs. 

Field Experience

Detailed performance evaluations 

are still being conducted for two-wheel

desiccant system (TWDS) technology

installed in several Federal facilities.

Several TWDS installations managed 

by U. S. Army Construction Engineering

Research Laboratories (USACERL) at

Defense Department sites are being 

evaluated; several other installations 

are in design stages. These systems use

steam from a gas-fired boiler for their

reactivation energy. In addition, the

Design and Construction Division of

Defense Commissary Agency (DCA) 

has installed over 70 desiccant-based 

systems in the past 10 years.

Six of the DCA units use heat recovery, and three have a TWDS design.

Several facility managers for desiccant

units were contacted to ascertain system

performance. Only one manager was

dissatisfied with the performance of the

unit. This manager reported that the

humidity levels in the conditioned space

were still high, and the sensible cooling

was inadequate. Both USACERL and

DCA plan to install more desiccant 

systems in the future.

Case Study

Information is available from the

Burger King demonstration site at

Aberdeen Proving Grounds, Maryland,

where some of the critical variables were

monitored after the desiccant system was

installed. Fast food restaurants, large

dining facilities, and other common areas

present a unique situation, because of the

high density of occupants. The effectiveness of the desiccant-based system at 

the Burger King site was evaluated as a

possible solution for other such facilities.

This demonstration site is open 

24-hours a day, seven days a week. 

The dining area initially had two 

packaged rooftop units (a 5-ton and a

7.5-ton) supplying 700 cfm of ventilation out of a total supply flow rate of

5,000 cfm. Although the peak design

load matched the equipment nominal

capacity (12.5-ton) for the dining area,

the components of the load (sensible and

latent) did not match the equipment

capacities. 

At the outset of the demonstration,

the nominal-capacity of the two units

was reduced from 12.5 tons to 10.5 tons,

and the total latent capacity was less

than the required design latent capacity.

This shortage was exacerbated by offdesign conditions, during which the

latent component of the total load 

did not drop off nearly as quickly as the

sensible component. Because of these

problems, the two packaged units were

unable to dehumidify and cool the air

simultaneously, resulting in frequent hot

and/or humid conditions in the dining

area. As a remedy, a nominal 1,600 cfm

TWDS manufactured by Engelhard/ICC

was installed. With this new system, the

unit has operated reliably. An improvement in the space conditions was noticed

by the restaurant employees and 

customers immediately. 

Implementation Barriers

A widespread use of the hybrid 

desiccant cooling system is impeded 

by a lack of familiarity with the 

technology as well as a lack of 

knowledge about its performance 

and cost-effectiveness. Use of hybrid

systems would be increased if there

were guidance and techniques for 

reducing the first cost of such systems,

performance documentation and 

confirmation, design tools such as 

user-friendly computer programs,

and utility incentives.
1

Federal

Technology

Alert

Two-Wheel Desiccant Dehumidification System

Technology for Dehumidification and Improving Indoor Air Quality

Abstract

Desiccant dehumidification 

technology provides a method of 

drying air before it enters a conditioned space. When combined with

conventional vapor compression 

systems, desiccant dehumidification

systems are a cost-effective means 

of supplying cool, dry, filtered air. 

In the last decade, desiccant 

dehumidification technology has

emerged as an alternative or as a 

supplement to conventional vapor

compression systems for cooling and

conditioning air in commercial and

institutional buildings. A typical

hybrid system (shown above) combines a desiccant system with a 

conventional vapor compression

cooling system. 

Desiccant-based systems are costeffective because they use low-grade

thermal sources to remove moisture

from the air. In general, the benefits

of desiccant-based systems are

greater where the thermal energy

required for regenerating the desiccant is readily available, the electricity price is high, and the latent load

fraction is high (>25%). If there is no

difference in energy costs, the factors

that influence the economy include

climate conditions (humidities) and

high outdoor-air requirements. 

In other situations, the important

variables that drive the economics

should be carefully evaluated. There

are, however, a few applications

where the technology’s benefits have

been so extensively demonstrated 

that no detailed analysis is required:

storage spaces, ice arenas, most

supermarket applications, military

commissaries, hospital operating

rooms, and as an add-on to existing

air conditioning systems with inadequate dehumidification capacity.

This Technology Alert provides

information and procedures that a

Federal energy manager needs in

evaluating the cost-effectiveness of 

a desiccant system. The process of

desiccant dehumidification and its

energy savings and other benefits are

explained. Guidelines are provided

for appropriate application and 

installation. In addition to a methodology of estimating energy savings

potential from desiccant system

installation, a case study is presented

to give the reader a sense of the actual

costs and energy savings. A listing 

of current manufacturers, technology

users, and references for further 

reading is also included at the end 

of this report.
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3

Contents

Abstract .................................................................................... 1

About the Technology.............................................................. 5

Application Domain

Energy Savings Mechanism

Other Benefits

Variations in System Design and Operating Modes

Variables Affecting Performance

Installation

Federal Sector Potential .......................................................... 10

Application .............................................................................. 10

Application Screening

Where to Use Hybrid Desiccant Cooling Systems

What to Avoid

Design and Equipment Integration

Cost

Utility Incentives and Support

Technology Performance ........................................................ 15

Field Performance

Maintenance

Desiccant Life

Other Impacts

How to Estimate Energy Savings Potential

Case Study .............................................................................. 16

Burger King Restaurant

Evaluation of the Two-Wheel 

Desiccant Demonstration System

Preliminary Monitored Data from the 

Two-Wheel Desiccant System

Implementation Barriers .......................................................... 18

The Technology in Perspective ................................................ 18

Acknowledgments.................................................................... 18

Manufacturers .......................................................................... 19

Commercial Desiccant System

Industrial and Military Desiccant System Manufacturers

Who is Using the Technology.................................................. 19

For Further Reading ................................................................ 20
4

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5

About the Technology

Although there are a variety of 

desiccant dehumidification technologies, the primary focus of this Technology Alert is the two-wheel desiccant system (TWDS). Except when

describing elements common to 

all desiccant-based systems, no 

other desiccant dehumidification

processes are discussed here.

Desiccant dehumidification 

technology has been used in military

storage and many industrial applications for more than 60 years (Harriman

III 1990). Continuous desiccant 

dehumidification process can be

achieved in a number of ways using

liquid spray-tower, solid packed

tower, rotating horizontal bed, multiple vertical bed, and rotating wheel.

The TWDS falls into the

rotating wheel category.

In such a TWDS, the

moist air stream, which

has a high vapor pressure, passes through a

rotating desiccant 

wheel. The desiccant,

which has a low vapor

pressure, adsorbs the

moisture until the 

desiccant is saturated.

Next, the saturated 

portion of the wheel

rotates into a hot air

stream, which is forced

through the wheel to

remove the moisture

from the desiccant. 

The dried desiccant is

rotated back into the

moist air stream, and the process

repeats itself. After it has been

regenerated (dried), the desiccant is

cooled to lower its vapor pressure. 

As shown in Figure 1, a TWDS

consists of a desiccant wheel, a rotary

heat exchanger (sometimes referred

to as a sensible heat wheel), a supply

fan, an exhaust fan, and a heat source

for regenerating the desiccant. The

desiccant wheel is made of finely

divided desiccant material, usually

silica gel, titanium silicates, or some

type of zeolite (a mineral containing

hydrous silicates). The desiccant

material is impregnated into a fibrous

support structure, which looks like

corrugated cardboard that has been

rolled into the shape of a wheel or

into a wheel-shaped rotor with a

lightweight structural honeycomb

core of man-made, fire-retardant

material. The rotary heat exchanger,

which exchanges (recovers) heat

rather than moisture, resembles the

desiccant wheel in appearance and

design.(a) Any form of thermal energy

stream (usually at 180°F or above)

can be used to dry and regenerate 

the desiccant, including electricresistance heaters, solar hot water

coils, heat reclaim coils, hot water 

or steam from boilers, or natural gas

burners. Most commercial applications use either direct- or indirectfired natural gas burners. The actual

desiccant cycle is explained later in

this section. 

The TWDS can control or lower

humidity, but its ability to lower 

sensible heat is limited. Therefore,

in most commercial applications the

Fig. 1. Schematic of the Two-Wheel Desiccant System

(a) For some applications heat pipes are used instead of a rotary heat exchanger. The function of a heat pipe is similar to that of a rotary heat

exchanger, i.e., it transfers heat from process air stream to the reactivation air stream. Although the efficiency of a heat pipe is not as high as a

rotary heat exchanger, it is used in applications where the exhaust air may be contaminated. Because the heat pipe is a stationary device,

particulates from one air stream do not mix with the other air stream. Desiccant systems with heat pipes are generally used in hospitals and 

laboratory applications.

Exhaust (Return)

Air for

Reactivation

Rotary

Heat

Exchanger

Filter

Direct-Fired Gas

Reactivation Heater

+

- 100

100

Process (Supply)

Air to Conditioned

Space
6

TWDS is supplemented with either a

vapor compression or an evaporative

cooling system. These “hybrid”

systems are described later in the

Technology Alert (henceforth, much

of the discussion in this Technology

Alert will be focused on the hybrid

systems).

Although the technology has been

proven in industrial environments,

there is limited field test data to 

evaluate the long-term performance

of the TWDS in commercial building

applications. The first cost of the

hybrid desiccant system is generally

higher than that of conventional 

systems, but this is offset by lower

operating costs in certain applications

(Manley et al. 1985; Burns et al.

1985; Cohen and Slosberg 1988;

Marciniak et al. 1991; Novosel 1996).

The TWDS offers many other benefits in addition to operational costsavings. These benefits will be 

highlighted later in the section.

Most commercial systems are

designed to maximize the energy cost

savings and minimize the initial cost.

To optimize the benefits of hybrid

systems, an understanding of the

impacts of the technology, the load

being served (latent versus sensible),

and the climate in which it is operating are all essential parameters.

These topics are reviewed below.

Application Domain

Desiccant systems have been

widely used in applications where the

prime consideration is special system

requirements rather than energy 

efficiency or competitive pricing

(such as the military and industrial

sectors). They have been successful

in these instances because there are

no practical alternative processes that

are capable of providing low moisture

levels (less than 30°F dew-point), low

microbial growth, or improved indoor

air quality. In the residential and the

commercial building sectors, desiccant technology currently competes

with the well-established conventional vapor compression technology.

Unfamiliarity with the technology,

and lack of assurance and education

about the performance and costeffectiveness of hybrid desiccant

cooling systems, impede implementation of such systems (Mei et al.

1992). 

Although no firm shipment 

numbers are available, the building

sector has seen significant growth in

installation of hybrid systems in the

past few years. This indicates

increased awareness that hybrid 

desiccant systems can provide both

temperature and humidity control and

in some applications use less energy

than conventional vapor compression

systems. The applications where the

benefits have been extensively

demonstrated include dry storage

spaces, ice arenas, most supermarket

applications, military commissaries,

hospital operating rooms, schools,

fast-food restaurants, unheated 

warehouses, and as an add-on to

existing air conditioning systems with

inadequate dehumidification capacity.

The following would encourage

increased use of hybrid systems:

guidance and techniques for reducing

the first cost of hybrid desiccant

cooling systems; performance documentation and confirmation by an

agency such as the Department of

Energy (to encourage government

facilities’ managers and their architects and engineers to objectively

evaluate desiccant technology for

large- and medium-sized building

projects); development of design

tools (such as a user-friendly computer program) to enable designers 

to easily evaluate economic tradeoffs

and design hybrid desiccant cooling

systems based on actual performance

data; and utility incentives 

(Mei 1992).

Energy Savings Mechanism

The energy-saving mechanism of 

a TWDS can best be understood by

comparing the dehumidification and

cooling process of the conventional

and the desiccant-based systems.

Both systems can be operated in 

various modes (recirculation, pure

ventilation, and mixed). These

modes will be discussed later in the

Technology Alert, but to illustrate the

energy savings feature, it is assumed

that both systems take in 100% 

outdoor air. 

The following steps describe the

psychrometric process for a hybrid

desiccant dehumidification and 

supplemental cooling system (the 

letters correspond to state points on

the psychrometric chart in Figure 2).

Dehumidification

• A: Intake—hot and humid 

outdoor air enters the desiccant

wheel at point A on the 

psychrometric chart 

(Figure 2 (a)).

• A-B: Dehumidification—as the

moisture from the outdoor air is

removed by sorption, the heat

generated when the water is

sorbed (akin to condensation)

remains in the air stream, increasing the air stream’s sensible load.

There is a slight increase in the

enthalpy (i.e., the energy content

of the air stream increases), when

latent heat is being converted into

sensible heat. At state B, the air

is hot and dry and cannot be

directly used to cool the conditioned area.

Cooling

• B-C: Heat loss or post-cooling—

the dehumidified outdoor air

enters the rotary heat wheel,

where it exchanges heat with the

exhaust (return) air stream from

the conditioned space. In this

process, the hot and dry outdoor

air cools down, and the cold

exhaust air is pre-heated for 

reactivating the desiccant wheel.

• C-D: Supplemental cooling—the

air leaving the rotary heat wheel

is colder than the air leaving the

desiccant wheel, but further 

cooling is often required before it
7

can enter the conditioned space.

This can be achieved by using a

conventional direct-expansion

vapor compression cooling 

system.

• D-E: Space cooling load—the

exhaust air leaving the conditioned space is at state E.

Regeneration

• E-H: Heat recovery—the 

exhaust air stream enters the

rotary heat wheel where it

exchanges heat with the hot 

and dry air leaving the desiccant

wheel. Part of the heat lost in

step B-C is recovered by this

process (Figure 2 (b)).

• H-I: Heat addition—the hot

exhaust air is further heated to

increase the vapor pressure at 

the desiccant.

• I-J: Reactivation—the hot

exhaust air stream dries and 

reactivates the saturated 

desiccant.

For comparing the above process

with that of a conventional system,

the following steps for cooling and

dehumidification with a conventional

vapor compression system are 

shown on the psychrometric chart

(Figure 2(a)):

Sensible cooling

• A: Intake—hot and humid 

outdoor air enters the evaporator

coil of a conventional vapor 

compression system at point A 

on the psychrometric chart

(Figure 2 (a)).

• A-F: Sensible cooling—the hot

and humid outdoor air stream is

cooled until it reaches saturation.

At this point, the air is cold

enough to be used in the conditioned space, but cannot be 

circulated because it is saturated

with moisture. To remove 

moisture, the air must be 

cooled to below its dew-point

temperature.

Latent cooling (dehumidification)

and reheat

• F-G: Dehumidification—the

evaporator continues to cool 

the saturated air stream and 

condenses the moisture, further

reducing the dry-bulb and the

humidity. If the humidity 

requirement is low (less than 

40 grains/lb of dry air), the air

must be cooled to less than 43°F

in order to condense enough

moisture. In this state, it is too

cold to be circulated to conditioned space.

• G-D: Reheat—the cold, dry air

stream is mixed with hot air or

reheated to the desired circulation

temperature (state D).

• D-E: Cooling load—the exhaust

air leaves the conditioned space

(state E).

The psychrometric processes

shown in Figure 2 highlight the 

differences in the way dehumidification is accomplished by the two 

systems. The amount of energy saved

depends primarily on the ability of

the hybrid system to shift part of the

cooling load (dehumidification load)

to a low-grade thermal source and to

eliminate reheat (step G-D). The fan

power is slightly increased because of

increased air pressure drop through

the desiccant and sensible wheels.

The amount of energy saved and the

reduction in electric demand depend

on several factors. The key factors

are discussed later.

Other Benefits

Desiccant-based systems in 

general, and the hybrid system in

particular, offer several benefits

besides energy conservation.

• Desiccant systems often permit

reductions in the size of the 

conventional system (vapor 

compression unit), because part

of the cooling load (dehumidification load) is shifted to the 

desiccant system. Size reduction

not only saves energy, but it 

also decreases electrical demand

and may reduce initial capital 

investment. 

Fig. 2. (a) Comparison of a Hybrid Desiccant-Based Dehumidification and 

Supplemental Cooling Process with a Conventional Dehumidification and Cooling Process (b) Desiccant Reactivation Process

(a) (b)
8

• In some hybrid systems, the

vapor compression system can 

be replaced with less expensive

direct or indirect evaporative

cooling systems. 

• Hybrid systems permit independent control of both temperature

and humidity. In conventional

cooling systems, only temperature is controlled directly; the

humidity is allowed to vary. 

• Desiccant-based systems can

reduce the moisture much below

the 40°F dew-point temperature,

while the conventional cooling

systems can only dehumidify the

air to temperatures above the

40°F dew-point temperature. 

• Desiccant-based systems can

improve indoor air quality

because of precise humidity 

control. Where conventional 

systems are used in humid 

climates, there is potential for

microbial growth in the ducts 

and condensate drain pans

because of inadequate moisture

removal. This is not a problem

for a desiccant-based system

because there is typically very 

little water on a post-desiccant

cooling coil or, subsequently, in

the drain pan and the air distribution ducts.

• Hybrid systems can provide yearround comfort (the boiler used 

for reactivation can be used for

comfort heating) and the heat

wheel can be used to recover

energy all year.

• As Federal facilities move to

replace CFC-11, CFC-12, and

CFC-22 refrigerants with HCFC123 and HCFC-134a, which only

provide about 90% of the existing

capacity, desiccant systems can

be used to replace the capacity.

Variations in System Design and

Operating Modes

Hybrid systems have several 

possible configurations and operating

modes, some of which are discussed

in this section. System configurations vary based on the type of 

desiccant used for humidity control,

type of cooling used for temperature

control, type of cooling used for precooling, and method of reactivation.

For the TWDS, the choice of desiccant is limited to solid desiccants

embedded in a rotating wheel. 

Regardless of the system configuration, some supplemental cooling

will be needed in most commercial

building applications. This can be

achieved in several ways: conventional vapor compression (direct

expansion), chilled water coils, direct

evaporative cooling pads and indirect

evaporative cooling coils. 

• Supplemental cooling options:

The heat wheel provides part of

the sensible cooling, but in most

cases is not sufficient to meet the

sensible load requirements of the

conditioned space. Therefore, the

TWDS is usually supplemented

with additional sensible cooling.

Indirect evaporative cooling can

be used, but the capacity of such

systems is limited. The process

air (air leaving the wheel) may be

cooled with direct evaporative

cooling pads, but this method

introduces additional moisture

into the conditioned spaces. A

third, commonly used option 

is to cool the process air using

conventional vapor compression

cooling systems.

All the options listed above for

supplemental cooling can also be

used for pre-cooling the air before 

it enters the desiccant wheel. 

• Heat source for reactivation:

The most commonly used heat

sources are direct- and indirectfired natural gas heaters, and gas

fired boilers. Direct-fired natural

gas heaters burn gas directly in

the reactivation air stream.

Therefore, the thermal efficiency

is high (90% to 95%). Indirectfired natural gas burners burn the

natural gas outside the reactivation air stream, and the combustion heat is transferred to the

reactivation air stream through a

heat exchanger. Because a heat

exchanger comes between the

flame and the reactivation air

stream, heat transfer efficiency is

reduced to 80% or less. Gas-fired

boilers that circulate hot water or

steam through heating coils can

be used for both reactivation and

heating during winter. Thus, one

boiler can serve both heating and

reactivation needs (Harriman

1996). Other heat sources used

for reactivation include electric

resistance heaters, solar hot water

coils, heat reclaim coils, and hot

water or steam.

• Operating modes: There are

four possible operating modes:

recirculation, pure ventilation,

makeup, and mixed (Figure 3).

Process and regeneration air can

come from two sources: outdoor air

and/or exhaust air for regeneration,

and outdoor air and/or return air for

process. These possible sources for

the process and regeneration air

streams can be pure or mixed. In 

the recirculation mode, the source 

of process air is return air from the

conditioned space, and outdoor air is

used for regeneration (Figure 3a). In

the pure ventilation mode, the source

for process air (i.e., supply air) is

outdoor air and exhaust air is used 

for regeneration (Figure 3b). In the

makeup mode, the source of both

process air and regeneration air is

outdoor air (Figure 3c). In the mixed

mode, the source for process and
9

regeneration can be a combination 

of the other modes (Figure 3d). Sitespecific conditions and application

requirements will dictate or favor the

choice of a given operation mode, as

well as the economic viability of the

hybrid desiccant system.

Variables Affecting Performance

To achieve maximum benefit from

the desiccant system and for proper

sizing, a clear understanding of the

variables that affect the performance

is essential. Among the variables that

have a major impact on the sizing and

effectiveness of a desiccant system

are process air moisture, temperature,

and velocity through the desiccant;

reactivation air temperature; velocity

and moisture load of air passing

through the desiccant; amount of 

desiccant presented to the reactivation

and process air streams; and desiccant

adsorption properties (Harriman

expected when it leaves the 

system. If a constant level of

moisture in the air leaving the

system is critical, less air should

be processed through the system.

• Process air temperature. If all

other variables are held constant,

lower process air temperatures

mean less moisture in the air

leaving the desiccant wheel.

Higher process air temperatures

mean more moisture in the air

leaving the desiccant wheel. 

• Process air velocity. The more

slowly the air moves through the

desiccant wheel, the drier the

outlet air will be when it leaves

the desiccant wheel. Low air

velocity, therefore, is critical if

low humidity must be maintained

in the conditioned area. However, slower air velocities mean

1990). In any system, these variables

change because of weather, and 

variations in moisture load. It is 

useful for the system designer to

understand the effect of these normal

variations on the performance of the

dehumidifier.

• Process air moisture. If all other

variables are held constant, the

lower the moisture in the air 

entering the desiccant wheel, the

lower the moisture will be in the

air leaving the desiccant wheel.

From a design perspective, if the

incoming air is more moist than

expected, the process air will be

warmer than expected. Therefore, additional cooling will be

necessary if a constant temperature in the conditioned space is

critical. If the air is less moist

than expected when it enters 

the wheel, it will be drier than

Fig. 3. Various Operating Modes for the Hybrid Cooling System
10

bigger and more costly desiccant

wheels. The designer must install

an airflow-monitoring device 

and control the system to avoid

unplanned changes in velocity. 

If the moisture removal rate 

(pounds of moisture removed 

per hour) is more important than 

low humidity in the conditioned

space, then the air can pass

through the system at higher

velocities, and smaller, less 

costly equipment can be used.

• Reactivation air temperature.

The desiccant is dried and 

reactivated by a hot air stream.

The hotter the reactivation air, the

more easily the desiccant gives

up moisture. If dry air is required

in the conditioned space, high

reactivation air temperatures

(250°F) are generally the most

economical choice. If low

humidity is not a requirement,

inexpensive, low-grade heat

sources, such as waste heat,

cogeneration heat, or heat rejected from refrigeration condensers, can be used to reactivate

the desiccant at a low cost. In

this case, the desiccant wheel will

need to be larger than the one in

a high-temperature reactivation

system in order to produce the

same outlet condition in the

process air.

• Reactivation air moisture. In

general, the moisture level of 

reactivation air does not affect

solid-desiccant wheel performance. However, leakage from

the reactivation air stream to 

the process air stream will add

moisture to the process air

stream. The units are designed 

to minimize leakage by keeping 

a positive pressure differential

between the process and regeneration air streams and by placing

seals around the rotor assembly.

• Reactivation air velocity. The

amount of moisture removed

from the desiccant wheel is a

function of the reactivation 

airflow and the temperature 

difference between the reactivation air and the desiccant wheel.

The faster the reactivation air

flows, the greater the moisture

removal from the desiccant. If

the temperature of the reactivation air remains constant, it is a

waste of energy to increase the

airflow beyond the minimum

value necessary to remove the

moisture from the desiccant

wheel. 

• Amount of desiccant presented

to the air stream. Increasing 

the amount of desiccant that is

available to dry the air in a fixed

period of time increases the

moisture removal capacity of the

wheel and the amount of energy

used in reactivation. The increase can be accomplished by

increasing the wheel depth or

wheel rotation speed. Increasing

the wheel depth increases the

mass of the desiccant. This 

causes an increase in airflow

pressure and increases the 

temperature of the air leaving 

the wheel. More energy must

then be expended to cool this air

before it enters the conditioned

space. Increasing the wheel 

rotation speed increases the

amount of moisture removal

because the desiccant wheel

moves faster between process and

reactivation air streams. Again,

more energy must be expended

for both cooling and reactivation.

In general, manufacturers design

the units to optimize the relationship between energy expenditure

and moisture removal capacity.

Therefore, the manufacturer 

usually establishes the rotational

speeds and wheel depth

(Harriman 1990).

• Desiccant adsorption characteristics. At constant temperature,

each desiccant has a fixed 

capacity to sorb moisture. In

general, manufacturers design

units to optimize the moisture

removal capacity for specified

values of other variables such as

air flow rates and wheel speed. 

Installation

The desiccant systems are generally designed for outdoor installation.

Most commercial desiccant systems

are mounted on rooftops. The units

are installed on concrete pads located

as close as possible to the gas and

electrical interfaces. If the desiccant

unit is equipped with an evaporative

cooler, it will need water supply. 

In some large systems, a telephone

hookup may be needed to remotely

monitor the unit’s operation. Clearance may not be an issue for rooftopmounted units; units installed in

enclosed spaces must have sufficient

side access clearance for maintenance. 

Federal Sector

Potential

Although the desiccant technology

has been employed for several

decades, its use was limited to 

industrial and military sectors. The

market potential in those sectors was

estimated to be between $50 million

and $60 million in the early 1990s

(Mei et al. 1992). No concrete 

estimates are available either for 

the commercial buildings sector 

or for the Federal sector because

wider applications of the technology

are only now being investigated.

Application

This section addresses technical

aspects of the hybrid desiccant 

cooling technology. The range of

applications and climates in which

the technology can best be applied is
11

discussed. The advantages and 

limitations are enumerated. Design

and integration considerations of the

technology are discussed, including

costs, options, and installation details.

Utility incentives are also presented. 

Application Screening

Hybrid desiccant cooling systems

can be used in any building application because they provide precise

temperature and humidity control.

However, high initial costs ($5/cfm 

to $8/cfm) typically limit the use of

desiccant technology. 

Several techniques can be used to

estimate the annual energy consumption of hybrid desiccant cooling 

systems. The most accurate are those

that use computer simulations.

Although they produce more reliable

results than hand-calculation techniques, computer simulations are 

difficult and expensive to employ

routinely as initial screening tools,

and are therefore appropriate only

when additional details are required.

The bin method is another analytical tool for screening technology

applications. In general, a bin

method is a simple calculation 

procedure that is readily adaptable 

to a spreadsheet-type analysis and 

can be used to reasonably estimate

the energy consumption of a given

application in a specific location

(ASHRAE 1993, Chapter 28).

However, the bin method underestimates the latent load by about 30%,

because the methodology relies on

using dry-bulb temperature bins with

average coincident wet-bulb temperatures. An alternate approach is to use

joint frequency tables of dry-bulb

temperatures and humidity ratio. The

difficulty with this approach is that

hourly humidity ratios are not readily

available. 

Where to Use Hybrid Desiccant

Cooling Systems

Site-specific conditions and 

differing application requirements

must be understood before use of

desiccant-based hybrid systems in a

building can be justified on economic

grounds. A detailed analysis is 

generally required to compare the

cost-effectiveness of a hybrid system

with a conventional cooling system.

While it is difficult to generalize the

cost-effectiveness of the hybrid 

systems, there are a few applications

where cost-effectiveness is so well

established that detailed analysis is

not necessary. These include storage

spaces (where the space is dehumidified in summer and heated in winter),

ice arenas that operate in summer,

hospital operating rooms (where

humidity control and indoor air 

quality are critical), and most supermarkets. In a situation where the

existing conventional system is

unable to provide sufficient latent

capacity, a TWDS can be integrated

with the existing system to provide

the necessary latent capacity. In such

a situation, the first cost usually

favors a TWDS over a conventional

system. 

For other situations the key 

variables that drive costs should be

carefully evaluated. These include

maximum allowable moisture level 

in the conditioned space, ratio of the

latent cooling load to the sensible

cooling load, amount of fresh air

required, design outdoor dew-point

temperature, availability of exhaust

(return) air for post-cooling, local

electric demand and energy costs,

local gas cost, availability of cheap

regeneration heat, and the benefits 

of improved indoor air quality. The

following application criteria can 

be used (Figure 4):

• Level of Indoor Humidity:

Desiccant-based systems are the

most economical choice to 

dehumidify air below 40°F dewpoint, because condensate often

freezes on the coils of conventional cooling systems at temperatures below 40°F, thereby

reducing the coil moisture

removal capability. If the dewpoint requirement is between

40°F and 50°F, the cost-effectiveness of desiccant-based systems

depends on the other site-specific

conditions and requirements. If

the dew-point requirement is

greater than 50°F, a conventional

system is generally favored. In

most commercial building 

applications, deep drying (dewpoint <40°F) is rare. The recommended level of indoor dew-point

for a typical office building is

between 51°F and 57°F, while the

level for libraries and museums is

between 46°F and 54°F.

• High Latent Load Fraction

(>25%): Desiccant-based 

systems are more cost-effective

in reducing latent loads than are

conventional systems; therefore,

they are more attractive when the

latent loads as a fraction of the

total loads are high. In most

office buildings, the latent loads

are a small fraction of the total

load (less than 25%). However,

supermarkets, movie theaters,

schools, auditoriums and outsideair ventilation systems have a

much higher fraction of latent

loads.

• Fresh Air Intake: Some commercial buildings (schools,

hospitals, restaurants, and retail

establishments) require significant fresh air intake (greater than

20%) (ASHRAE Standard 62). If

the design dew-point temperature

and the frequency of high outdoor

dew-point temperatures are high,

desiccant-based systems may be

cost-effective. ASHRAE has

published 1%, 2%, and 5% 

occurrences of extreme dew-point

temperatures and mean coincident dry-bulb temperatures

(Colliver et al. 1995). It is 

difficult to get the actual frequency of occurrences of dewpoint temperatures for various
12

Fig. 4. Illustration of Initial Application Screening Process (Source: Harriman 1996)
13

climate regions, but frequency 

of mean coincident wet-bulb

(MCWB) temperatures are 

readily available to most Federal

energy managers (TM 5-785,

1978). However, calculations

using MCWB temperatures may

lead to underestimates of the

latent ventilation load.

• Availability of Exhaust 

(Return) Air for Post-Cooling:

If exhaust (return) air is available

for post-cooling (as shown in

Figure 3b), the sensible cooling

requirements are reduced. If the

fresh air requirements are high

(greater than 20%), desiccantbased systems that use a combination of fresh and exhaust air

can be more cost-effective than

conventional cooling systems. 

• Demand and Energy Costs:

High summer-time electric

demand and electricity cost,

coupled with a low summer gas

cost, can favor desiccant systems.

Low gas costs alone may not

make the desiccant-based systems

cost-effective, and other criteria

should be carefully evaluated.

• Availability of Cheap/Free

Reactivation Energy Source:

Availability of free or cheap 

reactivation heat will make 

desiccant-based systems more

attractive. In general, 180°F to

220°F temperatures are required

for regeneration. Some possible

sources of “waste” heat are 

condensers, engines, and gas 

turbine exhaust.

• Indoor Air Quality: Indoor air

quality is difficult to quantify 

in terms of economic benefit,

but is essential for many buildings (e.g., hospitals and nursing

homes). Desiccant-based systems improve indoor air quality

because they precisely control

moisture levels. Where conventional systems are used in humid

climates, there is potential for

microbial growth in the ducts and

condensate drain pans because 

of inadequate moisture removal.

This is not a problem for a 

desiccant-based system because

there is typically very little water

on a post-desiccant cooling coil

or, subsequently, in the drain pan

and the air distribution ducts.

When two or more of the key 

variables favor a hybrid system over 

a conventional system, the building

manager should request a detailed

analysis of the benefits of a hybrid

desiccant system.

What to Avoid

The hybrid desiccant cooling 

system may be less attractive 

• if the conditioned space dewpoint requirement is higher 

than 50°F 

• if the latent fraction of the total

cooling load is less than 25% 

• if the designers and installers 

are inexperienced. As with 

any heating or air-conditioning

system, approved calculation 

procedures should be used 

for sizing.

Design and Equipment

Integration

The purpose of this Technology

Alert is to familiarize the Federal

energy managers and Federal facility

engineers with the benefits and 

liabilities of using desiccant cooling

systems and their application to 

commercial facilities. Because any

desiccant-based system needs to be

optimized to a specific site condition,

it is beyond the scope of this Technology Alert to fully explain the design

requirements of a particular desiccant

cooling system. In general, the 

system should be designed by an

experienced designer and installer of

these systems. It is, however, important that the reader know the basic

steps in the design process, which 

are listed below:

• Determine local climatic design

conditions, e.g., design dry-bulb

and dew-point temperatures. 

• Establish the control levels 

(temperature and humidity).

• Determine building heating and

cooling loads (both latent and

sensible) at design conditions:

- determine number of occupants in the space

- calculate ventilation requirements for the occupants and

for appropriate building 

pressurization

- calculate latent loads from

ventilation, occupants,

infiltration

- size TWDS to meet the latent

load

- calculate sensible loads from

internal gain (including 

occupants), infiltration, and

ventilation

- size conventional cooling 

system to meet the internal

sensible load, plus the load to

cool the hot dry process air to

the required set point.

• Talk to several desiccant cooling

system manufacturers (a list of

manufacturers is provided at the

end of this Technology Alert) to

identify the most suitable types of

equipment. Select the alternative

HVAC system components,

including the indoor air-distribution system type, and size the

alternatives as required.

• Determine the monthly and 

annual building heating and 

cooling energy requirements.

• Estimate the cost of the hybrid

system and the alternative 

system.
14

• Perform life cycle cost (LCC)

analysis on the system design 

(or system design alternatives).

Equipment Selection. Unlike 

the conventional vapor compression

systems, the hybrid cooling systems

are selected based on the latent 

cooling load and not on sensible

cooling or heating loads, although 

the equipment is capable of providing

sensible cooling and heating. Having

selected the system based on the

latent cooling load, check its sensible

cooling and heating capacity. If the

sensible cooling capacity is inadequate, consider adding an external

system, such as vapor compression

equipment and/or a warm-air furnace,

to meet the additional load. Most

manufacturers of TWDS will provide

additional sensible cooling equipment

in a single package along with the

TWDS. The units are also optimized

for a small range of airflows and

moisture removal capability. Operating outside these ranges may damage

the desiccant wheel material or lead

to unreliable performance. TWDS

with a flow capacity of up to 30,000

cfm are now available in the market.

The following issues need to be

evaluated before integrating the 

desiccant system with an existing

HVAC system:

Size and Location. The desiccant

system must be placed near the 

existing HVAC air handler. For new

installations, consider the unit size,

weight, and clearance required for

safety, maintenance, and adequate

airflow. This information is generally

available in the manufacturer’s 

literature. The actual space requirement and the weight of the unit

depends on the capacity of the unit

and other components required to

meet the cooling and heating load 

of the building. Always locate the

desiccant system at the fresh air

intake of the existing system. Ensure

that these utilities are available:

electricity (for operating fans, motors,

compressor), natural gas or propane

(for regeneration, not required if

regeneration is accomplished by

using waste heat from another 

existing source; gas or propane 

connections may be desired as a

reserve fuel), and water (for 

evaporative cooling components).

Equipment warranties. The

prospective user should ask potential

suppliers, contractors and installers

about equipment warranties. The

parts for desiccant cooling systems

are generally guaranteed free from

manufacturing defects for 12 to 

18 months. Some manufacturers

offer extended warranties for up to 

5 years for the desiccant and the heat

wheels. The prospective user should

also ensure that the warranties of

other equipment (for condensing

units, etc.) are valid when a desiccant

cooling system is integrated with 

an existing HVAC system. If the 

performance of the system is to 

be monitored, most manufacturers

can install the sensors that need to 

be placed inside the desiccant unit

before the unit is shipped. This 

protects the customers from potentially voiding the warranty due to

damage to the equipment that could

occur during installation of sensors.

Cost

Because the desiccant systems are

sized based on the airflow rate (cfm),

the costs are typically given in terms

of $/cfm.(b) For large commercial

systems the cost of a TWDS is 

usually about $5/cfm, while smaller

units (less than 1,000 cfm) for 

residential application may cost up 

to $8/cfm. The installation costs can

vary based on specific site requirements. For a hybrid system, the

additional cost of vapor compression

systems must be factored in. As an

illustration, the following examples

have been reported from a technology

transfer workshop on desiccant 

cooling systems (Meckler et al.

1995).

• A 1,600-cfm TWDS (without

additional vapor compression

cooling) was installed at a cost of

between $5/cfm and $8/cfm at a

Burger King restaurant, Aberdeen

Proving Grounds, MD.

• A 140,000-cfm TWDS was 

installed at a cost of about $6/cfm

at the Medical College Building,

Athens, GA.(c)

Utility Incentives and Support

Several utilities are currently 

providing incentives for installing

desiccant dehumidification systems

under their commercial demand-side

management (DSM) program:

Brooklyn Union, Metropolitan Utility

District, Minnegasco Inc., and Mobile

Gas Services Corp (EUN 1996).

Facility managers are encouraged 

to check with their utility regarding

the availability of any custom rebate

programs. These programs are based

on the energy and demand savings,

not on the technology used. Other

sources of information include a 

publication reporting current 

DSM programs by Electric Power

Research Institute (EPRI 1993). This

report identified 2,321 DSM programs from 666 utilities.

(b) A general rule of thumb of 300 cfm/ton to 400 cfm/ton can be used to compare desiccant system cost with conventional vapor 

compression systems.

(c) Refer to the Desiccant Technology Transfer Workshop Manual, American Gas Cooling Center, Arlington, Virginia, for more details 

(Meckler et al. 1995).
15

Technology

Performance

Field Performance

The U. S. Army Construction

Engineering Research Laboratories

(USACERL) managed several TWDS

installations at Department of Defense (DoD) sites to demonstrate the

benefits of desiccant technology.

Although several TWDS have been

installed at DoD sites and other

Federal facilities, detailed performance evaluation of the technology

has not yet been performed. In the

summer of 1994, USACERL participated in the installation of a desiccant

system at a Burger King restaurant at

Aberdeen Proving Grounds (details 

of the installation are provided in the

Case Study section). Since then, four

systems have been installed at other

DoD sites and several more are in

various stages of design. The units

featured in the USACERL desiccant

technology demonstration program

are TWDS manufactured by

Engelhard/ICC. The source of 

reactivation energy for the Engelhard/

ICC units is steam from a gas-fired

boiler system.

In addition to the demonstrations

handled by USACERL, Design and

Construction Division of Defense

Commissary Agency (DCA) has

installed over 70 desiccant systems in

military commissaries in the United

States. Of these, only three installations are of the two-wheel design,

three more use a heat pipe for heat

recovery, and the rest are singlewheel desiccant systems. The six

units with heat recovery are targeted

for direct distribution systems over

frozen food aisles. All other sites

have units that maintain the entire

store to a design of 75°F dry-bulb

temperature and 48°F dew-point 

temperature. The desiccant units at

the DCA sites use direct-fired natural

gas burners. The first installation

funded by DCA was in the year 1983

at Lackland Air Force Base in Texas.

DCA plans to install eight more 

desiccant systems with heat pipes for

heat recovery in the future. DryCool

division of Munters Incentive Group

manufactured all systems installed 

by DCA. 

Several facility managers were

contacted to ascertain the performance of desiccant systems. Only

one manager was not satisfied with

the performance of the unit. This

manager reported that the humidity

levels in the conditioned space were

still too high, and that the sensible

cooling was inadequate because of

improper design.

Maintenance

Maintenance costs fall into two

categories:

• General air-conditioning 

maintenance. Typical yearly

maintenance costs for airconditioning systems range 

from $25/ton to $35/ton. 

• Maintenance of the desiccant

dehumidification components.

For the desiccant and heat

wheels, filters should be well

maintained and changed every 

2 months. The wheel can be 

vacuumed to remove dust from

the wheel face. The other parts

of the desiccant wheel that need

regular maintenance are the 

contact seals (5-year life), the

wheel drive assembly, the wheel

support bearing, fan, and fan belt.

No regular maintenance is

required for the desiccant material. The heat wheel, like the

desiccant wheel, needs very little

maintenance if the filters are well

maintained. The other parts of

the heat wheel that need regular

maintenance include the wheel

drive assembly and the wheel

support bearing. If the process

air is post-cooled using a direct 

or indirect evaporative cooler,

the regular maintenance for the

cooler includes flushing the pads

and sump frequently (every 

2 months), treating the makeup

water, and draining the water

supply pipe during winter months

(when dehumidification is not

needed and freezing is likely).

Desiccant Life

Usefulness of the desiccant 

material depends largely on the 

quantity and type of contamination in

the air streams. In a commercial airconditioning environment, desiccants

last between 10,000 hours and

100,000 hours before they need 

replacement (ASHRAE 1993, Chapter 19). Adsorbents (solid desiccants

used in TWDS) tend to be less 

reactive chemically and more sensitive to clogging, a function of the

type and particulate material in the air

stream. They may also be sensitive to

hydrothermal stress, which results

from thermal expansion and contraction of the desiccant material due to

rapid changes in desiccant moisture

content (ASHRAE 1993). Because

the application of TWDS in commercial air-conditioning is new, the longterm performance (over 10 years) 

of the desiccant wheel is not clear.

According to the manufacturers, a

well-maintained desiccant wheel will

last for approximately 100,000 hours

of operation (10 to 15 years). 

Other Impacts

The usual codes and regulations

for installing or servicing air-conditioning or refrigeration equipment

apply. No other special code compliance issues exist. There will be a

small increase in local emissions,

because of the use of a fossil fuel- 

(gas or propane) fired heater for 

regeneration. However, there will

also be a decrease in utility emissions, because of reduced electric

energy use. Desiccant systems

reduce the cooling load on the 

conventional system; therefore,

smaller conventional systems can be

used, reducing use of ozone-depleting

chlorofluorocarbons (CFCs).
16

How to Estimate Energy Savings

Potential

Estimation of energy savings from

use of TWDS is an intricate task,

because of the complexity involved 

in modeling the annual performance.

A spread-sheet analysis using the

ASHRAE bin method works well 

for the conventional system, but can

not be used for desiccant systems.

Several detailed energy analysis tools

are available to assess the annual 

performance of the desiccant systems

and compare it with alternative

options such as conventional systems.

Two such tools, DOE2.1E and

TRACE,(d) are detailed programs that

can model the annual performance 

of the solid desiccant-based systems.

For details of other models, refer to

the report by Mei et al. (1992). Most

manufacturers have developed their

own analytical models. These 

generally are proprietary and 

validation is often not clear. 

Case Study

Several desiccant-based systems

have been installed at DoD sites. A

partial performance monitoring has

been completed at one site, but no

historical utility billing information is

available for any of the demonstration

sites. This has made even a qualitative analysis of the billing data 

difficult. Information is available

from one demonstration site, where

some of the critical variables were

monitored after the desiccant system

was installed. The monitoring data

include outdoor dry-bulb temperature

and relative humidity, process drybulb temperature and relative humidity (supply), process air flow rate, run

time of the unit, regeneration air 

temperature, electricity consumption,

and regeneration gas consumption.

The facility, its systems, and 

the preliminary monitoring data are

presented in the following section.

Burger King Restaurant

One of USACERL’s demonstration systems was installed at a Burger

King restaurant at Aberdeen Proving

Ground (APG), Maryland. Fast food

restaurants, large dining facilities and

other common areas present a unique

situation, because of high occupant

density. USACERL wanted to 

evaluate the use of desiccant-based

systems as an air-conditioning 

solution for such facilities.

The building is an Army-owned

Burger King franchise that is representative of a typical fast food 

restaurant. It is open 24-hours a day,

seven days a week. Several rooftop

air-conditioning units serve the 

building (kitchen, dining area, and

bathrooms). The dining area was 

isolated for this study, because its

occupancy density is highest. 

Initially, the dining area had two

packaged rooftop units (5-ton and 

7.5-ton) supplying 700 cfm of 

ventilation out of a total supply flow

rate of 5,000 cfm. Although the peak

design load(e) matched the equipment

nominal capacity (12.5-ton) for the

dining area, the components of the

load (sensible and latent) did not

match the equipment capacities. At

the design conditions, the nominalcapacity of the two units was reduced

from 12.5 tons to 10.5 tons, approximately 13% below the design load

(because of supply fan reheat and

other losses). The total latent capacity of the units at the design conditions was also less than the required

design latent capacity (Meckler et 

al. 1995). This shortage was exacerbated by off-design conditions, in

which the latent component of the

total load did not drop off nearly as

quickly as the sensible component.

Because of these problems, the 

two packaged units were unable to

dehumidify and cool the air simultaneously, resulting in frequent hot and

humid conditions in the dinning area.

As a remedy, a nominal 1,600 cfm

TWDS manufactured by Engelhard/

ICC was installed in the year 1994 as

a collaboration between Engelhard/

ICC, APG, and USACERL to demonstrate desiccant technology under 

the Army’s Facilities Engineering

Applications Program (FEAP).

The installation of the TWDS 

was completed in the summer of

1994. Since then, the new system

handles the latent load from ventilation and internal gains, and has 

operated reliably as designed. 

Improvements in operating conditions

were immediately noticed by the

restaurant employees and customers.

Specifics of the system performance

are given below.

Evaluation of the Two-Wheel

Desiccant System Demonstration

The objective of this demonstration was to evaluate the costeffectiveness and energy conservation

potential of the TWDS as it conditioned the air to the appropriate 

comfort level for the dining area

occupants. The design concept was

to separate the sensible (internal

gains) and latent (ventilation and

internal latent) cooling functions.

The sensible cooling was handled by

the existing 7.5-ton rooftop unit(f) and

the latent cooling was accomplished

by installing a new TWDS, which

replaced the existing 5-ton rooftop

unit. By separating the cooling 

functions, the effectiveness of the

conventional vapor compression 

system and the desiccant-based 

system was maximized.

(d) A computer model developed by Trane Commercial Systems Group for building energy modeling.

(e) A design load analysis was performed by a design engineering firm using Trane Ultra Cooling Load Program. Refer to the Desiccant Technology

Transfer Workshop Manual, American Gas Cooling Center, Arlington, Virginia, for more details (Meckler et al. 1995).

(f) The ductwork was modified to distribute this capacity to the entire dining area.
17

The TWDS, as shown in Figure 1,

combines a rotary desiccant wheel

with a high-effectiveness rotary heatexchanger wheel. This combination,

described in the Energy Savings

Mechanism section, transfers some 

of the “sensible penalty” associated

with desiccant wheel over to the 

regeneration air stream. The unit

uses a propane-fired steam boiler 

for the remainder of the regeneration

heat, which is housed within the 

desiccant unit. The TWDS operates

in a make-up mode, shown in 

Figure 3c. The outside air is passed

through the desiccant-wheel where 

it is dehumidified and then cooled as

it passes through the sensible heatwheel. The warm dry air is directed

to the conditioned space by its own

concentric diffuser at ceiling level,

and the return air is cooled by the

existing 7.5-ton packaged rooftop

unit. The dry air from the TWDS and

the cool air streams only mix inside

the dining area.

Preliminary Monitored 

Data from the Two-Wheel

Desiccant System

Several variables were recorded 

at 15-minute data intervals from

August 1994 through January 1995.

The daily average outdoor air and

process air dry-bulb temperatures for

the cooling season are shown in

Figure 5. As the outdoor air was

dehumidified it became warmer.

Because the source for the regeneration air stream was also the outdoor

air, the process air dry-bulb temperature was higher than the outdoor air

temperature (the heat exchanger

effectiveness was less than 1). 

The daily average moisture content

for outdoor air and process air

streams for the cooling season are

shown in Figure 6. With the exception of the first two weeks of operation, the moisture content of the

process air stream stayed between 

40 and 60 grains. The daily average

electric demand was around 4 kW(g)

(Figure 7), and the daily average gas

consumption was around 30 ft3/h.

The gas consumption in October

reflects the nighttime heating energy

consumption (Figure 8).

(g) The conventional 5-ton system that was replaced with TWDS would have consumed between 6 kW and 7 kW.

Fig. 6. Daily Average Outdoor Air and Process Air Humidity Ratios

0

25

50

75

100

Aug 01Aug 04

Aug 07

Aug 10

Aug 13Aug 17Aug 20Aug 23

Aug 26Aug 29

Sep 01

Sep 04

Sep 07

Sep 10

Sep 13Sep 16Sep 19Sep 22Sep 25

Sep 28

Oct 01

Oct 04

Oct 07

Oct 10

Oct 13Oct 16Oct 19Oct 22Oct 25

Oct 28Oct 31

Day of Month

Temperature (oF)Outdoor Air Process Air (Supply Air)

Fig. 5. Daily Average Outdoor Air and Process Air Dry-Bulb Temperatures

0.00

20.00

40.00

60.00

80.00

100.00

120.00

140.00

Aug 01

Aug 04

Aug 07

Aug 10

Aug 13

Aug 17

Aug 20

Aug 23

Aug 26

Aug 29

Sep 01

Sep 04

Sep 07

Sep 10

Sep 13

Sep 16

Sep 19

Sep 22

Sep 25

Sep 28

Oct 01

Oct 04

Oct 07

Oct 10

Oct 13

Oct 16

Oct 19

Oct 22

Oct 25

Oct 28

Oct 31

Day of Month

Grains (lb/lba)

Outdoor Air Process Air
18

Implementation

Barriers

The first cost of a desiccant 

system is higher than that of a conventional system. To offset this 

disadvantage, innovative designs

using hybrid systems are often

required. According to Mei et al.

(1992), the impediments to use of

desiccant-based systems are lack of

familiarity with the technology and

lack of assurance and education about

the performance and cost-effectiveness of hybrid systems. This Technology Alert has addressed some of

these issues. Federal energy managers who are familiar with the TWDS

are listed below in the Who is Using

the Technology section. The reader 

is invited to ask questions and learn

more about the technology.

The Technology 

in Perspective

The desiccant technology in 

general and the two-wheel desiccant

system technology in particular has a

good potential in the Federal sector

because it not only reduces operating

costs but also improves indoor air

quality. The technology is especially

useful for conditioning storage

spaces, ice arenas that operate in

summer, hospital operating rooms,

and most supermarkets. In a situation

where the existing conventional 

system is unable to provide sufficient

latent capacity, a TWDS can be 

integrated with the existing system.

In such a situation, the first cost 

usually favors a TWDS over a 

conventional system. It is not 

economical to install a desiccant 

system in situations where the 

design space dew-point requirement

is higher than 50°F, or where the

latent to total capacity ratio is less

than 25%.

Acknowledgments

The author would like to thank 

the following individuals for their

valuable input in preparing this 

Technology Alert: James R. Brodrick

(Engelhard/ICC), Tom Durbin

(USACERL), Lew Harriman (Consultant), Larry J. Kelkar (Munter

DryCool), Douglas Kosar (GRI),

Kevin McGahey (AGCC), and Dave

Conover, Steve Parker, and Sriram

Somasundaram (all of PNNL). 

The author would also like to thank

Michaela Mann and Dave Payson

(PNNL) for editing the manuscript.

Fig. 7. Daily Average Electricity Demand Consumption

Fig. 8. Daily Average Gas Consumption

0

1

2

3

4

5

Aug 01Aug 04Aug 07Aug 10Aug 13Aug 17Aug 20Aug 23

Aug 26

Aug 29

Sep 01

Sep 04Sep 07Sep 10Sep 13Sep 16Sep 19Sep 22Sep 25

Sep 28

Oct 01

Oct 04Oct 07Oct 10Oct 13Oct 16Oct 19Oct 22Oct 25

Oct 28

Oct 31

Day of Month

Electric Demand (kW)

0

10

20

30

40

50

60

Aug 01Aug 04

Aug 07

Aug 10

Aug 13Aug 17Aug 20Aug 23

Aug 26

Aug 29

Sep 01

Sep 04

Sep 07

Sep 10

Sep 13Sep 16Sep 19Sep 22

Sep 25

Sep 28

Oct 01

Oct 04

Oct 07

Oct 10

Oct 13Oct 16Oct 19Oct 22

Oct 25

Oct 28

Oct 31

Day of Month

Gas Consumption (ft3/h)
19

Manufacturers

Commercial Desiccant System

Engelhard/ICC

441 North Fifth St.

Philadelphia, PA 19123

Phone: (215) 625-0700 Ext.130

Fax: (215) 592-8299 

Munters DryCool

16900 Jordan Road

Selma, TX 78154

Phone: (210) 651-5018

Fax: (210) 651-9085 

e-mail: cargo@munters.com

Web Page URL: http://

www.munters.com/

New Thermal Technologies Inc.

12900 Automobile Blvd.

Clearwater, FL 34622

Phone: (813) 571-1888

Fax: (813) 571-2242

Octagon Air Systems

1724 Koppers Road

Conley, GA 30017

Phone: (404) 608-8881

Fax: (404) 608-0880

Seasons 4, Inc.

4500 Industrial Access Road

Douglasville, GA 30134

Phone: (770) 489-0716

Fax: (770) 489-2938

SEMCO Incorporated

1800 East Pointe Drive,

Columbia, MO 65201-3508

Phone: (606) 236-3181

Fax: (606) 236-3184

e-mail:

Web Page URL:

http://www.semcoinc.com/

Industrial and Military

Desiccant System Manufacturers

Airflow Co., Dryomatic Div.

295 Bailes Ln

Frederick, MD 21701

Phone: (301) 695-6500

Fax: (301) 631-0396

Air Technology Systems Inc.

1572 Tilco Drive

Fredercik, MD 21701

Phone: (301) 620-2033

Fax: (301) 622-6421

Bry-Air Inc.

P O Box 269, Rt. 37 West

Sunbury, Ohio 43074

Phone: (614) 965-2974

Fax: (614) 965-5470

e-mail: bry-air@zip.com 

Web Page URL: http://

www.smartpages.com/bryair/

Comfort Enterprises

a unit of Herrmidifier Co.

P.O. Box 11148

Lancaster, PA 17605

Phone: (717) 394-8208

Fax: (717) 394-0612 

Des Champs Laboratories, Inc.

Route 130/Douglas Way,

P.O. Box 220,

Natural Bridge Station, VA 24579

Phone: (540) 291-1111

Fax: (540) 291-2222

Kathabar Systems Division,

Sommerset Technologies

P.O. Box 791,

New Brunswick, NJ 08903

Phone: (908) 356-6000

Fax: (908) 356-0643

e-mail: kathabar@kathabar.com

Web Page URL: http://

www.kathabar.com/

Miller-Picking Division,

York International,

P.O. Box 130

Johnstown, PA 15907

Phone: (814) 479-4023

Fax: (814) 479-2469

Munters Cargocaire

79 Monroe Street,

Amesbury, MA 01913

Phone: (888) 349-4335

Fax: (500) 338-4556 

e-mail: cargo@munters.com

Web Page URL: http://

www.munters.com/

Who is Using the

Technology

Aberdeen Proving Grounds, MD

Contact: Gary Testerman

Phone: (410) 278-5738

MacDill AFB, FL

Contact: Jim “Zack” Zaccari

Phone: (813) 828-5340

Navy Public Works Center,

Naval Air Station

Pensacola, FL

Contact: John Geurin

Phone: (904) 452-3127

Defense Commissary Agency,

Design and Construction Division

Contact: Jack Hernandez

Phone: (210) 671-4741

American Gas Cooling Center

(Gas Cooling Industry Sales & 

Marketing)

Contact: Kevin McGahey

Phone: (703) 841-8542

Web Page URL: http://

www.agcc.org/
20

For Further Reading

ASHRAE. 1993. “Chapter 19:

Sorbents and Desiccants.” ASHRAE

Handbook: Fundamentals. American

Society of Heating, Refrigerating, and

Air-Conditioning Engineers, Inc.

Atlanta, Georgia.

ASHRAE. 1993. “Chapter 28:

Energy Estimating Methods.”

ASHRAE Handbook: Fundamentals.

American Society of Heating, Refrigerating, and Air-Conditioning Engineers, Inc. Atlanta, Georgia.

Burns P R., J.W. Mitchell, W.A.

Beckman. 1985. Hybrid desiccant

cooling systems in supermarket

applications. ASHRAE Transactions,

91(1B): 457-468.

Cohen, B.M., and R.B. Slosberg.

1988. Application of gas-fired 

desiccant cooling systems. ASHRAE

Transactions, 94(1): 525-536.

Colliver, D.G., H. Zhang, R.S. Gates,

and K.T. Priddy. 1994. Determination of the 1%, 2.5%, and 5% occurrences of extreme dew-point temperatures and mean coincident dry-bulb

temperatures. ASHRAE Transactions, 101(2): 265-286.

Electric Power Research Institute

(EPRI). May 1993. 1992 Survey of

Utility Demand-Side Management

Programs. EPRI-TR 102193, Palo

Alto, California.

Energy User News (EUN). 1996.

Energy User News Matrix of Commercial and Industrial Utility Incentive Program. Energy User News,

July, pp. 50-52.

Harriman, III, L.G., Editor. 1990.

The Dehumidification Handbook.

2nd Edition, published by Munters

Cargocaire, Amesbury, Massachusetts.

Harriman, L. 1996. Applications

Engineering Manual for Desiccant

Systems. American Gas Cooling

Center, Arlington, Virginia.

Manley, D.L., K.L. Bowlen, B.M.

Cohen. 1985. Evaluation of gasfired desiccant-based space conditioning for supermarkets. ASHRAE

Transactions 91(1B): 447-456.

Marciniak, T J., R.N. Koopman, D.R.

Kosar. 1991. Gas-fired desiccant

dehumidification system in a quickservice restaurant. ASHRAE Transactions, 97(1): 657-666.

Meckler, M., R. Heimann, J. Fischer,

and K. McGahey. 1995. Desiccant

Technology Transfer Workshop

Manual. American Gas Cooling

Center, Arlington, Virginia.

Mei, V.C., F.C. Chen, Z. Lavan,

R.K. Collier Jr., and G. Meckler.

1992. An Assessment of Desiccant

Cooling and Dehumidification

Technology. Oak Ridge National

Laboratory, ORNL/Con-309,

Oak Ridge, Tennessee.

Novosel D. 1996. Advances in

Desiccant Technologies. Energy

Engineers, 93(1): 7-19.
The Federal Government is the largest energy consumer in the nation. Annually, in its 500,000 buildings and 8,000 locations worldwide,

it uses nearly two quadrillion Btu (quads) of energy, costing over $8 billion. This represents 2.5% of all primary energy consumption in

the United States. The Federal Energy Management Program was established in 1974 to provide direction, guidance, and assistance to

Federal agencies in planning and implementing energy management programs that will improve the energy efficiency and fuel flexibility

of the Federal infrastructure.

Over the years several Federal laws and Executive Orders have shaped FEMP's mission. These include the Energy Policy and Conservation Act of 1975; the National Energy Conservation and Policy Act of 1978; the Federal Energy Management Improvement Act of 1988;

and, most recently, Executive Order 12759 in 1991, the National Energy Policy Act of 1992 (EPACT), and Executive Order 12902 in

1994.

FEMP is currently involved in a wide range of energy-assessment activities, including conducting New Technology Demonstrations, to

hasten the penetration of energy-efficient technologies into the Federal marketplace.

Federal Energy Management Program

About the Federal Technology Alerts

The Energy Policy Act of 1992, and

subsequent Executive Orders, mandate

that energy consumption in the Federal

sector be reduced by 30% from 1985

levels by the year 2005. To achieve

this goal, the U.S. Department of

Energy’s Federal Energy Management

Program (FEMP) is sponsoring a 

series of programs to reduce energy

consumption at Federal installations

nationwide. One of these programs,

the New Technology Demonstration

Program (NTDP), is tasked to accelerate the introduction of energy-efficient

and renewable technologies into the

Federal sector and to improve the rate

of technology transfer.

As part of this effort FEMP is 

sponsoring a series of Federal Technology Alerts (FTAs) that provide 

summary information on candidate

energy-saving technologies developed

and manufactured in the United States.

The technologies featured in the

Technology Alerts have already 

entered the market and have some

experience but are not in general use 

in the Federal sector. Based on their

potential for energy, cost, and environmental benefits to the Federal sector,

the technologies are considered to be

leading candidates for immediate

Federal application.

The goal of the Technology Alerts 

is to improve the rate of technology

transfer of new energy-saving technologies within the Federal sector and

to provide the right people in the field

with accurate, up-to-date information

on the new technologies so that they

can make educated judgments on

whether the technologies are suitable

for their Federal sites.

Because the Technology Alerts are

cost-effective and timely to produce

(compared with awaiting the results 

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the short-term need of disseminating

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a timeframe that allows the rapid

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ultimately the saving of energy in the

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The information in the Technology

Alerts typically includes a description

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and field experience to date; a list of

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contact information. Attached 

appendixes provide supplemental 

information and example worksheets

on the technology.

FEMP sponsors publication of the

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the technology featured promises significant Federal-sector savings, the

Technology Alerts do not constitute

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provided by manufacturers. Nor do 

the Federal Technology Alerts attempt

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note the publication date on the back

cover, and consider the Alert as an

accurate picture of the technology and

its performance at the time of publication. Product innovations and the

entrance of new manufacturers or 

suppliers should be anticipated since

the date of publication. FEMP 

encourages interested Federal energy

and facility managers to contact the

manufacturers and other Federal sites

directly, and to use the worksheets in

the Technology Alerts to aid in their

purchasing decisions.

This report was sponsored by the United States Government. Neither the United States nor any agency or contractor thereof, nor any of

their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness,

or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned

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not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency or

contractor thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States

Government or any agency or contractor thereof.
For More Information

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General Contacts

Ted Collins

New Technology Demonstration Program 

Program Manager

Federal Energy Management Program

U.S. Department of Energy

1000 Independence Avenue, SW, EE-92

Washington, DC 20585

(202) 586-8017

Fax: (202) 586-3000

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P.O. Box 999, MSIN: K5-08

Richland, Washington 99352

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steven.parker@pnl.gov 

Produced for the U.S. Department

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National Laboratory 

April 1997