3.7.7.2 Desiccant

90.1-2019

Desiccant Type
Applicability	Systems with desiccant dehumidification
Definition	Describes the configuration of desiccant cooling equipment The following configurations for desiccant systems are allowed: A liquid desiccant dehumidifying unit A liquid desiccant dehumidifying unit combined with a gas-fired absorption chiller A solid desiccant dehumidifying unit No desiccant – the default, which indicates that no desiccant system is present
Units	List (see above)
Input Restrictions	As designed
Baseline Building	Not applicable

Desiccant Control Mode
Applicability	Systems with desiccant dehumidification
Definition	The method of controlling the operation of the desiccant unit. For liquid-based systems this can be either: Dry-bulb: The desiccant unit is turned on whenever the outside air dry-bulb exceeds a set limit. Evaporative cooling: Cycles the desiccant unit on when an evaporative cooler is on to maintain a dewpoint setpoint. Dewpoint: Cycles the desiccant unit on and off to maintain the dewpoint temperature of the supply air. For solid-based systems the following configurations are possible: Dehumidification only: The desiccant unit cycles on and off to maintain indoor humidity levels. Sensible heat exchanger plus regeneration: The desiccant unit includes a sensible heat exchanger to precool the hot, dry air leaving the desiccant unit. The air leaving the exhaust side of the heat exchanger is directed to the desiccant unit. Sensible heat exchanger: The desiccant unit includes a heat exchanger, but the air leaving the exhaust side of the heat exchanger is exhausted to the outdoors.
Units	List (see above)
Input Restrictions	As designed
Baseline Building	Not applicable

Desiccant Air Fraction
Applicability	Systems with desiccant dehumidification
Definition	The fraction of the supply air that passes through the desiccant unit. Typically either the minimum outside air fraction or all of the air passes through the desiccant system.
Units	Fraction (between 0 and 1)
Input Restrictions	As designed
Baseline Building	Not applicable

Desiccant Heat Source
Applicability	Systems with desiccant dehumidification
Definition	The source of heat that is used to dry out the desiccant. This can be either: Gas – hydronic: The regeneration heat load is met with a gas-fired heater. Hot water: The heat load is met with hot water from the plant.
Units	List (see above)
Input Restrictions	As designed
Baseline Building	Not applicable

Liquid Desiccant Performance Curves

Applicability

Systems with liquid-based desiccant dehumidification

Definition

A set of performance curves apply to liquid desiccant systems:

$$DESC\_T\_FTW=a+b\times T +c\times T^2 + d \times w + e \times w^2 +f\times T \times w$$

(44)

$$DESC\_W\_FTW=a+b\times T +c\times T^2 + d \times w + e \times w^2 +f\times T \times w$$

(45)

$$DESC\_Gas\_FTW=a+b\times T +c\times T^2 + d \times w + e \times w^2 +f\times T \times w$$

(46)

$$DESC\_kW\_FTW=a+b\times T +c\times T^2 + d \times w + e \times w^2 +f\times T \times w$$

(47)

Where:

DESC-T-FTW = Temperature leaving desiccant unit

DESC-W-FTW = Humidity ratio leaving desiccant unit

DESC-Gas-FTW = Gas usage of desiccant unit

DESC-kW-FTW = Electric usage of desiccant unit

T = Entering air temperature

w = Entering humidity ratio

Units

Data structure

Input Restrictions

As designed, default to values in Table 78

Baseline Building

Not applicable

Desiccant Dewpoint Temperature Setpoint
Applicability	Systems with desiccant dehumidification
Definition	The setpoint dewpoint temperature of the air leaving the desiccant system
Units	Degrees Fahrenheit (°F)
Input Restrictions	As designed. Defaults to 50°F.
Baseline Building	Not applicable

Desiccant Heat Exchanger Effectiveness
Applicability	Systems with desiccant dehumidification
Definition	The effectiveness of a sensible heat exchanger used with a desiccant system
Units	Fraction (between 0 and 1)
Input Restrictions	As designed
Baseline Building	Not applicable

Desiccant Heat Exchanger Pressure Drop
Applicability	Systems with desiccant dehumidification
Definition	The pressure drop across a sensible heat exchanger used with a desiccant system
Units	in. H₂O
Input Restrictions	As designed. Defaults to 1.0 in. H₂O.
Baseline Building	Not applicable