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  4. 3.8 HVAC Primary Systems

3.8.4 Fluid Economizers

Baseline Building Summary. None of the baseline building systems use a water-side economizer.

Water-Side Economizer Name
Applicability All water-side economizers
Definition The name of a water-side economizer for a cooling system
Units Text, unique
Input Restrictions Descriptive reference to the construction documents. The default is no water-side economizer.
Baseline Rules No water economizer
Water Economizer Type
Applicability All water-side economizers
Definition The type of water-side economizer. Choices include:
  • None
  • Heat exchanger in parallel with chillers. This would be used with an open cooling tower is often referred to as a non-integrated economizer, because the chillers are locked out when the plant is in economizer mode.
  • Heat exchanger in series with chillers. This would be used with an open cooling tower and is often referred to as an integrated, because the chillers can operate simultaneously with water economizer operation.
  • Direct water economizer. This would be used with a closed cooling tower. In this case, a heat exchanger is not needed. This type works only as a non-integrated economizer.
  • Thermo-cycle (also known as refrigerant migration). With thermo-cycle, bypass valves allow for the flow to vapor refrigerant to the condenser and allow gravity flow of liquid refrigerant to the evaporator without use of the compressor.  Only some chillers have this capability and capacity may be limited under this mode.  There is no additional piping; the cooler water from the tower is brought directly to the chiller(s) and the chiller(s) respond by shutting down the compressor and relying on thermal forces to drive the refrigerant. This method is also known as “thermosiphon” since thermal gradients passively move refrigerant between the evaporator and condenser.
Units List (see above)
Input Restrictions As designed
Baseline Rules No water economizer
Water-Side Economizer HX Effectiveness
Applicability Water-side economizers with an open cooling tower

Definition

The effectiveness of a water-side heat exchanger at design conditions. This is defined as:

(6.8.4-1)

$$ W\!SE_{eff} = \frac{t_{ea}-t_{la}}{t_{ea}-t_{ew}}$$where

WSEeff The effectiveness of the water-side economizer coil
tea The entering coil air dry-bulb temperature (°F)
tla The leaving coil air dry-bulb temperature (°F)
tea The entering coil water temperature (°F)
Units Ratio
Input Restrictions As designed. The default is 60 %.
Baseline Rules No water economizer
Water-Side Economizer Maximum Tdb
Applicability All water-side economizers
Definition The control temperature (outside air dry-bulb temperature) above which the water-side economizer is disabled.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed. Defaults to 65°F.
Baseline Rules No water economizer
Water-Side Economizer Maximum CWS
Applicability All water-side economizers
Definition The control temperature (condenser water supply temperature) above which the water-side economizer is disabled.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed. Defaults to 50°F.
Baseline Rules No water economizer
Water-Side Economizer CWS Setpoint
Applicability All water-side economizers
Definition The design condenser water supply temperature for the cooling tower in economizer mode.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed. Defaults to 45°F or 40°F for “Thermo Cycle.”
Baseline Rules No water economizer
Water-Side Economizer Availability Schedule
Applicability All water-side economizers
Definition A schedule which represents the availability of the water-side economizer
Units Data structure: schedule, on/off
Input Restrictions As designed
Baseline Rules No water economizer
Water-Side Economizer Auxiliary kW
Applicability Water-side economizers with an open tower
Definition The electrical input (pumps and auxiliaries) for a dedicated pump for the chilled water side of the heat exchanger. This power is in excess of the condenser water pumps and cooling tower fans for the system during water-side economizer operation.
Units KW or kW/ton
Input Restrictions As designed
Baseline Rules No water economizer

Baseline Building Summary. None of the baseline building systems use a water-side economizer. Additional inputs to those below may be required to simulate actual control sequences.

 

Water-Side Economizer Name

Applicability

All water-side economizers

Definition

The name of a water-side economizer for a cooling system

Units

Text, unique

Input Restrictions

Descriptive reference to the construction documents. The default is no water-side economizer.

Baseline Rules

No water economizer

 

Water Economizer Type

Applicability

All water-side economizers

Definition

The type of water-side economizer. Choices include:

  • None
  • Heat exchanger in parallel with chillers. This would be used with an open cooling tower is often referred to as a non-integrated economizer, because the chillers are locked out when the plant is in economizer mode.
  • Heat exchanger in series with chillers. This would be used with an open cooling tower and is often referred to as an integrated, because the chillers can operate simultaneously with water economizer operation.
  • Direct water economizer. This would be used with a closed cooling tower. In this case, a heat exchanger is not needed. This type works only as a non-integrated economizer.
  • Thermo-cycle (also known as refrigerant migration). With thermo-cycle, bypass valves allow for the flow to vapor refrigerant to the condenser and allow gravity flow of liquid refrigerant to the evaporator without use of the compressor.  Only some chillers have this capability and capacity may be limited under this mode.  There is no additional piping; the cooler water from the tower is brought directly to the chiller(s) and the chiller(s) respond by shutting down the compressor and relying on thermal forces to drive the refrigerant. This method is also known as “thermosiphon” since thermal gradients passively move refrigerant between the evaporator and condenser.

Units

List (see above)

Input Restrictions

As designed

Baseline Rules

No water economizer

 

Water-Side Economizer HX Effectiveness

Applicability

Water-side economizers with an open cooling tower

Definition

The effectiveness of a water-side heat exchanger at design conditions. This is defined as:

(Equation 3.8.4-1)

$$WSE_{eff} = \frac{t_{ea} - t_{la}} {t_{ea}-t_{ew}} $$

Where

WSEeff

The effectiveness of the water-side economizer coil

tea

The entering coil air dry-bulb temperature (°F)

tla

The leaving coil air dry-bulb temperature (°F)

tea

The entering coil water temperature (°F)

Units

Ratio

Input Restrictions

As designed. The default is 60 %.

Baseline Rules

No water economizer

 

Water-Side Economizer Maximum Tdb

Applicability

All water-side economizers

Definition

The control temperature (outside air dry-bulb temperature) above which the water-side economizer is disabled.

Units

Degrees Fahrenheit (°F)

Input Restrictions

As designed. Defaults to 65 °F.

Baseline Rules

No water economizer

 

Water-Side Economizer Maximum CWS

Applicability

All water-side economizers

Definition

The control temperature (condenser water supply temperature) above which the water-side economizer is disabled.

Units

Degrees Fahrenheit (°F)

Input Restrictions

As designed. Defaults to 50 °F.

Baseline Rules

No water economizer

 

Water-Side Economizer CWS Setpoint

Applicability

All water-side economizers

Definition

The design condenser water supply temperature for the cooling tower in economizer mode.

Units

Degrees Fahrenheit (°F)

Input Restrictions

As designed. Defaults to 45°F or 40°F for “Thermo Cycle.” If a strainer cycle is specified, changeover temperature from economizer to mechanical cooling shall be set at 50°F.

Baseline Rules

No water economizer

 

Water-Side Economizer Availability Schedule

Applicability

All water-side economizers

Definition

A schedule which represents the availability of the water-side economizer

Units

Data structure: schedule, on/off

Input Restrictions

As designed

Baseline Rules

No water economizer

 

Water-Side Economizer Auxiliary kW

Applicability

Water-side economizers with an open tower

Definition

The electrical input (pumps and auxiliaries) for a dedicated pump for the chilled water side of the heat exchanger. This power is in excess of the condenser water pumps and cooling tower fans for the system during water-side economizer operation.

Units

KW or kW/ton

Input Restrictions

As designed

Baseline Rules

No water economizer
90.1-2019

Baseline Building Summary:

Baseline building system 11 would include an integrated fluid economizer, meeting the requirements as specified in this section, also in Section 6.5.1.2 of Standard 90.1-2019. Additional inputs to those below may be required to simulate actual control sequences.

Fluid Economizer Name   
Applicability All fluid economizers
Definition The name of a fluid economizer for a cooling system
Units Text, unique
Input Restrictions As designed
Baseline Building A water side fluid economizer will be modeled for baseline system 11, for computer rooms

 

 

Water Economizer Type  
Applicability All fluid economizers
Definition

The type of fluid economizer. Choices include:

· None

· Heat exchanger in parallel with chillers: This would be used with an open cooling tower combined with a heat exchanger or evaporative cooler (closed circuit cooling tower) and is a non-integrated economizer, because the chillers are locked out when the plant is in economizer mode.

· Heat exchanger in series with chillers: This would be used with an open cooling tower and heat exchanger or evaporative cooler (closed circuit cooling tower) and is integrated because the piping is arranged so the chilled water return is precooled and chillers can operate simultaneously with water economizer operation. Depending on the proportion of water economizer capacity compared to chiller capacity, the water economizer heat exchanger may see the full chilled water flow, or be in a “sidecar” arrangement where only a portion of the chilled water flow goes through the heat exchanger.

· Direct water economizer: In this system, the condenser and chilled-water systems are connected. When the outdoor wet bulb temperature is low enough, cold water from the cooling tower is routed directly into the chilled-water loop. This would be used with filtration of the condenser water. In this case, a heat exchanger is not needed. This type can work as either an integrated or a non-integrated economizer, depending on piping arrangement. Although the strainer cycle is the most efficient water economizer option, it greatly increases the risk of fouling in the chilled-water system and cooling coils with the same type of contamination that is common in open cooling-tower systems. A strainer or filter can be used to minimize this contamination, but the potential for fouling prevents widespread use of the strainer-cycle system (Trane, 2016).
Units List (see above)
Input Restrictions

As designed
Baseline Building

The baseline water side economizer should be a ‘heat exchanger in series with chillers’.

It shall be modeled for HVAC system 11 that serve computer rooms. The baseline system will be modeled with a heat exchanger in series with the chiller that pre-cools the chilled water return. The flow through the heat exchanger shall match the required water economizer capacity. The fluid economizer shall be capable of providing up to 100% of the expected system cooling load at outdoor air temperatures listed in Table 93.

Table 93. Fluid Economizer Sizing Dry-Bulb and Wet-Bulb Requirements for Computer Rooms

Climate Zone Dry Bulb °F Wet Bulb °F
0A NR NR
0B NR NR
1A NR NR
1B NR NR
2A 40 35
2B 35 30
3A 40 35
3B 30 25
3C 30 25
4A 40 35
4B 30 25
4C 30 25
5A 40 35
5B 30 25
5C 30 25

6A

 35

30
6B 30 25
7 30 25
8 30 25

 

Fluid Economizer Approach   
Applicability All fluid economizers
Definition The design temperature difference between the chilled water temperature leaving the heat exchanger and the condenser water (tower leaving) inlet to the heat exchanger.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed. Defaults to 2°F.
Baseline Building This will be specified as 2°F for the baseline building.

 

Fluid Economizer Activation Temp. Difference   
Applicability All fluid economizers
Definition The minimum temperature difference between the tower leaving temperature and the chilled water return below which the fluid economizer is disabled.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed. Defaults to 3.5°F below the chilled water return temperature.
Baseline Building This will be specified as 3.5°F below the chilled water return temperature for the baseline building.

 

Fluid Economizer Tower Leaving Temp. Setpoint  
Applicability All fluid economizers
Definition The temperature setpoint for the water side economizer heat exchanger entering temperature (tower leaving temperature).
Units Degrees Fahrenheit (°F)
Input Restrictions As designed.
Baseline Building This will be the 2°F below the chilled water supply temperature for the baseline building.

 

Fluid Economizer Availability Schedule   
Applicability All fluid economizers
Definition A schedule that represents the availability of the fluid economizer
Units Data structure: schedule, on/off
Input Restrictions As designed.
Baseline Building The waterside economizer will be simulated to be ‘Always On’ for the baseline building.

 

Fluid Economizer Hydronic Pressure Drop  
Applicability All fluid economizers
Definition Pressure drop of the pre-cooling coils of the fluid to water heat exchanger.
Units ft of water
Input Restrictions As designed
Baseline Building Refer to Section 3.7.5 for baseline system hydronic pressure drop requirements.

Baseline Building Summary. None of the baseline building systems use a water-side economizer.

Water-Side Economizer Name
Applicability All water-side economizers
Definition The name of a water-side economizer for a cooling system
Units Text, unique
Input Restrictions Descriptive reference to the construction documents. The default is no water-side economizer.
Baseline Rules No water economizer
Water Economizer Type
Applicability All water-side economizers
Definition The type of water-side economizer. Choices include:
  • None
  • Heat exchanger in parallel with chillers. This would be used with an open cooling tower is often referred to as a non-integrated economizer, because the chillers are locked out when the plant is in economizer mode.
  • Heat exchanger in series with chillers. This would be used with an open cooling tower and is often referred to as an integrated, because the chillers can operate simultaneously with water economizer operation.
  • Direct water economizer. This would be used with a closed cooling tower. In this case, a heat exchanger is not needed. This type works only as a non-integrated economizer.
  • Thermo-cycle (also known as refrigerant migration). With thermo-cycle, bypass valves allow for the flow to vapor refrigerant to the condenser and allow gravity flow of liquid refrigerant to the evaporator without use of the compressor.  Only some chillers have this capability and capacity may be limited under this mode.  There is no additional piping; the cooler water from the tower is brought directly to the chiller(s) and the chiller(s) respond by shutting down the compressor and relying on thermal forces to drive the refrigerant. This method is also known as “thermosiphon” since thermal gradients passively move refrigerant between the evaporator and condenser.
Units List (see above)
Input Restrictions As designed
Water-Side Economizer HX Effectiveness
Applicability Water-side economizers with an open cooling tower

Definition

The effectiveness of a water-side heat exchanger at design conditions. This is defined as:

(6.8.4-1)

$$ W\!SE_{eff} = \frac{t_{ea}-t_{la}}{t_{ea}-t_{ew}}$$where

WSEeff The effectiveness of the water-side economizer coil
tea The entering coil air dry-bulb temperature (°F)
tla The leaving coil air dry-bulb temperature (°F)
tea The entering coil water temperature (°F)
Units Ratio
Input Restrictions As designed. The default is 60 %.
Water-Side Economizer Maximum Tdb
Applicability All water-side economizers
Definition The control temperature (outside air dry-bulb temperature) above which the water-side economizer is disabled.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed. Defaults to 65°F.
Water-Side Economizer Maximum CWS
Applicability All water-side economizers
Definition The control temperature (condenser water supply temperature) above which the water-side economizer is disabled.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed. Defaults to 50°F.
Water-Side Economizer CWS Setpoint
Applicability All water-side economizers
Definition The design condenser water supply temperature for the cooling tower in economizer mode.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed. Defaults to 45°F or 40°F for “Thermo Cycle.”
Water-Side Economizer Availability Schedule
Applicability All water-side economizers
Definition A schedule which represents the availability of the water-side economizer
Units Data structure: schedule, on/off
Input Restrictions As designed
Water-Side Economizer Auxiliary kW
Applicability Water-side economizers with an open tower
Definition The electrical input (pumps and auxiliaries) for a dedicated pump for the chilled water side of the heat exchanger. This power is in excess of the condenser water pumps and cooling tower fans for the system during water-side economizer operation.
Units KW or kW/ton
Input Restrictions As designed

Baseline Building Summary. None of the baseline building systems use a water-side economizer.

Water-Side Economizer Name
Applicability All water-side economizers
Definition The name of a water-side economizer for a cooling system
Units Text, unique
Input Restrictions Descriptive reference to the construction documents. The default is no water-side economizer.
Baseline Rules No water economizer
Water Economizer Type
Applicability All water-side economizers
Definition The type of water-side economizer. Choices include:
  • None
  • Heat exchanger in parallel with chillers. This would be used with an open cooling tower is often referred to as a non-integrated economizer, because the chillers are locked out when the plant is in economizer mode.
  • Heat exchanger in series with chillers. This would be used with an open cooling tower and is often referred to as an integrated, because the chillers can operate simultaneously with water economizer operation.
  • Direct water economizer. This would be used with a closed cooling tower. In this case, a heat exchanger is not needed. This type works only as a non-integrated economizer.
  • Thermo-cycle (also known as refrigerant migration). With thermo-cycle, bypass valves allow for the flow to vapor refrigerant to the condenser and allow gravity flow of liquid refrigerant to the evaporator without use of the compressor.  Only some chillers have this capability and capacity may be limited under this mode.  There is no additional piping; the cooler water from the tower is brought directly to the chiller(s) and the chiller(s) respond by shutting down the compressor and relying on thermal forces to drive the refrigerant. This method is also known as “thermosiphon” since thermal gradients passively move refrigerant between the evaporator and condenser.
Units List (see above)
Input Restrictions As designed
Water-Side Economizer HX Effectiveness
Applicability Water-side economizers with an open cooling tower

Definition

The effectiveness of a water-side heat exchanger at design conditions. This is defined as:

(6.8.4-1)

$$ W\!SE_{eff} = \frac{t_{ea}-t_{la}}{t_{ea}-t_{ew}}$$where

WSEeff The effectiveness of the water-side economizer coil
tea The entering coil air dry-bulb temperature (°F)
tla The leaving coil air dry-bulb temperature (°F)
tea The entering coil water temperature (°F)
Units Ratio
Input Restrictions As designed. The default is 60 %.
Water-Side Economizer Maximum Tdb
Applicability All water-side economizers
Definition The control temperature (outside air dry-bulb temperature) above which the water-side economizer is disabled.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed. Defaults to 65°F.
Water-Side Economizer Maximum CWS
Applicability All water-side economizers
Definition The control temperature (condenser water supply temperature) above which the water-side economizer is disabled.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed. Defaults to 50°F.
Water-Side Economizer CWS Setpoint
Applicability All water-side economizers
Definition The design condenser water supply temperature for the cooling tower in economizer mode.
Units Degrees Fahrenheit (°F)
Input Restrictions As designed. Defaults to 45°F or 40°F for “Thermo Cycle.”
Water-Side Economizer Availability Schedule
Applicability All water-side economizers
Definition A schedule which represents the availability of the water-side economizer
Units Data structure: schedule, on/off
Input Restrictions As designed
Water-Side Economizer Auxiliary kW
Applicability Water-side economizers with an open tower
Definition The electrical input (pumps and auxiliaries) for a dedicated pump for the chilled water side of the heat exchanger. This power is in excess of the condenser water pumps and cooling tower fans for the system during water-side economizer operation.
Units KW or kW/ton
Input Restrictions As designed