3.8.4 Fluid Economizers

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).

As designed

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

Applicability

All fluid economizers

Definition

The name of a fluid economizer for a cooling system

Units

Text, unique

Input Restrictions

As designed

Baseline Building

G3.2 New Construction/Major Alterations

A water side fluid economizer will be modeled for baseline system 11, for computer rooms.

G3.3 Minor Alterations

Economizers shall be modeled as minimally compliant with Standard 90.1-2022 Section 6.5.1 with the same type as the proposed. Where the proposed does not include an economizer, it is recommended that an air economizer be modeled in the baseline. However, according to Standard 90.1 Section 6.5.1.5, when applicable based on the scope of the retrofit and the language in Standard 90.1 Section 6.1.4, if the proposed design includes hydronic cooling and humidification systems designed to maintain inside humidity at a dew-point temperature greater than 35°F, then a fluid economizer is specifically required to be modeled in the baseline to meet 90.1-2022 Section 6.5.1.

Exceptions to 90.1 Section 6.5.1:

1. Individual fan-cooling units with a supply capacity less than the minimum listed in 90.1 Table 6.5.1-1
2. Chilled-water cooling systems without a fan or that use induced airflow, where the total capacity of these systems is less than 1,000,000 Btu/h in Climate Zones 0, 1B, and 2 through 4; less than 1,400,000 Btu/h in Climate Zones 5 through 8; or any size in Climate Zone 1A.
3. Systems that include nonparticulate air treatment as required by Standard 62.1, Section 6.2.1.
4. In hospitals and ambulatory surgery centers, where more than 75% of the air designed to be supplied by the system is to spaces that are required to be humidified above 35°F dew-point temperature to comply with applicable codes or accreditation standards; in all other buildings, where more than 25% of the air designed to be supplied by the system is to spaces that are designed to be humidified above 35°F dew-point temperature to satisfy process application needs. This exception does not apply to computer rooms.
5. Systems that include a condenser heat recovery system with a minimum capacity as defined in 90.1 Section 6.5.6.2.2.
6. Systems that serve residential spaces where the system capacity is less than five times the requirement listed in 90.1 Table 6.5.1-1.
7. Systems that serve spaces whose sensible cooling load at design conditions, excluding transmission less than or equal to transmission losses at an outdoor temperature of 60°F.
8. Systems expected to operate fewer than 20 hours per week.
9. Where the use of outdoor air for cooling will affect supermarket open refrigerated casework systems.
10. For comfort cooling, where the cooling efficiency meets or exceeds the efficiency improvement requirements in 90.1 Table 6.5.1-2.

11. Systems primarily serving computer rooms where<\p>

    a. the total design cooling load of all computer rooms in the building is less than 3,000,000 Btu/h and the building in which they are located is not served by a centralized chilled-water plant;

    b. the room total design cooling load is less than 600,000 Btu/h and the building in which they are located is served by a centralized chilled-water plant;

    c. the local water authority does not allow cooling towers; or

    d. less than 600,000 Btu/h of computer-room cooling equipment capacity is being added to an existing building.

12. Dedicated systems for computer rooms, where a minimum of 75% of the design load serves<\p>

    a. those spaces classified as an essential facility,

    b. those spaces having a design of Tier IV as defined by ANSI/TIA-942,

    c. those spaces classified under NFPA 70 Article 708—Critical Operations Power Systems (COPS)

    d. those spaces where core clearing and settlement services are performed such that their failure to settle pending financial transactions could present systemic risk as described in “The Interagency Paper on Sound Practices to Strengthen the Resilience of the U.S. Financial System” (April 7, 2003).

Exception: the baseline and proposed should be modeled identically if, based on the requirements of 90.1-2022 Section 6.1.4 and the scope of the alteration, 90.1-2022 Section 6.5.1 requirements are inapplicable.

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

G3.2 New Construction/Major Alterations

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 117.

Table 117. Fluid Economizer Sizing Dry-Bulb and Wet-Bulb Requirements for Computer Rooms

G3.3 Minor Alterations

Same as proposed. If there is no fluid economizer in the proposed and it is required to be modeled in the baseline, then the baseline water side economizer is recommended to be modeled as a ‘heat exchanger in series with chillers’.

If it is required by Standard 90.1-2022 Section 6.1.4, based on the scope of the alteration, to meet Standard 90.1-2022 Section 6.5.1.2.1, then the fluid economizer shall be capable of providing up to 100% of the expected system cooling load at outdoor air temperatures listed in Table 118.

Table 118. Fluid Economizer Sizing Dry-Bulb and Wet-Bulb Requirements for Computer Rooms

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

G3.2 New Construction/Major Alterations

This will be specified as 2°F for the baseline building.

G3.3 Minor Alterations

Same as proposed. If there is no fluid economizer in the proposed and it is required to be modeled in the baseline, then use 2°F for the baseline building.

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

G3.2 New Construction/Major Alterations

This will be specified as 3.5°F below the chilled water return temperature for the baseline building.

G3.3 Minor Alterations

Same as proposed. If there is no fluid economizer in the proposed and it is required to be modeled in the baseline, then use 3.5°F below the chilled water return temperature for the baseline building.

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

G3.2 New Construction/Major Alterations

This will be the 2°F below the chilled water supply temperature for the baseline building.

G3.3 Minor Alterations

Same as proposed. If there is no fluid economizer in the proposed and it is required to be modeled in the baseline, then use 2°F below the chilled water supply temperature for the baseline building.

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

G3.2 New Construction/Major Alterations

The waterside economizer will be simulated to be ‘Always On’ for the baseline building.

G3.3 Minor Alterations

The availability schedule in conjunction with controls must be modeled such that the requirements of Standard 90.1-2022 Sections 6.5.1.3 and 6.5.1.4 are met in the baseline.

The requirements of 90.1-2022 Section 6.5.1.3 make it such that the economizer should be modeled such that it is integrated with the mechanical cooling systems and such that it is able to provide partial cooling even when additional mechanical cooling is required to meet the remainder of the cooling load.

The requirements of 90.1-2022 Section 6.5.1.4 require that the economizer controls shall be such that economizer operation does not increase the building heating energy use during normal operation.

Exception to 6.5.1.4: Economizers on VAV systems that cause zone-level heating to increase due to a reduction in supply air temperature.

Exception: the baseline and proposed should be modeled identically if, based on the requirements of 90.1-2022 Section 6.1.4 and the scope of the alteration, 90.1-2022 Section 6.5.1.3 and 6.5.1.4 requirements are inapplicable.

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 3.8.5 for baseline system hydronic pressure drop requirements.