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"Free" Cooling Using Water Economizers

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engineers newsletter volume 37–3 • providing insights for today’s hvac system designer "free" cooling using Standard 90.1 requirements Water Economizers The following sections of the Standard 90.1 prescriptive path are relevant for water economizers.[4] from the editor … Why economize? In past newsletters, we addressed the ability to accomplish "free" cooling by There are times of the year when a Section 6.5.1 Economizers. "Each artfully rearranging traditional cooling cooling system…" over the size equipment [1] and other ways to achieve system can use outdoor conditions to byproduct cooling virtually for free.[2] cool the building or process using the thresholds shown below "that has a fan We’ve also provided newsletters that standard cooling components to shall include either an air or water discussed air economizers and energy distribute its cooling effect. economizer." There are nine exceptions: code requirements.[3] small individual fan cooling units, In this EN, Susanna Hanson (Trane The most prevalent technique is an air spaces humidified above 35°F (2ºC) applications engineer) focuses on the economizer. When the temperature, or dew point for processes, systems with traditional methods of water-side enthalpy, of the outdoor air is low, condenser heat recovery, some economizing and associated energy code requirements. cooler outdoor air is used to reduce the residential applications, and systems temperature (or enthalpy) of air entering with minimal hours or cooling loads. the cooling coil. This can reduce or There is also an option to use higher eliminate mechanical cooling for much Let’s start with some definitions. efficiency equipment in lieu of an of the year in many climates. economizer for unitary system sizes Economize: V. Tr. To use or manage below 760 kBtu/h (63 tons). There is no with thrift: the need to economize Why water economize? Some high efficiency trade-off option for scarce energy resources. applications reduce or negate the applied systems. - economizer n. benefits of air economizers or simply The requirements vary by system size make them impractical. In these cases, and climate zone (Table 6.5.1 and The variety of ways to be thrifty with a water economizer can reduce Figure 1). HVAC energy explains why there are so compressor run hours and energy use. many components of equipment and Some examples include: systems called "economizers". For the Table 6.5.1 Minimum system size for which an economizer is required purposes of this discussion, we will • Minimum humidity requirements - focus on the water economizer as bringing in additional cold, dry Cooling capacity for outdoor air can increase which an economizer is defined by ASHRAE Standard 90.1- Climate zones required 2007 (Standard 90.1) section 3.2. humidification loads. 1a, 1b, 2a, 3a, 4a No economizer required • System limitations - some air Economizer, water: A system by handling units are easily equipped 2b, 5a, 6a, 7, 8 >135,000 Btu/h which the supply air of a cooling with economizers while others are 3b, 3c, 4b, 4c, 5b, 5c, 6b >65,000 Btu/h system is cooled indirectly with water not. This can be related to space, that is itself cooled by heat or mass distance to the outside air intake, transfer to the environment without increased return or exhaust fan Section 6.5.1.2.1 Design Capacity. the use of mechanical cooling. sizing, duct routing, or duct space. The section states that "water economizer systems shall be capable of • Dedicated outdoor air systems - a cooling supply air by indirect separate outdoor air unit is typically evaporation and providing up to 100% sized to serve only the ventilation of the expected system cooling load at loads. Another system to handle outdoor air temperatures of 50°F (10ºC) other building loads may be served dry bulb/45°F (7ºC) wet bulb and below." by chilled water. unless dehumidification loads cannot be met in which case, the water economizer system "must satisfy 100% of the expected system cooling load at 45°F dry bulb/40°F (4ºC) wet bulb." © 2008 Trane All rights reserved ● 1 Figure 1. Standard 90.1 regional economizer requirements based on cooling system capacity growth and corrosion. Maintaining this circuit involves frequently providing the same proper water treatment as the cooling tower. Economizer required if system > 135,000 Btu/h Evaporative cooler with air-cooled chiller. A variation on the water economizer for air-cooled chillers is to Economizer use an evaporative cooler in series with required if system the air-cooled chiller. The air-cooled > 65,000 Btu/h chiller will come on once the evaporative cooler can no longer handle the load by itself. Dry cooler with air-cooled chiller. Another variation for air-cooled chillers is a dry cooler, or closed-circuit cooling tower, in series with the air-cooled Expected building loads can vary chiller. widely for a given dry-bulb and wet- Types of water bulb combination (e.g. night versus economizers Because of their high approach day, occupied versus unoccupied), so temperature, dry coolers may not be this requirement is somewhat unclear. Strainer cycle. In this system, the able to meet the requirement of 100% A reasonable assumption is that the condenser and chilled-water systems of the cooling load at either 50ºF/45ºF intent of the Standard is 100% of the are connected. When the outdoor wet- or 45ºF/40ºF (see previous discussion normal daytime operation cooling load. bulb temperature is low enough, cold of Section 6.5.1.2.1). water from the cooling tower is routed Section 6.5.1.2.2 Maximum Pressure directly into the chilled-water loop. Plate-and-frame heat exchanger. In Drop. "Precooling coils and water-to- this type of water economizer, the water heat exchangers used as part of Although the strainer cycle is the most water from the cooling tower is kept a water economizer system shall either efficient water economizer option, it separate from the chilled-water loop by have a water-side pressure drop of less greatly increases the risk of fouling in a plate-and-frame heat exchanger. This than 15 ft of water or a secondary loop the chilled-water system and cooling is a popular configuration because it shall be created so that the coil or heat coils with the same type of can achieve high heat-transfer exchanger pressure drop is not seen by contamination that is common in open- efficiency without cross- the circulating pumps when the cooling-tower systems. A strainer or contamination. With the addition of a system is in the normal cooling filter can be used to minimize this second condenser-water pump and (noneconomizer) mode." contamination, but the potential for proper piping modifications, this heat fouling prevents widespread use of the exchanger can operate simultaneously A sidestream application (see Figure 3) strainer-cycle system. with the chiller. As much heat as has no maximum pressure drop, since possible is rejected through the heat it is not seen by the circulating pumps Indirect evaporative precooling exchanger while the chiller handles the when in normal cooling mode. coil. For water-cooled, self-contained rest of the cooling load. direct-expansion (DX) systems (like Section 6.5.1.3 Integrated floor-by-floor systems), a precooling Plate-and-frame heat exchangers Economizer Control. "Economizer coil upstream of the refrigerant coil is isolate the building loop from the water systems shall be integrated with the supplied with cold water from the in the open cooling tower loop, but mechanical cooling system and be cooling tower. This indirect evaporative they must be cleaned, typically capable of providing partial cooling precooling-coil circuit is similar to the annually. The labor and parts for even when additional mechanical strainer cycle because it interfaces an cleaning and reassemby (e.g. cooling is required to meet the open circuit directly with the air gasketing) is an expense that should be remainder of the cooling load." delivered to the zones. factored into the life-cycle cost of this option. This is only required in climate zones Indirect evaporative cooling-coil 3b, 3c, 4b, 4c, and 5c. However, circuits, connected directly to an open integrated control is worth considering cooling tower, must be maintained even in cases where it is not required. properly to minimize fouling, microbial 2 ● Trane Engineers Newsletter volume 37–3 providing insights for today’s HVAC system designer Free-cooling chiller. Another method fewer pipes, pumps and fittings and no The water economizer must either of "free" cooling is to transfer heat heat exchanger are required. create the leaving chilled-water between the cooling tower water and setpoint, or the plant must mix colder- the chilled water inside a centrifugal If the free-cooling chiller is part of a than-required water from the operating chiller through the use of refrigerant traditional parallel chiller lineup, chiller with warmer water from the migration, also known as a simultaneous operation is not possible economizer to satisfy the chilled-water thermosiphon. When the temperature without blending leaving chilled-water setpoint. This is seen as undesirable as of the water from the cooling tower is temperatures. It is common for it may use as much energy as it saves. colder than the desired chilled-water designers to embrace a heat exchanger The chiller uses 1.0 to 2.5% more temperature, the compressor is turned in series with a chiller but less energy per degree of colder water off and automatic shutoff valves inside common to embrace chillers in series. created. In this case, since the the chiller refrigerant circuit are If simultaneous operation is desired, temperature is then raised by the opened. Because refrigerant vapor the free-cooling chiller should be warmer water from the economizer, no migrates to the area with the lowest installed in the same location as the pump energy savings are available to temperature, refrigerant boils in the heat exchanger in Figure 3.
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