Chilled Water Plant Solutions: Thermosyphon Free-Cooling
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Certain facilities operate their chilled water network throughout the year due to continuous cooling demands in the winter. There are numerous chiller plant configurations that allow for winter chilled water production, each with its advantages and disadvantages. One configuration that allows for superior operating efficiency, reliable cooling operations, and low O&M costs is a flooded chiller equipped with thermosyphon free-cooling capabilities.
Types of Free-Cooling Operations
There exist numerous types of free-cooling operations that will create the required chilled water load. Each method has its advantages and disadvantages.
Winter Chiller Operation:
Throughout the winter months a chiller is operating to develop a chilled water load. This is the most inefficient and expensive method but is still utilized in the industry. The chiller will operate at low part-loads and potentially cycle often (worse on non-VSD compressors). Additional maintenance would be required due to longer operating hours. For water-cooled chiller plants, cooling towers will have to be winterized. The extended use of the cooling tower will result in added water consumption thus added chemical usage.
A Strainer Cycle:
A strainer cycle refers to a chilled water network which is directly open to the condenser water network. Since cooling is directly transferred this system is efficient however, there is a high risk of contamination and fouling in the chilled water system and cooling coils.
A strainer filter is utilized to minimize contamination, but fouling occurs frequently, which reduces efficiency and increase maintenance.
Evaporative Cooler with an Air-Cooled Chiller:
An evaporative cooler is installed in series with an air-cooled chiller. The evaporative cooler will deliver the winter cooling load and air-cooled chiller will replace the cooler once load cannot be satisfied. A disadvantage on an evaporative cooler is that it requires water to operate effectively. In addition, these coolers are also breeding grounds for mosquitoes and even legionella.
Dry-Cooler with an Air-Cooled Chiller:
A dry-cooler is a variation on the evaporative cooler design. The difference is that the dry-cooler does not utilize water to deliver the cooling load. Unlike the evaporative coolers where heat is transferred via latent and sensible heat, dry-coolers only transfer via sensible heat. This limits the operating range of the dry-cooler to ~45°F and is less efficient than an evaporative cooler.
Free-Cooling Heat Exchanger:
A plate and frame heat exchanger can be installed between the condenser water network and chilled water network. This configuration can achieve a high heat transfer without any network cross-contamination. The main disadvantage is that the heat exchanger must be maintained annually to maintain efficiency.
Thermosyphon Free-Cooling Chiller:
Thermosyphon is a process of passive heat exchange based on natural convection. In free-cooling operations, the air-cooled chiller will gain heat in the evaporator and transfer heat to the refrigerant. The liquid refrigerant will then gain heat causing the liquid to expand and change phase to a gaseous state. Convection and the phase change propel the heated refrigerant gas upwards to air-cooled condenser, bypassing the compressor. The condenser cools the refrigerant, condensing the refrigerant gas into a liquid. Gravity assists the cooled refrigerant to return to the evaporator, bypassing the expansion valve. Due to the low hydraulic resistance, the thermosyphon effect develops a substantial refrigerant flow. The flooded heat-exchanger type chillers are more efficient in free-cooling thermosyphon mode since there is more refrigerant to propel the cycle.
Mechanical Cooling Mode
Thermosyphon Cooling Mode
Thermosyphon chillers boast superior operating efficiency during the winter. The only operating component is the condenser fans, when needed. An advantage of this system is that there are less moving parts and equipment needed for the system to operate properly. In addition, due to the lack of an evaporative cooling tower/cooler, there is no water or chemical treatment consumption, resulting in additional savings. Legionella and cross-contamination is not an issue for this system. Due to the lack of a heat exchanger, maintenance costs are lower than other systems.
THERMOSYPHON FREE-COOLING EQUATES TO SUPERIOR PERFORMANCE
Airtron Canada has successfully implemented three projects utilizing Smardt chillers with flooded evaporators, oil-less centrifugal compressors, variable speed drives, and the thermosyphon Free-cooling options. Two projects involved packaged air-cooled chillers, while the latter was a split chiller plant with a remote air-cooled condensing unit. The greatest savings observed were from the split chiller plant, where the winter kW/Ton were superior and where the transition point from mechanical to thermosyphon was increased.
In order to verify performance, tests were performed on the split chiller plant. Meters were installed in order to trend performance. The following is the performance results from the split chiller plant equipped with a thermosyphon free-cooling kit. As shown below, the plant was successful in saving 38.5 kW at an outdoor temperature of 32°F, which is the operating upper limit of the thermosyphon mode. The graph depicts the observable operating performance ranges for the same chiller operating in thermosyphon mode and in standard mechanical cooling mode.
EXAMPLE PROJECT: CMHC: ENHANCED CHILLER PLANT DESIGN
Airtron Canada was retained to implement a complete redesign of the entire chiller plant the Canadian Mortgage and Housing Corporation (CMHC) national office in Ottawa.
Existing System Design:
CMHC is Canada’s authority on housing for more than 70 years. Their national office is the location of their main datacentre complex and offices. The facility was originally designed such that each datacentre was to be cooled by a Hiross CRAC unit, which is equipped with a dual coil design (DX coil/compressor package and glycol cooling coil).
Winter cooling was satisfied by the glycol cooling coil, which was cooled via the main building loop. This loop was satisfied by the main building’s aged rooftop air-cooled chillers. In the summer, the DX coil (utilizing R-22 refrigerant) cooled the CRAC unit with the aid of a compressor, and heat was transferred to the glycol loop. The glycol loop served as the condenser loop in summer time and was cooled via rooftop dry-coolers.
Enhanced Chiller Plant Design:
Rather than replacing aged equipment with more efficient systems, CMHC opted for a complete redesign of the cooling operations, in order to align to their global objectives. The glycol loop was drained and replaced with water, removing 4,625 gallons of glycol. The thirteen CRAC unit were properly disposed of their refrigerant (R-22) and related equipment. The new design involved that the cooling coil satisfy the CRAC cooling requirements. Two new Smardt chiller split-units were implemented including the Thermosyphon Free-cooling package and rooftop condenser unit. A heat exchanger was installed and connected to the main building cooling loop, in the event the chillers were to fail. The dry-coolers and air-cooled chillers were removed completely from the facility.
The project successfully generated significant energy savings, substantial maintenance savings, enhancing global redundancy, critical equipment asset renewal, and the complete elimination of glycol and of the OPD and GWP refrigerant R-22. The following are photos of a sample split-unit chiller project including Thermosyphon Free-Cooling.
WHY AIRTRON CANADA?
- We have over 50 years of experience providing Heating, Ventilation, and Air-Conditioning (HVAC) Services for commercial, industrial, and institutional buildings
- Turn-key Implementation & Commissioning of Energy Efficient Chiller Plants and Various Improvements
- Energy Incentive Applications and Procurement including Savings Measurement and Verification
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