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In this two-part blog series, we will take a deep dive into the world of hydraulic circuit requirements. The first blog, presented here, looks at the minimum water volume, how to protect your circuit, installation methods, techniques, tips and tricks, and finally, considering multiple machines.
Our UK Cooling and Heating Product Manager and local expert Stefan Lay, has provided information on key points before discussing optimisation and efficiency in our next blog post.
Minimum Water Volume
Chillers and heat pumps require a minimum amount of water to operate correctly. Each refrigerant circuit is powered by one or more compressors which has safety tolerances to consider, such as its minimum run time. When a compressor starts it must run for a minimum amount of time (it must also remain off for a prescribed period) in the interest of proper lubrication and by limiting the amount of starts per hour for maximum durability. A good amount of water ensures that this can be achieved and promotes temperature stability around setpoint through increased inertia. Think of a cup of water vs a large tank of water and a compressor able to produce 25kW of cooling at minimum load. The compressor starts and must run for 3 mins. We over-cool the cup of water due to its volume vs 25kW energy available for 3 mins and potentially freeze the water. This scenario results in unachievable setpoint stability during low load operation. With a tank of water, there is enough inertia in the system to allow the compressor to work outside of this minimum run time and into normal regulation with no risk of over shooting setpoints. This is one reason why it is critical to properly understand not only your peak demands but also your minimum and contemporary demands when sizing such equipment.
With an ASHP, the same considerations exist for minimum run times but conversely, high temperatures are experienced opposed to freezing. In addition, we must also factor in the impact of defrosting the air coils. During a defrost cycle the machine instead produces cooler water at its outlet where a good amount of water limits temperature deviation, promoting system stability and efficiency. Remember the cup of water vs a large tank of water, let’s assume they hold heated water… Operating conditions trigger defrost mode so the machine starts to produce cooler water at its outlet. The temperature drops quickly in the cup of water due to its volume resulting in a big deviation from the starting condition/setpoint and consequently, more work is involved to regain setpoint. In the tank of water, this temperature reduction is limited due to its increased volume/inertia and therefore less work is required to get back to setpoint and there is less of a deviation from setpoint.
Protection
Hydronic Chillers and Heat Pumps commonly use either a plate heat exchanger or a shell and tube heat exchanger on the user water side pending the capacity of the machine. In both cases it is important to ensure that the heat exchangers are protected from debris in the water circuit, which can affect performance due to reduced heat exchange capabilities and moreover in the case of plate HXs, cause blockages which can lead to catastrophic failure in terms of cooling operation where local freezing can rupture internal plates. This is considered mandatory to uphold machine warranty where a <=1mm mesh strainer must be used at the inlet of each heat exchanger. Water quality also plays a big part in system performance and durability so make sure water circuits are cleaned and properly dosed where required. It is also important to consider adding glycol to such water circuits to protect them from freezing where recommendation to include this should be considered if you are using the equipment to produce a temperature of less than 5°C or if you intended to operate the equipment in an ambient condition less than 5°C.
Installation
In addition to actually working as intended, best practice dictates that the physical pipework installation should aid in easy maintenance including future optimisation/fault-finding endeavours. But how are you meant to understand what is happening if you are unable to measure operation? Ensure that there are suitable points around the system to be able to take relevant pressure, temperature or water flow readings. Gauges and thermometers on pipes although functional probably for their first years, soon become untrustworthy or fail completely through being exposed to the elements. Test points should be included at the machine inlet and outlet at minimum so that engineers can use calibrated equipment when monitoring operation. Ensure that there is a way to measure water flow through the machine, maybe through a commissioning plate or DRV if used, for example. Ensure that the water strainer can be easily isolated for maintenance purposes instead of having to drain down large portions of the system for a simple inspection. Especially important if glycol or expensive dosing chemicals are in use. Verify the installation requirements of the external flow switch (if included), as it may require space of at least 5 x pipe diameters from any other component or bend in the system to ensure good operation. You may need to consider an installation point for the flow switch outside of the predefined ATEX zone when using A3 refrigerant equipment. An “as built” water circuit schematic should be readily available for review in the event of a problem, as trying to physically trace a maze of pipework through the building can be difficult. Consult your manufacturer/project support team should any queries arise to save on costly rework situations.
Multiple machines
When implementing multiple machines in parallel, it is important to manage water flow correctly to avoid mixing water temperatures which leads to machines having to work harder to achieve setpoint (increasing operational costs). Flow rate requirements should be balanced where possible vs the amount of energy needed/produced to ensure the most efficient operation. This can be achieved through implementing variable water flow circuits, where the logic to control this can be included within the build of the machine to dramatically simplify installation, although, functions such as our MultiLogic system can offer control even in constant flow situations where on board water pumps are only activated if their associated compressors are working, further optimising operation and reducing energy waste. MultiLogic also manages defrost requests in multiple machines ensuring that this impact is at a minimum and removes the need for the external BMS to rationalise when and how to enable multiple machines. This functionality also allows us to reduce the amount of water needed in the system by physically managing multiple requests for machine defrost cycles.
