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HEATING & CHILLING
necessary pressure and run at what is referred to as dead-head pressure, or limit. When this state occurs, the pump’s life can be reduced; liquid ceases to flow and the liquid in the pump becomes hot, eventually vaporising and disrupting the pump’s ability to cool leading to excessive wear to bearings, seals, and impellers. Determining the pressure loss across a
system requires siting pressure gauges at the process’s inlet and outlet, then applying pump pressure to obtain values at the desired flow rate. Flow rate: Inadequate flow through the
process will yield inadequate heat transfer so the flow will not remove the heat necessary for safe operation of the process. As the fluid temperature increases beyond the setpoint, the surface/component temperatures also continue to rise until a steady-state temperature that is greater than the initial setpoint is reached. Most chiller systems will detail the pressure
and flow requirements. When specifying the necessary heat load removal as part of the design, it is important to account for all hoses, fittings, connections, and elevation changes integral to the system. These ancillary features can significantly increase pressure requirements if not sized appropriately. An air-cooled chiller’s ability to dissipate
heat is affected by the ambient temperature. This is because the refrigeration system uses the ambient air/refrigerant temperature gradient to induce heat transfer for the condensation process. A rising ambient air temperature decreases the temperature differential (∆T) and reduces the total heat transfer. If the chiller uses a liquid-cooled condenser,
high ambient temperatures can still have negative effects on key components such as the compressor, pump, and electronics. These
components generate heat during operation, and elevated temperatures will shorten their lifetime. As a guideline, the typical maximum ambient temperature for non-exterior rated chillers is 40°C. Spatial Constraints: In order to maintain the
proper ambient air temperature, it is important to provide adequate air circulation space around the chiller. Without this, air rapidly heats it up, affecting chiller performance and potentially damaging the chiller unit.
WHY SIZE IS IMPORTANT
Selecting a correctly sized chiller is a crucial decision. An undersized chiller will never be able to properly cool the process equipment and the process water temperature will not be stable. In contrast, an oversized chiller will never be able to run at its most efficient level and prove more costly to operate. To determine the correct size of unit for the
application it is necessary to know the rate of flow and the heat energy that the process equipment is adding to the cooling medium, i.e., the change in temperature between the inlet and outlet water, expressed as the ∆T. The formula for calculation purposes is:
Heat energy per second (or more commonly known as Power) = mass flow rate × specific heat capacity × change in temperature (∆T)’ The specific heat capacity of the water is nominally expressed as 4.2 kJ/kg K but if it contains a percentage of glycol additives that value is increased to 4.8 kJ/kg. K Note: 1K = 1°C and the density of water is 1 i.e.,
1l of water volume = 1kg of water mass Here is an example of the formula
application to determine the correct kW sized chiller to handle a water flow rate of 2.36l/s (8.5 m3
(Flow Rate) X 5°C (∆T) X 4.2 kJ /kg K (Specific Heat Capacity of pure water). Chiller size required = 49.6 kW
/hr) with a temperature change of 5°C: Heat Energy per second (kJ/s or kW) = 2.36l/s
Alternatively, the heat load to be cooled
may already be known in which case the formula can be re-arranged to determine the temperature difference (∆T) that can be attained with different flow rates (achievable with different pump sizes). There may be other circumstances that
can influence size choice, including planning for future plant expansion, exposure to high ambient temperatures, or location at high altitudes. In the latest generation of industrial chillers,
ease of maintenance, operational safety, and intelligent control and connectivity are prominent features of their designs. For example, they are constructed with
IP54-rated, sound-attenuated canopies that allow chillers to operate indoors or outdoors, even at ambient temperatures down to - 45°C. They are specifically designed for easy access to the installed components. Wide canopy doors and intelligent layout reduces maintenance time and allows for easy inspection to prevent breakdowns. New models on the market feature a range
of safety devices, such as flow and level switches, thermal probes, pressure probes, crankcase heating and strainers which allow the chiller to operate securely. Additionally, a fully hermetically sealed refrigeration system prevents refrigerant gas from leaking and requires zero maintenance. UK F-Gas Regulations do require an annual, and on larger refrigeration systems, bi-annual inspection by a F-Gas certified engineer. In these new designs, a touch screen
controller operates with energy-efficient algorithms, combines all the chiller sensors into one system and issues warnings in case of deviation from the operating parameters.
Atlas Copco
https://www.atlascopco.com/en- uk/compressors/customer-offers/how-to-sele ct-the-right-industrial- chiller?amc_cid=em_AC-Chiller-guide_na
DECEMBER 2021/JANUARY 2022 | PROCESS & CONTROL 21
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