Four Parameters an Experienced Engineer Checks First — Before Capacity and Price
Introduction
Selecting a chiller for an industrial or infrastructure facility is rarely just a matter of determining the required capacity and selecting equipment at a suitable price level. In the real world, a chiller does not operate at an abstract design point, but in a complex set of dynamic refrigeration loads, variable climatic factors, hydraulic regimes and electrical constraints. It is these parameters that form the actual environment in which the compressor, evaporator and control system must operate around the clock and without fail. For this reason, an experienced engineer does not start his work by selecting capacity from a catalogue, but by analysing the four primary characteristics of the facility, which define the limits of the future system’s performance.
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Refrigeration load profile as a primary constraint for correct selection
Correct chiller capacity selection is not possible without analysing the time structure of the heat fluxes. The maximum load gives only a single point, while the chiller system must be able to operate dynamically under varying condensing temperatures, circuit flow rates and partial loads. Professional selection starts with a study of how the load is formed and distributed over time, not with the peak value.
Fig.1 – Example of daily variation of heat load of a dairy plant with display of three-zone electricity tariff: red – 1 zone 11 UAH/kWh, yellow – 2 zone 8 UAH/kWh, green – 1 zone 5 UAH/kWh.
The graph in Fig. 1 clearly demonstrates why the analysis of the temporal distribution of heat fluxes in can be a determining parameter even before choosing the capacity of the refrigeration plant. The dairy plant is characterised by a pronounced daily non-uniformity: in the red tariff zone (electricity cost 11 UAH/kWh) the heat load reaches maximum values, while in the green zone with a minimum tariff of 5 UAH/kWh the actual load decreases to 50-60% of the peak level. If the equipment is selected directly according to the daytime maximum, the installed cooling capacity should cover this very peak, although it lasts for a limited time – usually 3-4 hours a day.
The profile analysis allows to quantify the amount of stored cold. For example, if the peak load exceeds the average level by 200-250 kW for 4 hours, the total cold deficit is about 800-1000 kWh. It is this volume that can be transferred to the ice accumulator and generated in the night green zone, when the cost of electricity is 2.2 times lower. In this mode, the chiller operates at night at nominal or close to nominal capacity, forming a reserve of cold, and during the day – either completely switched off or covers only the base load.
From an engineering point of view, this reduces the installed capacity of the equipment by 20-30%. If without accumulation a unit was required, for example, for 1000 kW, then with the use of an ice accumulator the calculated refrigeration capacity can be reduced to 700-800 kW, since the peak component of the load is compensated by the accumulated cold. At the same time, the electricity consumption profile also changes: a significant part of electricity is transferred to the third tariff zone, which, given the tariff difference of 6 UAH/kWh, gives direct OPEX savings of tens of per cent in the annual balance.
Equipment manufacturers use the load profile as a mandatory parameter. The Trane TRACE and YORKworks programmes simulate load distributions for a year and show that for most facilities 70-85% of the time the chiller operates in the range of 30-65% of capacity. Under these conditions, Daikin inverter scroll compressors provide stable operation up to 15-20% of rating, whereas mechanical slider screw compressors have EER degradation in the 15-35% zone. Thus, the choice of compressor is not determined by the peak load, but by the prevailing range of partial modes. In other words, consider a multi-compressor unit where the number of compressors in operation will increase proportionally with load.
As shown in Fig.2, in the zone of low and medium loads (modes A-B) the unit operates with one compressor with frequency control, providing a smooth change of cooling capacity without frequent starts and stops. However, when the load rises sharply and the effective range of one compressor is exceeded (transition to modes C-D), connection of a second compressor allows the control to remain in the zone of stable overheating, permissible currents and high energy efficiency.
Fig.2 – Example of an effective loading sequence on a two-compressor refrigeration system
If the facility is characterised by significant variations – e.g. from 30-40% to 90-100% of capacity over short time intervals – a single compressor chiller, even with an inverter, will either operate at the control limit or go into cyclic mode. The multi-compressor scheme (2-4 compressors) allows the control range to be divided into several overlapping zones, where each compressor operates closer to its optimum point. In practice, this results in 30-50% fewer starts, more stable evaporator operation at variable flow rates and a 10-20% increase in seasonal SEER efficiency compared to an equivalent single compressor solution at the same installed capacity.
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Climatic conditions of the facility
The climatology of the site determines the actual thermal performance of the chiller and is the second fundamental parameter that is analysed before selecting the capacity and cost of the equipment. The ratings given in the AHRI or Eurovent catalogues reflect the behaviour of the unit under fixed conditions – for example, 35 °C outside air for an air-cooled unit or 30/35 °C for a condensing water machine circuit. However, the actual operating behaviour is shaped by local climatic factors: temperature amplitude, humidity, frequency of extremes and wind load patterns.
Even a small deviation of the outside air temperature significantly changes the operating point of condensation. A 1 °C increase in air temperature increases the required condensing pressure by about 10-15 kPa, which leads to a 1.5-2.5 % decrease in refrigeration capacity and a 2-4 % increase in compressor energy consumption. For microchannel condensers, the sensitivity is even higher due to the limited depth of fan control and high heat flux density. Therefore, the actual climatic data of the facility influences the choice of heat exchange surface, fan configuration and condensing control algorithms.
In regions with summer extremes of 40… 43 °C (south of Ukraine, Azerbaijan, Turkey) the share of operation in the zone of reduced efficiency can reach 30-40 % of the annual time. Under such conditions, the use of a standard air cooler without increased condenser surface leads to excessive compressor currents, premature high-pressure shutdowns and reduced motor life.
Air humidity affects condenser behaviour indirectly: at high humidity, the wet bulb temperature rises and the reduction of condensing temperature by evaporative cooling becomes less effective. This is critical for chillers with integrated adiabatic panels, where humidification efficiency can vary from 70-80 % in dry climates to 40-50 % in high humidity conditions. Without accurate climate data, an engineer may misjudge the potential for seasonal savings and incorrectly select the type of condensing system.
As a result, facility climatology determines:
– the required condensing surface and number of fans;
– the choice between air, water, adiabatic or hybrid cooling;
– compressor operating temperature range and allowable currents;
– condensing pressure control algorithm;
– actual seasonal EER/SEER, not the nameplate value.
Hydraulic system architecture as a critical factor in operability
The hydraulic configuration of the refrigeration system determines the actual operating conditions of the evaporator and compressor/control loop. Regardless of the correctly calculated capacity and climatology, the chiller will not operate consistently if the actual flow rate, head and circulation pattern do not match the requirements of the refrigeration cycle. Therefore, a professional engineer analyses the hydraulics before selecting the model, cost and even the type of compressor.
The performance of evaporators is sensitive to flow and hydraulic conditions. Plate heat exchangers require a minimum flow rate to maintain a turbulent Reynolds number to ensure stable heat transfer and avoid subcooled zones. Practically, this translates into a minimum flow rate of about 0.8-1.2 m³/h per 10 kW load.
An example of decreasing cooling efficiency can also be given by heat exchangers of extrusion thermoplastic machines, where, when the flow rate drops and the flow switches to the laminar flow regime, the performance drops critically (Fig. 3a, 3b).
Shell-and-tube evaporators are more tolerant to flow rate reduction, but their heat transfer also degrades when the flow velocity drops below 0.3-0.4 m/s. Therefore, correct selection of the refrigeration machine is impossible without checking the actual hydraulic parameters of the system.
Fig.3a – Cooling with water (or air) is much more efficient with turbulent flow than with laminar flow. In laminar flow, a thin fluid boundary layer tends to remain stationary on the surface of the extrudate, isolating it from the main coolant flow. Turbulence destroys this layer by exposing the extrudate to the temperature of the main coolant.
Fig.3b – In the transition from laminar to turbulent flow, the heat flux from the heated surface effectively doubles. This occurs even with very small increases in coolant flow rate, so it is critical in any cooling system to know the exact flow rate and resulting Reynolds number.
The complexity of hydraulic analyses increases in systems with multiple cooling units. When units with different minimum flow characteristics and different types of evaporators are operated in parallel, the operating ranges need to be harmonised. For example, a machine room with two chillers: one is a Daikin 250 kW inverter scroll chiller with a minimum flow rate of 32 m³/h, the other is a Climaveneta 300 kW screw chiller with a minimum flow rate of 42 m³/h. If the system reduces the flow rate to 60-65 m³/h during the night period, one of the chillers will be forced to operate in a zone close to the minimum allowable, causing an increase in ΔT, deterioration of heat transfer and increased overheating. Adjusting the control logic without recalculating the hydraulics does not solve the problem.
Electrical constraints and influence of electrical infrastructure on chiller configuration
The main limiting factor is the available line power and allowable inrush current. Compressors have significant starting loads: a 300-350 kW screw compressor can have a starting current of about 450-600 A, while large scroll units have a starting current of 280-350 A. If the electrical network of the facility allows short-term voltage dips of only 8-10 per cent, direct start-up becomes impossible. That is why manufacturers – Trane, Daikin, Johnson Controls – offer several variants of starting devices: soft-start, VFD-start, autotransformer start. Each variant reduces inrush current by 30-70%, but requires appropriate protection, thermal stability of cables and correct setting of electronic modules.
Fig. 4 – Inrush current versus operating current (FLA – Full Load Amperes) for different starting methods
The quality of the mains supply has a direct influence on compressor life and control logic. If the voltage deviates ±10 % from the nominal voltage, motor torque is reduced, current increases and winding cooling is impaired. In networks with frequent voltage dips of 15-20 % (typical for industrial zones of Ukraine, especially when working on temporary lines) screw compressors can go into emergency shutdown by “Undervoltage”, and electronic TRVs lose stability of regulation due to an error in the power supply of controllers. That is why manufacturers specify permissible ranges of supply voltage, for example 3×400 V ±10 % for Daikin EWAD or ±15 % for Trane RTA series, and violation of these ranges makes it impossible to correctly select a chiller, even if its capacity is suitable for the heat load.
Harmonic distortion (THD) is a separate problem. Compressor and fan frequency converters are a source of higher harmonics, and in unstable or polluted networks, the total THD can exceed the permitted values of EN 61000-3-12. Higher THD causes winding overheating and step torque fluctuations, which reduces compressor life. For industrial applications with a large number of non-linear loads, a harmonic analysis of the mains network is required prior to chiller selection. Mains filters may need to be installed, which will change the cost and layout of the equipment.
Electrical constraints also affect the choice of compressor technology. Inverter-controlled scroll compressors have low inrush current and high resistance to voltage fluctuations, but their unit capacity is limited. For facilities with insufficient dedicated capacity and stringent energy quality requirements, inverter systems are often the only viable option. Screw compressors offer high unit capacity but have higher demands on network quality and current parameters. Centrifugal compressors with magnetic bearings (e.g. Daikin Turbocor range) have low starting currents, but are extremely sensitive to voltage dips due to the need for a stable supply to the active rotor positioning system.
Emergency power supply (diesel generators, UPS) is also a determining factor. The generator set is limited in the multiplicity of starting currents (often no more than 2.5-3× the rating), so direct starting of the chiller on a DGU is not possible. A 300 kW screw compressor with an LRA of 500 A requires a 1600-1800 kVA generator for direct start, whereas 700-800 kVA may be sufficient for VFD start. If the generator is the only source of emergency power, the choice of chiller configuration is actually determined by the electrical characteristics of the chiller, not just the cooling capacity.
Conclusion
The correct chiller selection is not possible without first analysing four fundamental parameters: refrigeration load structure, site climatology, hydraulic architecture and electrical constraints. These factors determine the actual operating points of the equipment, the allowable capacity range, compressor configuration, heat exchanger requirements, control algorithms and operational reliability limits. If an engineer starts selection with rated capacity and price, bypassing these basic parameters, the system turns out to be designed according to formal values, but is not capable of stable operation in real conditions of the facility. Therefore, a professional approach to chiller system design is always built “from the bottom up”: from the technical constraints of the environment to the capacity, configuration and cost of the equipment.
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Author of the article:
Dmytro Lychak, CEO of the company
18.12.2025