Key considerations in chiller selection

The chiller is the heart of a facility's cooling system. A wrongly selected unit either fails to meet demand or runs inefficiently because it is oversized. In both cases, operating costs rise for years. In this article, we summarise the questions we often encounter on site and the key criteria we consider during selection.

Starting from the right load

The first step in selection is to determine the actual cooling load. In comfort applications, the building envelope, internal gains and fresh air load are calculated. In process applications, the heat rejection of machinery, operating hours and diversity are decisive. If a unit is being replaced in an existing facility, measured load data should be used rather than the nameplate value of the old unit.

• We assess the peak load and the load distribution over the year separately.
• We prefer to meet future capacity increases with a modular configuration rather than oversizing the unit.
• In critical facilities, we clarify redundancy requirements (e.g. N+1) from the outset.
• For process cooling, we determine the required water temperature and tolerance together with the process manager.

An oversized chiller cycles on and off frequently. This both shortens compressor life and makes temperature control more difficult.

Air-cooled or water-cooled?

Air-cooled chillers are relatively simple to install and do not require a cooling tower or condenser water circuit. Water-cooled chillers generally operate at higher efficiency, but they bring the need for a cooling tower, water treatment and additional maintenance. When deciding, we look at the following:

• Capacity and annual operating hours
• Roof or site space, and structural load capacity
• Water supply, water quality and water treatment options
• The competence of the maintenance team and the facility's operating structure

In facilities with year-round cooling demand, a free cooling option should also be considered. When the outdoor temperature is low, compressor load decreases, creating significant energy saving potential.

Part-load efficiency, not nominal efficiency

A chiller does not run at full load for most of the year. Looking only at the nominal EER value can therefore be misleading. Values that show seasonal and part-load efficiency (indicators such as SEER or IPLV) give a more meaningful picture for comparison. It is also important that the performance values given in catalogues have been independently verified; certification programmes such as Eurovent can be taken as a reference in this respect.

The compressor type also affects part-load behaviour. Scroll, screw and centrifugal compressors each suit different capacity ranges. Variable-speed drives generally improve part-load efficiency. In selection, the soundest approach is to compare the annual energy consumption of different types over the facility's load profile.

Hydraulic configuration and refrigerant

A chiller does not work on its own, but together with the hydraulic system it is connected to. Many problems on site stem not from the unit but from the hydraulic design. We therefore address the following at the selection stage:

• Choice of constant-flow, variable-flow or primary-secondary system
• The minimum evaporator flow permitted by the manufacturer
• The system water volume required for stable operation and, if necessary, a buffer tank
• Use of glycol in applications with a risk of freezing, and the related capacity correction

Refrigerant selection is also a long-term decision. F-gas regulations are progressively restricting the use of refrigerants with a high global warming potential (GWP). In new investments, units using low-GWP refrigerants and the service conditions of these refrigerants need to be assessed in light of current legislation.

Layout, noise and maintenance access

Where the unit is placed directly affects its efficiency and ease of maintenance. In air-cooled units, hot air recirculation leads to capacity loss. Clearances from surrounding walls and neighbouring units should therefore be planned according to the manufacturer's recommendations. If there are homes or offices near the installation area, noise levels should also be examined.

Service access should be left to the compressor, heat exchanger and control panel. Vibration isolation and flexible connections reduce vibration transmitted to the structure. Communication between the chiller and the building management system makes remote monitoring and fault tracking much easier.

When comparing quotes

Comparing different quotes on price alone can lead to surprises later. It is necessary to check whether the quotes have been prepared under the same conditions. At this stage, we look for answers to the following questions:

• Are the capacity values given for the same water inlet/outlet temperatures and the same outdoor design temperature?
• If glycol is to be used, have the capacity and efficiency values been corrected accordingly?
• Are accessories such as pumps, buffer tanks, flow switches and communication cards included in the scope?
• How will commissioning, spare parts supply and service support be provided?

Alongside the initial investment cost, the energy and maintenance costs the unit will incur over its lifetime should also be assessed. In most cases, operating expenses account for the decisive share of the total cost.

Chiller selection gives sound results when load calculation, unit type, hydraulic system and operating conditions are addressed together. Testing, adjusting and balancing after installation then verifies that the selected unit operates at its design performance. At Ekura, we can support you through this process, from analysing your facility's needs to commissioning.