IPLV in Chillers: Meaning, Formula and Worked Example

A chiller's full-load efficiency tells you how it performs on the hottest afternoon of the year. For the other 99 % of its running hours it is partly loaded, with cooler condenser water or air. IPLV (Integrated Part Load Value) was created to express that seasonal performance in one number, so that buyers can compare machines fairly.

What IPLV measures

AHRI Standard 550/590 defines four rating points. The chiller is tested at each one with a fixed leaving chilled water temperature of 6.7 °C (44 °F) and progressively cooler condenser conditions, reflecting the fact that part load usually coincides with milder weather.

The weightings come from a study of building load profiles: a typical comfort chiller spends only about 1 % of its hours at full load and nearly 90 % between 50 % and 75 %.

The IPLV formula

For efficiency in COP or EER (higher is better):

IPLV = 0.01 A + 0.42 B + 0.45 C + 0.12 D

For efficiency in kW/TR (lower is better):

IPLV = 1 ÷ (0.01/A + 0.42/B + 0.45/C + 0.12/D)

where A, B, C and D are the efficiencies at 100 %, 75 %, 50 % and 25 % load.

Worked example

A water cooled screw chiller has these test results:

In kW/TR: IPLV = 1 ÷ (0.01/0.62 + 0.42/0.52 + 0.45/0.46 + 0.12/0.50) = 1 ÷ (0.0161 + 0.8077 + 0.9783 + 0.2400) = 1 ÷ 2.0421 = 0.49 kW/TR.

In COP: IPLV = 0.01 × 5.67 + 0.42 × 6.76 + 0.45 × 7.65 + 0.12 × 7.03 = 7.18, which is 3.517 ÷ 0.49, the same answer. The conversions between kW/TR, COP and EER are in chiller efficiency: COP and kW per TR.

Note that the IPLV (0.49) is much better than the full-load figure (0.62). That is normal, and it is why a quoted "0.49 kW/TR" must always be labelled as IPLV or full load.

IPLV vs NPLV

NPLV (Non-standard Part Load Value) uses the same weightings but at the project's own design conditions. If your plant needs 10 °C chilled water, or your cooling tower can only give 32 °C water in summer, the standard IPLV does not describe your installation and the supplier should give NPLV instead. In India, where design condenser water is often 32 °C rather than 29.4 °C, NPLV is the more honest figure.

What improves IPLV

• Variable speed drives on screw and centrifugal compressors, which cut power sharply at part load.
• Multiple compressors so that part load is met by fewer machines running near their best point. See screw vs scroll chillers.
• Electronic expansion valves and floating head-pressure control, which exploit cool condenser water.
• Generous heat exchangers, which become even more effective at reduced flow of heat.

Why IPLV can mislead for process chillers

IPLV assumes a building: load falls when the weather cools. A process chiller serving an extruder, a reactor or a dairy has a load set by production, often constant day and night, summer and winter, and frequently at a temperature far from 6.7 °C. For such duties:

• Compare full-load kW/TR at your actual outlet temperature and design ambient.
• If the load genuinely varies, ask for efficiency at your own load points and weight them by your own hours.
• Remember that IPLV does not exist as a standard figure for a −20 °C brine chiller; a number quoted for one has no test basis.

The practical difference between the two kinds of machine is explained in process chiller vs HVAC chiller.

Using IPLV when buying

• Ask whether the figure is IPLV, NPLV or full load, and in which unit.
• Ask for the four individual part-load points, not only the result.
• Estimate annual energy as average load (TR) × running hours × IPLV (kW/TR) for comfort duty, or with full-load kW/TR for steady process duty.
• Weigh the energy difference over 10–15 years against the price difference. Power is usually the largest lifetime cost of a chiller, as discussed in the chiller price guide.

When you ask Teqtive to quote a screw or scroll chiller, send us your load profile and ask for the power figures at your actual operating conditions.

Frequently asked questions