What is De-Rate and Delta T


Delta T is the expression used in the HVAC and HVACR industries, to determine the difference between return fluid temperature and delivery fluid temperature, within a conventional heating or cooling system.

Delta-T is easily discernible by its Greek symbol ΔT.  Δ is the 4th letter in the Greek alphabet also a mathematical symbol for “change” or “the change”.

How Delta T, Works for You.

When applied to commercial or industrial heating and cooling this simple ΔT explains how well a heating or cooling system is able to perform a change in temperature when in use.

A good example being a commonly used setup for heating is 80/60, this means a flow delivery temperature to the process of 80 degrees Celsius. On return to the heat source the flow temperature of the water is now 60 degrees Celsius, therefore a 20-degree ΔT is applied and this is an acceptable heat loss on a conventional boiler.

Although ΔT will alter significantly between different heating applications.

However there is a change when it comes to cooling, ΔT is normally much smaller within a cooling system.

The wider the gap becomes between flow and return temperatures. The harder the chiller or cooling device has to work. this may indicate that the  chiller or cooling device installed is not able to perform this task. Or is an indication that it may of developed a fault.

Using the example based on expected Eurovent conditions, that being a 35-degree ambient ( outdoor ) air, and a flow water temperature of 7 degrees Celsius. The return water temperature of 12 degrees Celsius, the ΔT between the two temperatures would then be 5 degrees Celsius.

What is chiller De-Rate

De-rate is the slidable scale to calculate a chillers capacity in any given condition. This all begins with the OEM specification.
If we use Eurovent conditions as an example a 1000kw Chiller is built with the capacity to remove 1000kwW of heat in specific conditions, leaving water temperature (LWT) 7 degC returning at 12degC.

If you change any of these conditions, the chiller changes in predictable ways, this is called de rating.

We will Calculate Your Delta T for You

A common mistake is not factoring the leaving water temperature. The colder the chiller has to perform, the less kilowatt energy it can remove and the harder it has to work to achieve this.
To answer the question you need to answer, in order to size your chiller correctly.
” Is what does the kW output de-rate to when its LWT is minus 10 degrees”.

Let’s say for this example the chiller you have de-rates to 550kW in 35 degC ambient ( outdoor air ) with a LWT of -10 then you would require 2 of the 1000kW chillers to provide 1000kW at -10.

Likewise, the same applies in the exact opposite direction when the requirement is outside the eurovent conditions of your chiller.
An example is a 250kw specified in EV (* eurovent conditions ) 7/12 35DegC will remove 250kW, if your leaving water LWT was +15 and not 7 degC your chiller capacity actually increases , so your 250kW chiller might actually remove 300kW instead! * these figures are not based on an actual chiller,
Please consult your manufacturer for this information or Give us a call and we’ll be happy to help.

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