The Disadvantage of Higher Delta T in Chilled Water Systems
In many cases, higher delta T results in a higher AVERAGE temperature of the chilled water as it goes through the cooling coil at your load. In these cases, the cooling coil cannot perform as much cooling.
For example, it was once common for chilled water to be 44'F supply temperature and 54'F return temperature. The delta T was 10 degrees (54 - 44 = 10). The average temperature was 49'F ( ( 44 + 54 ) / 2 = 49 ).
Suppose instead you want a delta T of 15 degrees. There are numerous ways you can select new supply and return temperatures to achieve this new delta T. Most likely you will lower the supply temperature a little and raise the return temperature a little.
But lowering the supply temperature is a disadvantage. Lowering the chilled water supply temperature increases the work the chiller must do. This increases chiller energy use (kWh), chiller peak power (kW), and increased wear and tear on the chiller.
So, continuing the example, let's avoid the disadvantage of lowering the supply temperature, and instead achieve a delta T of 15 degrees by increasing the return temperature by 5 degrees. Now we will have a supply temperature of 44'F and a return temperature of 59'F ( 54 + 5 = 59 ).
The average temperature is now 51.5 'F ( ( 44 + 59 ) / 2 = 51.5 ).
You have raised the average temperature in your cooling coils from 49'F to 51.5 'F. Your cooling coils will not cool as much, they will not dehumidify as much (if you were using them for dehumidification).
The solution to this is a larger coil. But that is a disadvantage, if the lower delta T allowed you to use a smaller coil. Or maybe this is a retrofit project where you wanted to continue using an existing coil. Now you have to check whether the existing coil will do the job with the warmer average water temperature. Even a larger coil that gives you equal total cooling may not give you sufficient dehumidification using warmer average water temperature.
The other solution is to lower the chilled water supply temperature so that the average temperature does not increase. As described above, this has its disadvantages.
This is not to advocate against higher delta T. Higher delta T allows lower chilled water flow which can mean lower first costs in pump sizing and pipe sizing, and lower pumping energy (kWh) costs, lower pumping peak power (kW), and lower wear and tear on the pumps. These advantages may outweigh the disadvantages.
I posted this because I had a hard time finding a simple explanation of what is the disadvantage of going to higher delta T.
5/14/2015