Why Air Differential Pressure on Chilled water coil not a standard practice?

[jackrumorz]

Would anyone explain why is it not a standard practice to use Differential Pressure Transmitter or Switch across Chilled water coil in AHU?

We have been using Air DPT across filter for years, but I have not come across one for CHW coil in AHUs. Please explain if it's beneficial to have Air DPT across AHU Chilled water coil, with advantages and disadvantages.

Regards.

[orion242]

If your changing your filters, what would be the point?

if your ignoring the filter dp, why add another on the coil?

[jackrumorz]

If the filter is changed, the pressure drop at the filter section is addressed. There are still possibilities of pressure drop across coil which could result in more fan power consumption. Eg. Aluminium fins on the coil are not properly maintained/damage to the fins, condensate from the coil etc.

I agree, if the filter is ignored, no need to add another DP across coil.

[XcelTech]

If the filter is changed, the pressure drop at the filter section is addressed. There are still possibilities of pressure drop across coil which could result in more fan power consumption. Eg. Aluminium fins on the coil are not properly maintained/damage to the fins, condensate from the coil etc.

I agree, if the filter is ignored, no need to add another DP across coil.

The short answer, its another input and most customers want it cheap, that means less man hours and fewer points.

But if you want to go that way, why not use Current Transducers (not switches) on fans, supply and return air temps, supply and return chilled water, and supply and return hot water.

I can write a lot of alarming code to tell the owner what needs to be fix with those inputs. I also have to a controller with 7 inputs and I still don't cover all the inputs I should have on an AHU.

[jackrumorz]

The short answer, its another input and most customers want it cheap, that means less man hours and fewer points.

But if you want to go that way, why not use Current Transducers (not switches) on fans, supply and return air temps, supply and return chilled water, and supply and return hot water.

I can write a lot of alarming code to tell the owner what needs to be fix with those inputs. I also have to a controller with 7 inputs and I still don't cover all the inputs I should have on an AHU.

Thank you. We are considering Supply & Return Air Temp, Chilled water return temp (supply is taken at building entry). We do not have Hot water coil here in India. The question is does it justify to add DPT across the coil alone? In other words, what are the possibilities of pressure drop across the Chilled water coil and what are the chances for it to occur or how frequently it will occur.

[jbeckham]

If your coils are becoming clogged, your filter arrangements are improperly applied and/or your filter maintenance schedule is wrong... I don't see the point in measuring the coil DP, since you should really be way, way out in front of addressing this potential condition. If you want more accuracy in your control, you could always install air flow stations in the AHUs. However, more accuracy comes at a higher premium on initial install and maintenance costs.

[WAYNE3298]

It is not likely that you can get an accurate pressure drop reading across the coil. If you could the dp will change with coil moisture load even if airflow does not. Stratification within an air handler with few exceptions is present which means as airflow changes your dp reading will change disproportionately. You would also have to know the actual pressure drop and corresponding airflow when balanced. The design dp provided by the manufacturer is not always correct. The only real value with coil dp would be to alarm a significantly plugged coil but the added value would cost more than it is worth.

[orion242]

Exactly.

Good PM, no need to measure coil DP shouldn't change much. Crap PM, no need for sensor....your coils will be plugged and no one will care.

[browntg]

I don't think there is much of a reason to install a DP on the coil is becoming clogged you can see it in a loss of coil performance. the DAT will go up because you are not getting thee heat transfer you should because of a dirty coil.

[stanbyyourword]

DP sensing is done on DX cooling unit to prove airflow , to prevent icing and liquid floodblack , hardly necessary for a chilled water system.