Hello,
I have some questions regarding the functioning of the software VE and the functioning of some HVAC components.
The Cooling coil component needs a parameter named “Coil Contact factor” to be defined.
This contact factor is used to describe the way air flows over the coil, and is used to calculate the balance of sensible to latent heat removal of the air passing over the coil. Several tests on this parameter show that reducing the contact factor implies increasing the Latent Load and the dehumidification.
Contact coil factor 0.1 0.5 0.7 0.91
Sensible (kW) 38.0 38.0 38.0 38.0
Latent (kW) 17.0 6.36 1.5 0.7
Latent/Sensible 45% 17% 4% 2%
If we look at the formula describing the contact factor, we observe that fixing the contact factor to one single value during all the simulation implies that the temperature of the Apparatus Dew Point (ADP) varies depending on the temperature of the air entering the cooling coil .
contact factor= (x1-x2)/(x1-x3)=(h1-h2)/(h1-h3)~(t1-t2)/(t1-t3)
I don’t understand how the ADP temperature is related to the Chilled water supply temperature and to the Delta T in the definition of the chilled loop.
Moreover, I tried to modify the value of the chilled water supply temperature and the value of the Delta T in the definition of the chilled loop, it doesn’t seem to have an impact on the Latent Loads. Only the contact coil factor changes the ratio Latent loads/Sensible Loads. How can it be explained?
Thank you very much for your attention,
Bests regards,
coil contatc factor
Re: coil contatc factor
Section 3.2.7.1 of the ApHVAC User Guide explains the coil contact factor in detail and is of use in clearing up your questions.
The contact factor specifies what percentage of the air comes in direct contact with the coil. This air is cooled along a constant moisture content line until the saturation curve is met, at which point cooling follows the saturation line. The rest of the air is unaffected and mixed with the cooled air upon leaving the coil. So if you have 1,000 cfm and a contact factor of 0.8, 800 cfm gets ideally cooled and 200 cfm is unaffected.
If the contact factor - and thus airflow - is specified as above, then coil temperature will vary to meet specified coil leaving air conditions. Coil temperature is modified by changing chilled water loop temperature. If the contact factor is higher, say 0.9, there is now 900 cfm that gets cooled. To get the same air conditions leaving the coil as when only 800 cfm was cooled, a warmer coil is used. If the contact factor is lower, say 0.7, the opposite is true. Only 700 cfm has direct contact with the coil so the coil must be colder in order to achieve the leaving air conditions specified.
Of course, a colder coil results in more dehumidification. Thus, when you lower the contact factor in the VE, you see no change in sensible heat transfer, but an increase in latent heat transfer.
This also explains why the coil temperature changes when the incoming air temperature changes. If the contact factor is constant, the only way to adjust performance to achieve constant leaving air conditions is to modify the coil temperature. This, of course, requires a modification in chilled water temperature flowing through the coil.
Whether contact factor is specified or derived in the VE depends on what type of cooling coil you use - advanced or simplified - and whether or not you are manually sizing or autosizing. The ApHVAC User Guide clarifies which variables are derived in each case.
Thanks,
Megan
The contact factor specifies what percentage of the air comes in direct contact with the coil. This air is cooled along a constant moisture content line until the saturation curve is met, at which point cooling follows the saturation line. The rest of the air is unaffected and mixed with the cooled air upon leaving the coil. So if you have 1,000 cfm and a contact factor of 0.8, 800 cfm gets ideally cooled and 200 cfm is unaffected.
If the contact factor - and thus airflow - is specified as above, then coil temperature will vary to meet specified coil leaving air conditions. Coil temperature is modified by changing chilled water loop temperature. If the contact factor is higher, say 0.9, there is now 900 cfm that gets cooled. To get the same air conditions leaving the coil as when only 800 cfm was cooled, a warmer coil is used. If the contact factor is lower, say 0.7, the opposite is true. Only 700 cfm has direct contact with the coil so the coil must be colder in order to achieve the leaving air conditions specified.
Of course, a colder coil results in more dehumidification. Thus, when you lower the contact factor in the VE, you see no change in sensible heat transfer, but an increase in latent heat transfer.
This also explains why the coil temperature changes when the incoming air temperature changes. If the contact factor is constant, the only way to adjust performance to achieve constant leaving air conditions is to modify the coil temperature. This, of course, requires a modification in chilled water temperature flowing through the coil.
Whether contact factor is specified or derived in the VE depends on what type of cooling coil you use - advanced or simplified - and whether or not you are manually sizing or autosizing. The ApHVAC User Guide clarifies which variables are derived in each case.
Thanks,
Megan