Sensible Capacity change with Entering DBT

[EnOm]

Hi
I have questions regarding how a fan coil unit's capacity changes with the Dry Bulb temperature of the air entering it.

DBE: Dry Bulb Temperature of air entering coil (F)
DR: Wet Bulb Depression Ratio

I am trying to select a unit from a catalogue and the following correction equation is given to get the unit's capacity at a DBE different than the one that the figures in the performance table are based on.

Correction Factor = 1.08 x CFM x(1-DR)x(New DBE - 80) , where 80 is the DBE the performance table is based on.

I noticed that increasing DBE results in a higher unit capacity, as opposed to my expectations which were that the higher the temperature of the air entering the unit, the less capacity it should be able to handle.

Based on this, just theoretically speaking, if I have a room that has a certain heat load, and my fan coil unit's capacity falls just short of this heat load, I can overcome the shortage in capacity by injecting hot air from the outside to raise the return air stream's dry bulb temperature? Is the hotter air adding energy to the refrigeration cycle somewhere?

My other question is the catalogue does not specify which capacity, sensible or latent is the one affected by the change in DBE. I'm assuming it is added on to the sensible capacity since it's a factor of DBE with no change in latent capacity. Is this true?

Thank you.

 

[Drazen]

If cooling water temperature is the same, increasing enter air DB will increase temperature difference, and with the same water flow rate, heat transmission capacity will increase.

Have in mind that this applies to stationary condition, i. e. level of balance you want to achieve by selecting specific design DB temperature. If you introduce outside air, you would increase coil load even more, but you would also increase space load, so you would likely not be able to reach design temperature.

Your goal is to achieve design temperature, not to increase coil transmission abilities, isn't it, so your general thinking went into somewhat wrong direction. There are several factors you can play with, each one affecting another, but if you do not focus on your primary goal, reaching interior design conditions, you may stay somewhat lost until you grasp whole feeling of it, which comes with practice.

Question two, yes, that should basically affect sensible capacity only. If DB is raised, but humidity ratio remains the same, dew point temperature will remain the same as well, and if we assume that cooling surface temperature will also remain the same, latent capacity will not change. Your space latent load, however will decrease with the conditions mentioned.

 

[EnOm]

@Drazen

Thank you for your answer.
Introducing outside air will not increase my space load because I am introducing it directly at the coil. So it will affect the coil load only. So in that sense I guess the theoretical situation still holds true?

I do agree with you that there are several factors and changing one will affect the others. This is issue is not stopping my work or anything, I was just intrigued by the concept that hotter airflow into the coil will actually raise the unit's cooling capacity. I suppose I need more knowledge about the refrigeration cycle to be able to figure this one out.

Regards

 

[Drazen]

that is matter of heat transfer, irrelevant of whether you are using direct evaporation or hydronic cycle.

terms that you are using might be more precise than mine - usually i use term space load for total space load, meaning ventilation + skin load. anyhow, you are right, coil load will increase and you do not want to do that without purpose.

if you want to get good grip of parameters, try to find some air cooling heat exchanger software, there was one swedish called coils for windows, but it is not available any more. plate heat exchanger software may help as well.

theoretically looking at it, all processes are described by non-linear differential equations of second order, which do not heave real solutions (real as mathematical term) as there are much more unknown variables than boundary conditions, so only iteration can lead to approximate solution.

 

[EnergyProfessional]

Enom: I think where you are wrong is that you can add more warmer air to the coil to increase its capacity. But this doesn't help you cooling the space because you also added more load to the coil (more than the increase in capacity).

for example if you space is 75°F and you recirculate it through the coil to cool it to 55°F at 1000 cfm you get a certain cooling capacity for both the space and the coil if your coil has 45°F water at a given flow. Now don't recirculate but use 100% outside air at 90°F. this will increase the coil capacity since the heat transfer from the 90°F air to the water is much greater. so far you are correct. but you also need to provide more power to cool the water (since your return water now will be warmer).

and the increase in coil capacity is not so big that you cool the air to 55°F, but only to 65°F, this gives you much less cooling at the 1000 cfm flowrate.

I just made up the numbers, with cooling you also need to consider humidity etc. but from what I understand you want to do you don't get more cooling, you get less at higher energy cost.

 

[EnOm]

Thank you all for the replies. I apologize for my late response, I was extremely busy this last week.

@Drazen. I did just that, starting to get a much clearer picture.

@HerrKaLeun: What you said is correct. To follow up on what you said, the cooling load that is actually reaching the space can be calculated from the unit airflow rate and the temperature difference between the space design temperature and the supply temperature. The now lower (dT) will result in a smaller heat transfer and hence less sensible capacity.

Best Regards.