Ventilation air treatment in hot and humid climate 1

[SAK9]

I was recently looking at the design of HVAC system for an apartment building in the Middle East. The apartments are cooled by chilled water fan coil units. There are couple of central chilled water Air Handlers on rooftop that conditions the ventilation air and distributes to various apartments through ductwork that drops down from top to bottom. Ventilation air is cooled down to 12C and then reheated(by use of electric heaters) to 24C before being distributed to the apartments. I could not quite understand why the air needs to be reheated in this instance. If air was supplied at 12C it would have provided some cooling and more importantly significant amount of electrical energy could have been saved! Any thoughts on why this inefficient design is used in those parts of the world?

 

[berkshire]

Re heat is often used to get the relative humidity down.
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[IRstuff]

The pre-cooling condenses the humidity out of the air, not dissimilar to a normal A/C's evaporation coils needing a moisture outlet for the same reason. Then air is then warmed up, but the RH will be very low, so it's the combination of cooler air and dryer air that helps keep the occupants cooled, since the lower RH allows body sweat easily evaporate, cooling the body.

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[dbill74]

Consider the entering air/mixed air temperature at the room FCUs. If the ventilation air is not reheated, when it mixes with the return air from the rooms, the mixed air temperature will lower than if the vent air is reheated. If the coil EAT is too low to start with (starting RH will be higher than the coil is designed for), you will freeze the coils.

Another issue is that today's building codes mandate a continuous flow of ventilation air. When the room thermostat is not calling for cooling and you introduce non-reheated ventilation air, it will cool the space to uncomfortable levels.

Without reheat, the air from the DOAS is going to be at 100% RH (or very near). This can cause mold problems inside the ductwork, or if the ductwork is metal, premature rusting and corrosion.

Colder air in the ductwork will also require additional insulation to prevent condensation forming on the exterior of the insulation.

Reheating the air is unfortunately a necessary energy usage to prevent other problems when dealing with high humidity outside air.

 

[SAK9]

Since posting the query I have done some more research and learnt the following :

Reheat is not required in the flowing situations :

[ul]
[li]If the ventilation air is directly introduced into the room.[/li]
[li]Cold ventilation air is not likely to overcool the space when it is unoccupied[/li]
[/ul]

 

[quark]

As suggested in previous posts, the reheat is for RH Control. When our sensible heat ratio is less than 90%, you may not be able to cool the area and reduce RH with our reheat. A simple way to check is to draw a line on psychrometric chart (at SHR) and see if the room entry point and room return point can be able to reduce moisture to required level. If this is not possible, you will extra cool the air and reheat to bring it back to room entry point.

 

[urgross]

If you want to control sensible temperature and RH at the same time a reheat, or desiccant, is needed. Going thru that now. Thinking about using an APR, but haven't used it before. Anyone here had any experience using them? Last time that came up (on a different site) I used a reheat. No problems since.

 

[GT-EGR]

As long as you have a constant source of heat gain in the space that will equal the amount of reheat the ventilation air would have received in the ductwork, then yes it’s a waste of energy to reheat that air.

But ventilation is typically delivered neutral because that source of internal heat gain may not always exist in every single space that it serves. Whether or not it’s day night summer winter occupied or unoccupied.

Spaces are cooled and simultaneously dehumidified by 12C air regularly, from all types of systems - because it is being provided to offset a specific heat gain. Otherwise you’ll just end up with a cold and high RH room.

Even if it is ducted into the return it may or may not cause an issue, depends on how the unit cools, and if fan operation and cooling cycle on is controlled separately or together. 12 C on its own won’t freeze up anything, and 12C mixed with a much larger quantity of room air also won’t freeze anything.

 

[MV Rupesh]

When the air is hot, it can contain a large amount of moisture vapour, ... Dry air - exists when all of the contaminants and water vapor have been removed .... Enthalpy is the measure of heat energy in the air due to sensible heat or latent heat. ... RH should not be confused with absolute humidity which is defined as the actual.All the comment are correct
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