Today, as scientists are still struggling to confirm, whether Sars Cov – 2 virus is airborne or not, several concerns on running air conditioning systems in buildings are being raised.
With temperature rising and offices opening after lockdown, human comfort in occupied space is important as there is a direct correlation in human comfort and the immune system of our body. Hence, switching off air conditioning is seemingly not the right approach to address preventing virus spread, if any, through air conditioning.
Ventilation of conditioned spaces to dilute the indoor air pollutants in the space is becoming savior during this pandemic. Many International papers are now recommending increasing ventilation in occupied spaces and even suggesting 100% outdoor air systems.This increased amount of ventilation should definitely solve IAQ related problems. However, the inability to maintain the right humidity by our HVAC systems may lead us to other problems. Mold & Mildew are serious dilemma in itself, which are caused by lack of humidity control. Also, the growth of viruses & bacteria can augment if proper humidity is not maintained in the conditioned spaces. The question is “Have we Traded one problem with the other”.
Figure 1: Ventilation Load index Mumbai
Shown above is the load profile (refer Figure 1) of the outside air for the city of Mumbai. The curve defines the load of outside air in terms of the sensible and latent load in tonnes for the entire year. Clearly, one can see that latent load of the city is quite high and is around 78% of the cumulative fresh
air load.
Similarly, if one studies the profile, the cumulative loads (i.e cooling & dehumidification only) for major cities of India, one can see that latent load component of the fresh air is quite high (ranges from 60% – 85 %) (refer Figure 2).
With outside air bringing in high amount of latent energy, RH management becomes difficult. HVAC fraternity was first sounded the wake up call for IAQ & RH control, when the bacteria spread by a hotel air-conditioning system, had killed 34 people and ailed more than 200 people at American Region Convention, Philadelphia (US), around twenty years ago.
Lack of RH control leads to growth of Mold and Mildew which lead to various health related issues. Mold release tiny spores to reproduce. These spores then waft through the Indoor Air and start developing in damp areas. They can cause several problems like rashes, asthma, running nose, respiratory problems including serious diseases like hypersensitivity pneumonites, etc. (refer Figure 4).
Latent Loads
If RH control is so critical and important let us examine the latent load profiles in a building. Comparing the sources of the latent load, it is clear that the largest contribution to latent component is the outside air. The chart (Figure 3) below clearly shows that the almost 50% – 70 % of internal latent load comes from Ventilation
Figure 3: Latent Load Characteristic
Source : Lewis Harriman
Figure 4: An example of passive humidity moisture control in a 3 ton Unit.
Source: Dehumidification Equipment Advances by Lewis g Harriman III
Rh Control Challenge
Why is it so difficult to manage RH? The answer lies in the fact that the sensible and latent loads don’t peak at the same time. Hence in moderate weather the sensible loads are reduced but latent remains high. With Ventilation bringing in the most of internal latent load one needs to study the ability of conventional cooling system to control RH during moderate weather conditions. A thermostat driven cooling coil will experience great difficulty in managing the RH in low sensible load periods. i.e. off peak periods of the day or monsoon weather in most of India. The problem is that in moderate weather, the outside temperature drops but the moisture level remains high. And, with the ambient temperature dropping, the sensible loads drop. Hence, the return air temperature is quickly achieved thereby triggering the thermostat to switch off the compressor in the constant volume DX cooling system, allowing them to operate only for short periods. As a result the moment the compressor turns off, the coil stops dehumidifying and moisture remaining on its surface re-evaporates back into the supply air. (refer Figure 4).
Even in immaculately designed central plant system, where designing for very low ADP’s and reheat, one faces difficulty in controlling RH during low sensible loads. Such systems do have better RH control, as compared to constant volume, DX cooling systems but are highly energy intensive (involve sub cooling and then reheat) and are increasingly getting banned in many countries.
With Moderate weather being a considerable part of season during Monsoons and throughout the year in many coastal cities, this problem needs some immediate attention. Consider the grain levels in the following data at the outside temperature of 75OF (i.e moderate weather) one can see considerable number of hours) where the grain level is more than 65 gr/lb (moisture level generally maintained inside).
On studying the hourly data of outside air, one can see that RH control definitely is a problem in the moderate weather, which is almost 2500 hrs in a year for city like Atlanta in US.
Control Strategies
Traditionally, one would immediately talk about a system with low ADP. i.e., having low chilled water temperature, high row deeps
(8 row or deeper) and reheat with active energy.
Such systems do help but are highly inefficient and drain lot of energy. Figure 5 illustrates the fact that one has to first sub-cool and then add active reheat wasting energy twice.
DOAS Approach
Constant Volume, mixed air, HVAC units are generally selected with sufficient cooling capacity to handle dry bulb design and are controlled by thermostat, which matches the sensible cooling capacity of the coil with the sensible cooling need of the space. But, when it is cool and rainy outside, latent cooling load can approach or even exceed sensible cooling load. To overcome this problem, one needs to divide the load into the two components i.e. ‘Sensible’ and ‘Latent’ and handle them separately. This approach commonly referred to as the “Divide and conquer” deals with both components separately.
As already identified, majority of the internal latent load is coming from the outside air. Hence, It is important that the latent load of this air be handled separately. The DOAS approach works on this principle only. It removes all the latent load being brought by the outside air at the source and processes the same to a very low dew point, thereby enabling it to take care of the rest of the internal latent load too.
The internal cooling devices are then limited to sensible cooling only. If one wants to draw an analogy its like making the “Naughtiest Boy” of the class as the “Class Monitor”.
This approach now opens up a whole new world of innovative designing and helps the designer to maintain the right RH throughout the year irrespective of the weather pattern outside. The IAQ and RH management both get resolved and one is able to overcome all obstacles being seen by the conventional systems.
The designer can now increase the chilled water temperature (CHW) feeding the Internal Air Handling units (AHU’s), reduce the row deeps (as ADP’s can be increased) and can optimize between the air volume and ADP (by CHW temperature), as one now is not limited by Sensible Heat Factor (SHF) any longer, to get the maximum energy and space efficiency.
Part 2 of it will be continued in next issue of Cooling India.
Rahul Aeron,
Assistant Vice President, Sales,
DRI, Pahwa Group.
