Energy recovery from HVAC exhaust is an important matter on the point of view of energy efficiency.
In pharmaceutical industry, biotechnology, research labs and hospitals, 100% fresh air is very common and most times mandatory. In a 100% fresh air system the exhaust air stream contains a big amount of thermal energy thus to recover this energy is a very important matter for the energy balance.
Energy recovery on exhaust air stream is a well known subject on standard HVAC. Among the different recovery systems, plate heat exchangers could be the system with the best ratio comparing energy recovered, investment cost and maintenance cost. Recovery ratio in a plate exchanger is bigger than 50%, free maintenance and not expensive installation cost make them very appreciate for energy recovery on standard HVAC. However, in some fields of pharmaceutical industry, containment labs and especially in hospitals, plate heat exchangers have a bad reputation and sometimes they area expressly forbidden. Why a simple, cheap and efficient device is banned in fields in which the energy saving could be decisive?
2. The Black Legend
Mainly the rejection for plate exchangers is because plate’s manufacturers admit they are not totally airtight and a leakage rate of 0,1% could be expected with pressure differences of 400Pa. (Note that there are many architectural elements, also used in hospitals with leakage ratios higher than this value and their manufacturers and users consider them “totally airtight”) Nevertheless in some pharmaceutical and hospital fields this leakage ratio, particularly for the case of plate exchangers, is declared “unacceptable” and plate exchangers are banned without further defense.
3. Breaking the Myth
Without wanting to discuss whether 0,1% is a lot or a little, has nobody asked where the leakage is going to? Or where could the leakage be directed to?Because, what the matter is if the fresh air is mixed, in a proportion of 0,1%, with the “contaminated” air and then it is exhausted outside? Why it should be that “contaminated” air is mixed with the fresh air stream and go back inside?
What happen inside a plate exchanger is subject to the physical laws, if there is a leakage inside the air will go from the higher pressure side to the lower pressure side. Thus, if it is possible to assure that the “contaminated” air remains at lower pressure than fresh air there will not be any possibility of contamination. Note that inside of containment areas a differential pressure of 10-15 Pa between “contaminate” and “no contaminate” areas is considered safer enough. And both areas are separated by means of doors with a leakage ratio well high of 0,1%
Nevertheless, it is advisable to analize what is happening inside of the exchanger. In order to get an efficient interchange ratio it is necessary a high turbulence level in the air stream and this means pressure drop. In fact pressure drops lower than 100 Pa produce a poor performance and the bigger the pressure drop the bigger the energy recovered.
Operating pressures inside of a plate exchanger range from 150 to 250Pa. Less than 150 Pa the exchanger is inefficient, over 250 Pa the fan consumption could be too high, thus values from 175 to 200 Pa could be considered adequate. Due to the exchanger construction there may be some areas where the pressure pattern changes. On figure 1 it can be seen that, despite of the fact the contaminate stream (red) has lower pressure than fresh air stream (blue), there is an area inside of the exchanger (blue line) where the “contaminate” air has a higher pressure than fresh air and then the contamination of the fresh air with exhaust air could be possible .
For this reason it is very important selecting the pressure drop inside of the exchanger related with the pressure drops out side of the exchanger, in order to avoid “inverse” pressure areas.
Figure 2 shows the most common scheme for a exhaust system with plate exchanger. On exhaust side there are HEPA filters with pre-filters, normally at least F8 plus the duct network. On the fresh air side there is just the filter that protects the exchanger, normally F8. Then it is clear that exhaust side has a pressure level quite lower tan fresh air side.
Putting these values on the exchanger scheme it (Figure 3) can be seen that the minimum differential pressure between both streams is 150Pa, always from fresh air to contaminated air, never on the opposite way, even taking into account the possibility of theoretical Venturi effects.
Whit a proper installation of plate exchangers the differential pressure between “contaminated air” and fresh air could be kept on the level of 75-100Pa (on the worst part of the exchanger) This is at least ten times the pressure considered “safer” inside of the containment areas. On the other hand the exhaust air stream is filtered by HEPA, it means the air is 99,999% cleaner than the environmental inside of the containment area. After that the leakage rate is just 0,1% and finally plate exchangers is the cheapest recovery system for installation and maintenance, with recovery ratios higher of 50%. So…
Who’s afraid of plate heat exchangers?