INGENIEUR A tongue-in-cheek Guide for the HVAC Engineer :
FEATURE
INGENIEUR A tongue-in-cheek Guide for the HVAC Engineer :
Designing for the ‘ Four Seasons ’ in Malaysia
FEATURE
By Ir . Au Chong Hun
It has been said that Malaysia is endowed with ‘ four seasons ’ — the hot and humid season , the rainy season , the haze season and the durian season ! All these four seasons have direct bearing on the HVAC engineer .
The hot and humid season is probably where we spend most of our design efforts in . When we design , we estimate the maximum sensible and latent cooling load that would ever be anticipated in a space . We like to think being in the tropics we only have this one season . One peak design condition — one size fits all . Then we cross check with the golden rule : the rule-of-thumb . Then we can go to sleep with a peace of mind assuming that as long as it is cold enough , there is no problem .
Along comes the rainy season . Days when the sun is not as hot . Days when the outdoor air moisture grains per pound shoot up 20 points compared to the design day . Rainy season represents the season of part loads . It can also disguise itself in the form of bad business , low occupancy , after-hours or night operation , or any other of the one hundred and one occasions . This is the season when our part load operability and performance is put to test .
But then no worries . That is why we have controls . We have installed dry bulb temperature sensors and valves . Modulate the valve when the space temperature reaches the set point . It used to work fine in the good old days . That was because previously , the sensible load was substantial and dominant . We had a natural occurring “ heater ” to keep the relative humidity ( RH ) within acceptable range . Today , sensible loads are saying goodbye — we have state-of-theart facades to ensure they don ’ t come in , LED lighting which takes away 10W / m 2 and low-plug loads , thanks to the green lease guide , etc . All these green awareness and knowledge actually works and people are switching to laptops and even iPads ( anybody still remember CRT ?) and are behaving themselves by not leaving the computer on after work , giving the excuse of waiting for automatic updates when in actual fact they were just plain lazy . All of a sudden , without all these “ fixed deposits ” we are no longer sure ( in fact we never did ) whether a 24 0 C thermostat reading on your BMS actually means 24 0 C @ 65 % RH or 24 0 C @ 85 % RH . The valve should close or modulate in response to the former condition but not the latter .
So , designers today need to think harder as what worked previously might not work today anymore . Probably start by being realistic and not assuming the high quantum of sensible heat load which may not be there anymore . Or be it installing a relatively inexpensive RH sensor as an extra parameter into the control equation , making full use of the fan VSD ( Variable Speed Drive ) which probably is already there , pre-treating outdoor air to a condition so that it can help out in dehumidification , decouple sensible and latent cooling ( whatever that is ), incorporating active or passive dehumidification devices , recover unwanted heat for reheat from the return or the condenser water or even gas-reheat for DX systems . The designer today simply needs to put in more brain juice . But please don ’ t use this as an excuse to put back the dinosaur electric heaters !
Your variable primary flow chiller plant may have been sized for maximum efficiency during peak load but then you realize that you struggle to
56 VOL 94 APRIL-JUNE 2023