What purpose should we keep in mind when designing Fresh Air Handling Units (FAHUs)? This was a question that Rajat Upadhyay, Consulting Sales Manager at SKM Air Conditioning, posed to those who attended the technical seminar his company organised on November 24, in Dubai, UAE. The attendees offered varying answers, including ventilating and cooling the indoor space and maintaining the thermal comfort of building occupants. Their answers, noted Upadhyay, while good, missed one of the most critical roles of FAHUs, and that is dehumidification.

“It’s very simple,” he said. “Our Fresh Air Handling Units are not only all about cooling but also dehumidification. After all, where does air supplied to the FAHU go? It goes to the indoor unit, whatever we have, and if it has a Fan Coil Unit (FCU), then it goes to the Fan Coil Unit. It goes to the chilled beam. And what is the primary function of the indoor units? To cool the air, yes, but there is a fundamental difference between the outdoor and the indoor units.”

Elaborating on his last statement, he said: “The indoor unit is mainly designed to remove the sensible load. If you get more latent load on the indoor unit, it would mean more moisture, and then you’d have a problem in the building, particularly fungus growth and increase in humidity. Therefore, we need to make sure, whenever we are supplying air from the outdoor unit to the indoor, the air should be dry. That way, your indoor unit does not fall short or become ineffective.”

Not all about the RH

The problem with the industry, Upadhyay pointed out, was that in discussions on moisture, the focus was often limited to Relative Humidity. “When we talk about moisture,” he said, “the only thing on our minds is RH. We develop our designs with the goal of maintaining 55% or 60% RH in the room, at 24 degrees C. That is our primary target during the design phase of the system. Most of the time, we don’t think about dew point and absolute humidity. But we should, because if the system is not effectively and properly designed to remove the moisture in the air, it would produce the kind of issues I mentioned earlier, resulting in a poorly performing building – a problem that would be difficult to resolve.”

Trying to illustrate his point, he presented two conditions to his audience, and asked them to vote on which one they regarded as the more comfortable condition. The first condition involved a temperature of 55 degrees F (12.78 degrees C), an RH of 100% and a dew point of 55 degrees F (12.78 degrees C). The second, on the other hand, had a temperature of 95 degrees F (35 degrees C), an RH of 50% and a dew point of 74 degrees F (23.33 degrees C). Of those who voted, a majority went for the second condition. They were, however, wrong, revealed Upadhyay.

“Most people would think that 100% RH is uncomfortable for human beings and that 50% RH is better for us,” he said. “But that is incorrect. Let’s look at dew point. What is it? Dew point is the point where moisture in the air starts to undergo condensation. When your dew point is high, air starts condensing much quicker than if the dew point was low. So absolute moisture and dew point – those two are extremely important and just as critical as RH.”

An oversized issue

Disclosing that all his information came from the ASHRAE database and was backed by research, he pointed out that other than lack of attention paid to dew point and absolute humidity, oversizing was another common design error in the industry, and it was one that also affected the air quality of an indoor space.

“Moisture problems can also be the result of oversizing the equipment,” said Upadhyay. “As a supplier, we get enquiries from contractors or designers who think that bigger is better. ‘Why 1.5 TR? Make it 2.2’, they’d say. Their reasoning is that the equipment should not fall short. That is not a practical approach, though.”

Explaining his claim, he added: “A slightly under-designed system is actually better, because it will work continuously. But if you oversize, your FCU will automatically switch on or off, depending on whether or not the set-point temperature has been reached. So your FCU could be working for 40 minutes and then switch off after 10 minutes, and when it does, moisture will be back in the air and into the room.”

In connection with the scenario he had described, Upadhyay emphasised the advantage of using a DC inverter FCU: “The role of the DC inverter Fan Coil Unit is very important, because it matches the blower speed or blower rpm to your requirement. So whenever you are reaching your indoor set-point temperature, the blower will immediately reduce the rpm, and the chilled water flow modulating valve is controlled. Your unit is, thus, working with your cooling and helping improve humidity. That’s why I say that with the inverter FCU, you have a better RH and better temperature control than with a 3-speed or 4-speed FCU.”

Going back to an earlier point for emphasis, he continued: “You should always remember that over-designing is not good for both your system and your design. Be precise in your calculation, or if it’s not possible, then go for being a bit undersized. Never go for a higher sized design, because it will do nothing but over-cool the air and create more problems.”

A new vision

While Upadhyay’s presentation on FAHUs and their impact on IAQ was the main agenda of the seminar, it wasn’t the only one. The attendees – described by SKM as a mix of consultants, contractors and developers – also got an opportunity to listen to presentations on the company’s Applied Product and Unitary Product divisions.

“The aim of the seminar,” said Nabil Abouseido, SKM’s Marketing and Business Development Director, “was to let both our old and new customers learn about our new vision, which we put together almost a year ago.”

The vision, he clarified, was to transform SKM into a multinational company and increase its market share in three different categories. “We divide our business into three categories: applied sales, which consist of the chilled water and the special, bigger DX units; then unitary business, which is the standard residential and white commercial applications; and the third one is the oil and gas sector,” he said. “The seminar was held to explain to the audience our vision as well as inform them on how to contact or deal with SKM. Unitary, for example, mainly goes through dealer networks and partners.”

On the company’s aim to become multinational, he had this to say: “We are looking into being a multinational company and expand in areas where we’ve never been there, like the post-Soviet states as well as in Turkey and East Europe. Perhaps later on, we’ll also look into West Europe. We may work with some people in the industry to join forces.”

Although Abouseido admitted that none of those had been finalised, he was quick to point out that plans were definitely in development. “Everything’s being planned and in the works,” he said, adding, “Business is tough and requires us to fight, but that’s okay, because at SKM, we are good fighters.”