It’s not the heat, it’s the humidity!

Addressing comfort, space conditioning and make up air in commercial kitchens.

I’m sure you’ve heard this before, somewhere.  It implies the heat isn’t that big a deal for comfort, it’s more about the humidity that accompanies it.  Well, that’s sort of true.  Don’t try to tell anyone during summer in Las Vegas on a 106-degree dry day that the heat doesn’t impact their comfort, though!

Humans are affected differently by our surrounding temperature and humidity, depending on several variables.  Two important ones are what we are wearing and what we are doing.  If we are wearing layers of clothes (think of a uniform), we would be warmer than someone else nearby with shorts and a t-shirt.  If we were stacking heavy boxes all day, we would be warmer than someone else who was sitting at their desk and typing emails.  It’s the individual perception of the temperature around them based on these variables that determines the comfort. 

What does this have to do with Kitchen Ventilation, you might ask?  Well, in a kitchen, these same tenets hold true.  Also, a comfortable kitchen is a more productive, enjoyable place to work with less employee turn-over.  This means the restaurant is more likely to be successful.  Who starts a business without wanting to be successful?

To get all the cooking generated grease, smoke and odor out of the building, kitchens exhaust a lot of air.  Thousands of cfm worth.  That air must be replaced to keep the space balanced (see earlier articles on “It Hangs in The Balance”).  When replacing air in winter, it’s easy to add heat and make the air comfortable for the kitchen.  In the summer, this is a little more challenging since we have the heat and that pesky humidity to worry about. 

In a kitchen, where there are numerous heat sources and people actively working to produce meals, folks tend to be more comfortable at a little cooler temperature and/or a lower humidity.  To make that happen, we need to be careful with the HVAC system in the kitchen (keeping the space temperature comfortable), and the accompanying make up air being brought into the space (keeping the space balanced).  These two devices are both affecting the air in the kitchen space.  During the initial design phase of the project, there is an expectation set for the kitchen space – this is called the “design conditions,” and this dictates the equipment that is selected for the project in order to ensure this design condition is met throughout the year.  The design conditions are frequently expressed as one condition for summer (cooling season) and one condition for winter (heating season).  A common design condition can exist, and it is usually around 75 degrees F with 50% RH.  Any equipment selected to meet this criterion must co-exist and cooperate with the other equipment to make the design expectation happen.

Some smart engineers have come up with a way to illustrate the properties of air if we know two pieces of information.  It really doesn’t matter what two pieces if we have two.  They called this illustration a “psychrometric chart.”  Figure 1 is a sample of this chart.  I know, it looks like a mess, and you can hardly read it.  We will simplify this chart and explain how to understand controlling the heat and the humidity so that the kitchen is a great place to create gastronomic masterpieces!

The psychrometric chart tells us about the quality of air, and by extension, about comfort in the space.  You’ll see on the simplified chart in figure 2 an area marked off that illustrates where people tend to be comfortable when they are actively working.   Again, some smart people in ASHRAE came up with these zones.  The bottom of the graph is the “dry bulb” temperature, which is essentially the temperature you read on any standard thermostat.  The curved lines with percentages swooping from the lower left up to the rest of the graph are the relative humidity lines.  This is the percentage of moisture in the air compared to the most moisture that the air can hold.  On the left, the curved line at an angle from top right to bottom left is the “wet bulb” line, which is a measurement of how saturated the air is. 

Now, if we get a little nerdy and look at what happens in kitchens, we will see there are competing interests at hand.  We expect the HVAC unit for the kitchen should be capable of maintaining comfort in the commercial kitchen, even on a hot summer day.  It’s important to remember, too, that HVAC units are most efficient when they are “right sized” for the application and not too big or too small.  The last thing we want to do with a right sized HVAC unit is to pile more work on its plate, which makes it not work properly.  How would we make that mistake?  By bringing in make up air that is warmer and more humid than it should be! 

The above scenario occurs since many kitchen ventilation manufacturers tend to compromise on the design to reduce the tonnage (capacity and cost). That is done when discharge temperatures are higher (around 85 degrees F).  Typically, that would be insufficient to remove the moisture effectively.  It’s certainly a more attractive price with this approach to sell a conditioned make up air unit, but to what benefit?  These units provide a lower first cost because they don’t have the tonnage (capacity), to cool the incoming make up air enough on hot days.  What the commercial kitchen ventilation manufacturers fail to acknowledge is that the space design conditions, and comfort, will not be met when this compromised design is installed, begging the question “why did we buy this unit?” for the lifecycle of the business. 

A properly designed make up air unit will deliver outdoor air cool enough to keep from taxing the HVAC unit for the commercial kitchen.  Using figure 3, if we look at Miami, FL, for instance, the 92-degree dry bulb and 78-degree wet bulb outdoor air can be plotted on the psychrometric chart.  This is the red dot intersection of the two green lines that are plotted from the starting points.  We can then show cooling – the light blue line going horizontally to the left and then curving downward to its discharge condition from the make up air.  When the blue line goes from horizontal to sloping downward, moisture is being removed from the air, which will help us introduce less of it into the kitchen space.  Notice that this example shows the discharge being 70 degrees F dry bulb, 70 degrees F wet bulb.

If we approximate the mixed condition of the air coming from the HVAC unit and the make up air discharge, we’ll see that it ends up within the box of comfort that we showed earlier (figure 4, green dot on the orange line). 

In figure 5, which is the same example, but with a compromised design and lower cooling capacity make up air unit, we can see that the discharge temperature will only achieve 85 degrees F, placing the mixed airstream (blue dot on orange line) outside the comfort zone for the space.  This means the space won’t be comfortable, people will be hot and humid.

When selecting a kitchen make up air unit with cooling, it’s important to get the design right, then work on the pricing.  Without the correct design, it’s a fool’s errand.  If the design constraints need to be changed, then change them in discussion with ownership while setting realistic expectations.  If the equipment that provides a good working climate for the task of preparing food isn’t correctly selected, everything suffers – the employees, the customers, and ultimately the profits. 

As we say in the restaurant industry, there is no free lunch. Proper design is based on the physics of air. When one variable changes, it changes another. Your design engineer can go into greater detail and show you the trade-offs from designing a comfortable energy efficient space or a compromised one.  So, yes, when it comes to humidity, you need to sweat the details. 

Published by:
Chris Lowell
Regional Sales Manager, Halton Company
Follow Chris Lowell on LinkedIn

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Our recommended next article would be: How not to ruin a good thing, Make-up Air 101

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