In this post, I am going to define a few terms related to economizer processes so we are all “on the same page” for what will follow as I move forward with the W7212 discussion. I am writing this at a fairly basic level, assuming that the reader has limited knowledge of HVAC processes and terms.
Generally the post builds using one topic as the foundation supporting the next. Those with more familiarity may want to just skip this post or use the Contents section below to jump to a particular topic of interest. There is a “Back to Contents” link at the end of each section that you can use to jump back to the contents if you want.
- The Modern Building; A Year Round Need for Cooling
- A Source of Cold Air
- Meeting the Need for Cooling with Cold Outdoor Air
- More Outdoor Air than You Need to Ventilate
- Relief Fans vs. Return Fans
- Free Cooling
- Free Cooling
- Integrated vs. Non-integrated Economizers
- What’s Enthalpy?
- Integrated Economizers and Mechanical Cooling Equipment Loads at Low Outdoor Temperatures
- Integrated Economizers and Mechanical Cooling Equipment Loads when its Hot and Humid
The Modern Building; A Year Round Need for Cooling
For starters, we need to recognize that in a modern building, because of the equipment, lights, and other heat generating processes that are going on, there can be a net cooling load on the facility, even in the dead of winter. For example, consider this floor in a high-rise office building in San Francisco on a January day.
While the perimeter zones (orange highlight) may be influenced by the frigid 40°F outdoor conditions that can exist in San Francisco in January, they also serve as a buffer from the ambient environment for the core zones (red highlight).
As a result, if you were to look at the hour by hour loads for the floor, you would see something like this on that 40°F January day.
Any time the red line in the graph above goes above 0 Btu’s per hour, you will need to provide cooling to the space if you don’t want it to overheat.
A Source of Cold Air
If you look at what is going on outside on the day associated with the load calculation above, you will discover that temperatures might be running in the low 40-50°F range, as illustrated by the light blue wide line below.
Generally speaking, the discharge temperature required by most HVAC systems serving office spaces like the ones we are talking about will be in the 52-65°F range. How you arrive at the specific value is a topic for a different discussion.
But if you accept the proposed range for this type of facility as being true and consider the outdoor air temperatures that exist at the time we are discussing (January in San Francisco), you will conclude that you should be able to get the discharge temperatures you need by bringing in outdoor air instead of running mechanical cooling.
Meeting the Need for Cooling with Cold Outdoor Air
Air Side Economizers
Specifically, if you blend the right amount of outdoor air with the warmer return air in the building, you should be able to get any supply temperature you want as long as what you want is a temperature somewhere in-between the current outdoor air and return air conditions.
If you did that, you would be cooling the building with out running mechanical refrigeration and would have created an air side economizer.
Water Side Economizers
You could also leverage low wet-bulb temperatures using a cooling tower to generate cold water and then use the cold water to cool the building instead of running a chiller.
I’m not going to be talking about water side economizers in this string of blog posts, but if you want to know more, take a look at Cool Tools, which is a free resource that includes a design guide focused on chilled water plant design along with some spreadsheet tools that will help you with your design process.
More Outdoor Air than You Need to Ventilate
Economizer equipped or not, most buildings will bring in some minimum amount of outdoor air for ventilation purposes, primarily to control contaminants. For the ventilation process to be effective, there has to be a way for the air that you bring into the building to exit the building; as Al Black, my mentor would say;
The goes intas gotta equal the goes outas
In other words, conservation of mass. Typically, ventilation air will go back out of the building via toilet exhaust or process exhaust systems.
Its important to realize that economizers bring in outdoor air beyond what is required for ventilation purposes. And economizers need to comply with the conservation of mass principle. As a result, they need some sort of system to get the air that is brought in by the economizer cycle back out of the building.
Some folks call the dampers that are required for this purpose the “exhaust” dampers. I prefer to call them the “relief” dampers to distinguish them from the systems that take the ventilation air back out of the building.
Relief Fans vs. Return Fans
A subtle but important detail that comes up if you start talking about economizers and the need to relieve the air that is brought into the building for the economizer process is the difference between a relief fan vs. a return fan. The following slides from the VAV class I teach summarize the differences.
In the context of a discussion about an economizer process, the difference between return and relief fans is that return fans will generally run any time the air handling system they serve is in operation because they are involved in addressing pressure drops that occur regardless of if the system is using an economizer process or not.
In contrast, a relief fan only will need to operate if a system is using its economizer process. In fact, the relief fan may not need to run until the economizer is bringing in enough outdoor air that it is over-pressurizing the building. When the system is on less than 100% outdoor air, building leakage may be sufficient to accommodate the extra air the economizer process brings into the facility.
Both the wet economizer and the air side economizer are referred to as “free cooling” in the industry because you provide some or all of the cooling you need with out running a mechanical refrigeration process. But like anything else, its important to recognize that its not totally free. For an air side economizer, some of the potentially “not free” items include:
- Dampers that are properly sized to provide the modulating control required by an economizer process will probably have more pressure drop associated with them than, for instance, the intake isolation damper provided on a 100% outdoor air system.
- Relief fans may be required, which will use energy that would have not otherwise been used had there not been an economizer process
For a wet economizer process:
- Pumps will need to run to distribute water to the cooling towers and to the air handling unit coils.
- The cooling tower fans will need to run, even though the chiller is not running. And because the desired supply temperatures are lower than what would be used for a chiller, the fans will probably run harder all other things being equal
My point here is to emphasis the word “Cooling”; its important to recognize that an economizer process is a cooling process. That means if all of the zones served by the economizer equipped system need heat (i.e. there is more energy leaving them via the walls, roof, and windows than entering them from the internal heat gains in the space like the lights and equipment), then the economizer process should be shut down and the system should only be bringing in the outdoor air required for ventilation.
Integrated vs. Non-integrated Economizers
At first blush, you might think that the term “integrated economizer” is a reference to the design integration perspective; in other words, the economizer process needs to be integrated with the other processes that are going on in the air handling system is is associated with. And that certainly would be true. In that context, there should only be integrated economizers.
But, if you are talking economizers with someone, there is actually a much more specific and different meaning to the terms “integrated economizer” and “non-integrated economizer”. Specifically, an integrated economizer cycle allows the air handling system to use outdoor air while running mechanical cooling as long as it provides an energy efficiency benefit.
In contrast, a non-integrated economizer process will not try to use outdoor air for cooling if it can not provide all of the required cooling solely with the outdoor air.
To clarify this lets consider an example. Specifically, lets say that we have a system that wants to make 58°F supply air, has an economizer, and is operating on a day when it is 57°F outside. (For the purpose of this discussion, we will ignore fan heat and assume leak-free dampers, just to keep it things simpler).
Outdoor Temperature Below Desired Set Point
Irrespective of if the economizer is integrated or non-integrated, at this point in time, the system would be using nearly 100% outdoor air, recirculating only enough return air to mix with the 57°F outdoor air to bring the mixed temperature up to the desired 58°F set point.
Outdoor Temperature At Desired Set Point
If the outdoor temperature is equal to the desired set point, both an integrated and non-integrated economizer will still look the same. The return dampers will be completely closed and the system will simply be moving air that is at the desired temperature from outside the building into the building.
Outdoor Temperature Above Desired Set Point
This is when the two processes will behave differently. When the outdoor air temperature is above the desired set point, say 59°F outside with a desired set point of 58°F, the integrated economizer cycle basically gives up on using outdoor for a source of cooling. It reverts to minimum outdoor air and provides what ever cooling is necessary by running the mechanical refrigeration equipment.
In contrast, the integrated economizer process continues to use the outdoor air stream when it is warmer outside than the current supply temperature set point. The reason is that until the enthalpy of the outdoor air is higher than the enthalpy of the return air, it will take less energy for the mechanical equipment to cool the 100% outdoor air stream vs. the return air plus minimum outdoor air stream.
For those who are not familiar, enthalpy is a scary sounding word that is referring to the total energy content of the air. In general terms, the energy content of the air is a function of two things:
- Sensible energy, which is the energy that we feel (sense, thus the name) as making things hotter or colder, and
- Latent energy, which is the energy associated with keeping the water content in a vapor state. Its not as readily visible (thus the name), at least not until we cool the air to the point of condensation, at which point we get a cloud and then a puddle.
Enthalpy is typically not measured directly. Rather, temperature and some other parameter that is an indicator of moisture content, like dew point or relative humidity or wet bulb temperature are measured and then enthalpy is calculated using equations of state or read from a psych chart.
There is a lot more that could be said about enthalpy, but for the purposes of this discussion, all you really need to understand is that:
- It’s a function of both temperature and humidity.
- Increasing temperature or increasing humidity or both will increase enthalpy and vice versa.
- If you have two air streams with two different enthalpies, it will take less energy to cool the air stream with the lower enthalpy, all other things being equal.
Integrated Economizers and Mechanical Cooling Equipment Loads at Low Outdoor Temperatures
If you are someone concerned with energy efficiency, then you are probably asking yourself:
Why in the world would anyone use a non-integrated economizer when you could save energy by using an integrated economizer?
That’s a good question and to understand the answer, you have to look closely at the load that the cooling coil will see under the two different operating cycles in the light of the ability of the mechanical equipment to match its capacity to the requirement.
Comparing the Cooling Capacity Requirements
If you plotted the non-integrated process when it was 59°F outside on a psych chart, it would look like this.
If you plot the same point in time for an integrated economizer process on a psych chart, it looks like this.
In the first chart, the load on the cooling coil is represented by the line between the “Mixed Air at MOA” point and the “Cooling Coil Discharge” point.
But in the lower chart, it is the difference between the “Outdoor Air” point and the “Cooling Coil Discharge” point. The two points are almost on top of each other, so the latter is a much shorter distance, which, on a psych chart, means a much smaller enthalpy change, which means a much smaller load.
If you put some numbers to this, you get the following result.
Specifically, the load on the cooling coil for our system at the point in time under consideration is reduced by an order of magnitude if we use an integrated economizer instead of a non-integrated economizer. And the energy savings associated with that is the attraction. But the problem is that our mechanical cooling system needs to be able to run efficiently and with out undue stress at the lower load condition for the energy savings to be realized and that is not an easy trick to accomplish.
Matching Capacity to Requirements
Virtually no current refrigeration machine can efficiently unload and operate from full capacity to 0% capacity. The specific lower limit will be a function of the technology. For instance:
- Reciprocating chillers with mechanical cylinder unloaders will be limited by how many cylinders they have unloaders installed on (the unloaders typically hold the valves open so that the compression stroke does not happen).
- Centrifugal chillers will be limited by the fluid mechanics of the impeller design interacting with the pre-rotation supplied by the inlet vanes and the speed vs. performance characteristic of the entire system if there also is a VFD involved.
- Packaged equipment with on-off type compressors will be limited by how many compressors there are and their sizes.
Considering the Packaged Equipment Case
To gain some insight into this issue, lets assume that the cooling coil in the system we have been using in our illustration has one 10 ton compressor and two 15 ton compressors for a total capacity of 40 tons. Each compressor can be turned on and off as the means of capacity control. By managing the various compressor options, the following levels of capacity are available:
- 0 tons (all compressors off)
- 10 tons (smallest compressor on)
- 15 tons (one larger compressor on)
- 25 tons (small compressor and one large compressor on)
- 30 tons (two larger compressors on)
- 40 tons (all compressors on)
On a 59°F day, if this system was operated with an integrated economizer cycle, its only choice would be to turn on 10 tons of capacity to handle the 2.3 ton load associated with cooling the 100% outdoor air stream from 59 to 58°F.
Of course, if you try to do that, you will over-cool significantly and the discharge temperature will plummet, quickly turning the compressor back off. But, since you really do need a little bit of cooling, the discharge temperature will rise right back up again and the cycle will repeat.
This is called short cycling the compressor and it can cause a number of problems.
- Discharge temperatures and thus, the conditions at the load are very unstable. At a minimum, this can lead to comfort and quality control problems. In a hot and humid environment, it can lead to indoor air quality problems with the portions of the system that are cooled during the compressor run cycle continue to condense water out of the air stream after the compressor cycles back off.
- The heat associated with the motor start inrush current is not given sufficient time to dissipate. This stresses the motor insulation, which can cause it to fail and lead to a motor failure.
- The lubrication cycle associated with the operation of the compressor never has time to establish itself, which can lead to a failure of the compressor itself.
The details of those issues are actually the target of a different series of blog posts that I am working on related to a string I started about solving a short cycling problem by creating a thermal flywheel. For the purposes of our discussion, you just need t o recognize that an integrated economizer cycle can create the potential for short cycling because at low outdoor air temperatures, the load on the cooling coil will be very small.
Integrated Economizers and Mechanical Cooling Equipment Loads when its Hot and Humid
Outdoor air temperature and humidity interact with an integrated economizer at another point. Remember that the reason we are using an integrated economizer process is that we are trying to save energy by selecting the air stream that will require the least energy to cool. So somehow, we have to figure out when it no longer makes sense to cool the outdoor airstream from an energy standpoint.
What’s the Right Change-over Set Point? It Depends
This is not a “one size fits all” discussion. Rather, it’s a discussion about “hot and humid” from the perspective of the conditions in the return air stream, which are usually set by the conditions in the space. For instance, consider this psych chart comparing a typical office space with the outdoor condition in our example with the conditions that might exist in an orthopedic surgery.
With an enthalpy of 22.9 Btu/lb, the outdoor air is dry both dry and cool compared to the air coming back from the office space at 26.4 Btu/lb. So if the economizer equipped system was serving the office on the day we have been discussing, then it would make sense from an energy standpoint to keep using outdoor air on an integrated economizer cycle to serve the space (as long as we didn’t ruin compressors doing it).
In contrast, when compared to the enthalpy in the surgery space (21.8 Btu/lb) the energy content of the outdoor air stream is higher than the return air stream coming back from the space. Even though the air is cooler outside (less sensible energy), it is more humid (more water vapor) than the surgery, thus it has a higher energy content. If we were to introduce it into the surgery, we would increase the humidity above the target that was likely set by the hospital licensing agency or the hospital’s infections control department.
Making the Right Decision
As you are probably guessing, the technically perfect way to decide if you should or should not use outdoor air for an integrated economizer process is to compare the enthalpy of the outdoor air with the enthalpy of the return air. Unfortunately, that is easier said than done. As you will recall, we don’t measure enthalpy; rather, we measure temperature and some indicator of humidity.
It turns out we have a hard time measuring humidity accurately enough to make a good decision. Maintenance plays into that too; humidity sensors are maintenance intensive. To gain some insight into the issues with humidity sensors, take a look at the NBCIP humidity sensor reports that are linked under Commissioning Resources on the right side of the opening page of the blog.
A more persistent way to make a decision that will be right most of the time is to compare the return air temperature to the outdoor air temperature and make the decision on that basis. You can even make a pretty good argument for simply comparing the outdoor temperature to a set point that is based on the conditions in the space and the characteristics of the climate at the location under consideration.
My point here is not to go into the details of all of those options. Rather it is simply to bring the subject to the table because it plays into the discussion that will follow. If you would like to know more about the various change over strategies:
- Chapter Three of the Functional Testing Guide includes a fairly detailed discussion of the topic along with other economizer related issues.
- Steve Taylor wrote a very comprehensive article on the merits of a dry-bulb temperature based economizer change over strategy in the November 2011 ASHRAE Journal.
- I discuss the topic in the Economizer Class we do at the Pacific Energy Center. The class should show up in the Spring Semester and will likely be a webinar in addition to a live class. So watch the PEC Classes web page if you are interested. Meanwhile, the slides from the most recent class are linked from the home page of the blog under the “Materials from Classes and Presentations” on the right side of the home page.
Having said all of that, I think I have laid the groundwork for discussing the questions that came up out of the lab. So I will start down that path with my next post by exploring how the specific location selected for the sensor used by the W7212 as its input for the economizer control process can make a very significant difference in how the system will perform.
Senior Engineer – Facility Dynamics Engineering