- Tech Tips
HVAC school demonstrates how to calculate target superheat for fixed orifice metering device AC systems using testo Smart Probes and then how to measure superheat. Featuring Bryan Orr.
If you don’t do a lot of commercial work you may see a system like the one above and wonder what the heck it is? It is a 100% fresh air unit or a makeup air unit and you will see them more and more as codes start to deal with the fact that balanced ventilation and conditioned outdoor air are an import part of a healthy and comfortable building.
What are they for?
Most commercial buildings require a good amount of outdoor air be brought into the building to dilute VOCs (Volatile organic compounds), reduce CO2 buildup and replenish oxygen levels.
In many cases, this fresh air requirement is accomplished through fresh air intakes on the HVAC equipment but increasingly building are using dedicated fresh air machines as ductless and VRF becomes more popular.
The type shown above is most often used in kitchens where there is significant exhaust air. When air is exhausted you must add back outdoor air to achieve balanced ventilation. In the past, we would see many restaurants and grocery stores pump in unconditioned makeup air into the kitchens and then we would see moisture issues and all that entails in humid climates as well as hot kitchens.
By filtering, cooling and dehumidifying the air as it travels through the makeup air intake we can control the temperature and dewpoint of the air and reduce these issues.
The refrigerant side of this particular type of machine functions very simply. It uses an air pressure switch to “prove” sufficient negative air pressure which acts as an indication to the cooling equipment that sufficient airflow is present. It then uses a thermostatic control that measures the temperature of the incoming air to turn the equipment on and off based on heat load.
These can also be set up to work off of enthalpy which is the total heat content of the air (humidity & temperature) rather than air temperature. It functions much like an economizer in this way, it can take advantage of the outdoor air when appropriate or run cooling and dehumidification where needed.
HVAC school shows how to use the Testo Smart Probes app and Testo 605i’s from the new expanded Testo Smart Probes case. We use the kit to calculate total system BTU removal during operation. Featuring Bryan Orr.
As we talked about in an earlier podcast, a TXV is designed to maintain a specified and constant superheat at the outlet of the evaporator coil. It does this through a balance of forces between the bulb pressure (opening force), Equalizer pressure (Closing force) and spring (Closing force). It is the spring pressure that can be adjusted on some valves, but why and when would this be done?
For the quick, cut to the chase version, turning the adjustment on the bottom of an adjustable valve clockwise = higher superheat and counterclockwise = lower superheat. However, before you start messing with the adjustment, I suggest you read on.
First, the valve must be an adjustable type, many valves on small equipment are not adjustable and have no hex cap at the base.
Here are some other items you need to consider first-
Before an expansion valve can function properly and do its job, it must have a full line of properly subcooled liquid refrigerant all the way to the inlet. On a split system checking the subcooling at the condensing unit is a good start but you also need to make sure there isn’t a significant temperature drop all the way up the expansion valve inlet. Keep in mind that some valves have a screen right at the valve inlet, so a restriction even at that point will cause operational issues.
Required Pressure Drop
For an expansion valve to function there needs to be a significant pressure differential between the evaporator` design pressure and the liquid pressure entering the expansion valve (In many cases 100 PSIG +). During cooler times of the year the outdoor condensing pressure/temperature may drop to the point that the required difference in pressure may not exist and in these cases, the valve may no longer be able to maintain the target superheat. While low ambient controls may be employed to rectify the issue in some cases, in many cases, you must simply be aware that the valve will not function as expected.
Improper Bulb Placement
Ensure that the bulb is mounted on the suction line flat and tight with a proper strap. It is never a bad idea to insulate the bulb, and anytime it is exposed to ambient air it is a necessity.
When to Consider Adjustment
Now you are at the point where you can consider whether that valve could use some adjustment. First, measure the superheat right at the evaporator outlet in the same general location as the TXV bulb and equalizer in most cases the superheat at that point should be 5-10 degrees but refer to manufacturers specs when in doubt. In some cases, you will not have a pressure port at the evaporator so you must rely on a pressure reading outside. Use common sense when assessing the situation and realize that there may be some pressure drop on a 100′ line set and there should be very little in a 10′ line set. Make some allowance according to the situation.
If the system is running VERY low or VERY high suction pressure and /or superheat readings that are way out of range, it is very unlikely that adjusting the valve will remedy it. Usually, valve adjustments are only for small superheat changes up or down.
The Forces at Play
The bulb pressure is the opening force of the valve, so when the bulb is warmer it exerts more opening force resulting in a more “open” orifice, and when it’s cooler it exerts less opening force resulting in a more “closed” orifice.
The equalizer is a closing force so the higher the suction line pressure, the more the valve is forced closed and the lower the suction line pressure the more the valve is forced open.
The spring is also a closing force and on an adjustable valve increasing the spring tension/force results in lower flow and higher superheat, decreasing the spring tension/force results in more flow and lower superheat. In short, counterclockwise = lower superheat, clockwise = higher superheat.
Making an Adjustment
Before you adjust anything, the system must have been running for a good long while, and you have observed that the superheat has stabilized. You then must check the entire system and surmise that everything else is functional, the valve is being provided with a fully liquid, properly subcooled, high enough pressure feed of refrigerant. If at that point you find it is out of range then you can make adjustments.
Adjusting a TXV / TEV is an advanced skill for a technician who has a good grasp on their readings and the forces at play. Tread carefully.
P.S. – Here is a great resource from Parker / Sporlan