Month: August 2018

Enthalpy is easy… it’s just a state function that depends only on the prevailing equilibrium state identified by the system’s internal energy, pressure, and volume. It is an extensive quantity. Simple.

Like most things, the scientific definition is as clear as mud. In HVAC/R we use enthalpy measurement to come up with the total heat change in a fluid, whether it’s refrigerant, water or air.

That total change in heat content or enthalpy change is called Delta H (ΔH) which is just another way of saying “total heat split” and it is generally measured in BTU/lb in the US.

In air, we need to use probes that measure humidity and temperature like the HUB2 probes shown above or the Testo 605i probes in order to calculate the enthalpy of the air. Air has both the energy associated with the temperature of the air as well as the latent heat stored in the water vapor.

UEI HUB Screenshot

If you want to use the Δto calculate the total heat added or removed from the air you would then use this formula to calculate BTUs of heat added or removed from the air.

Total Heat = (H1-H2) x 4.5 x CFM

In the case above it would be

Total Heat = (29.68 – 22.77) x 4.5 x 730 (CFM we measured)

so

29.68 – 22.77 = 6.91 Δ

6.91 x 4.5 x 730 = 22,699.35 BTU/hr

This total air enthalpy change is a required part of calculating total system capacity and is a pretty simple thing to understand.

Don’t confuse ΔH (Total Heat Change) with ΔT (Temperature Difference). ΔH includes both latent and sensible heat and is a measure of heat quantity in BTU/lb while ΔT only calculates temperature difference and isn’t converted to BTUs at all.

— Bryan

A good technician uses their senses before they use diagnosis tools. Is your suction line abnormally cold? Make sure the evaporator coil isn’t frozen and inspect for obvious airflow issues like a dirty filter or evaporator coil.

Is your liquid line abnormally warm to the touch? Could be a dirty condenser, condensing fan issue or overcharge.

Listen for abnormal motor and compressor noises, watch for signs of corrosion and oil for possible leaks.

Smell for signs of burning lacquer  which can signal burned motors or controls.

Listen for a blower that sounds like a train engine (If it’s an ECM it could be an airflow restriction) .

Train your senses to spot abnormalities and you will save time and catch issues before you need to pull out tools for confirmation.

This isn’t to say that you shouldn’t use diagnostic tools, just that you will save a lot of time if you use the best tools of all first.

–Bryan

The TXV powerhead or power element is the part of the valve that sits on top of the valve to which the sensing bulb is attached. The power head provides the opening force for the valve by translating force from the bulb to a diaphragm in the element that forces the valve open.

The sensing bulb and power element contain a mixed liquid/vapor charge (under most conditions), when the bulb temperature increases the pressure in the bulb increases which opens the valve (increases the orifice size, feeding more refrigerant into the evaporator). When the sensing bulb temperature decreases the pressure in the bulb decreases which closes the valve (decreases the orifice size, feeding less refrigerant into the evaporator).

In some cases, the powerhead and bulb can lose the charge, usually due to a cracked bulb tube. This causes the bulb and element to lose all of the pressure and the expansion valve will fail closed / evaporator will starve. The result will be abnormally high suction superheat at the evaporator outlet accompanied by low suction pressure, high subcooling and often freezing at the center to the outlet of the valve (on A/C applications).

A good practice is to first confirm that you have proper subcooling all the way to the valve inlet. This means checking liquid line temperature inside and out on split system.

Next, remove the sensing bulb and warm it in your hand with panels on and the system running. With a properly functioning element, the suction pressure will increase and the superheat will decrease. If you do not get this response then either the power element has lost its charge or the valve is severely blocked.

Keep in mind that some expansion valves are field adjustable. In this case, ensure that the valve is adjusted full counterclockwise (open) before condemning the valve as failed closed.

Keep in mind that some valves have replaceable power elements, check to see if you can replace the element instead of the entire valve to say time and expense for the customer when applicable.

 

— Bryan

 

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