Total Heat of Rejection – Short #152

In this short podcast, Bryan talks about THOR, total heat of rejection. He explains what it is and why we should care about it when working on HVAC/R systems. THOR is another aspect of pressure-enthalpy calculations, along with net refrigeration effect (NRE) and total heat of compression.

When we talk about system capacity, we’re often referring to heat absorbed in the evaporator coil (NRE).  Heating is on the opposite side of the coin; when we bring heat into a home, we care more about how much heat is rejected than absorbed. That’s where THOR comes in.

More heat is rejected at the condenser than absorbed in the evaporator. The total heat content increases due to additional heat being absorbed in the suction line. Compressors also have motors that aren’t 100% efficient, so a bit of inefficiency also adds a small amount of heat to the refrigerant (in a system operating normally). All of that heat adds up to the total heat of rejection (THOR).

Even though a higher total heat of rejection is desirable when we want heat pumps to bring heat into the home, we don’t want our compression ratios and discharge temperatures to get too high. We have to avoid oil breakdown and other negative effects. So, modern heat pumps use variable frequency drive technologies or liquid or vapor injection to get a lot of capacity out of the compressor without overheating it.

The effective THOR only happens in the condenser. Some heat rejection may occur in the discharge line, but none of that is of use to us when we need to bring heat indoors.

Check out Eugene Silberstein’s book, Pressure Enthalpy Without Tears, at https://escogroup.org/shop/itemdetail.aspx?ID=1445.

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