HEAT PUMPS PART 1 - CAPACITY AND FLOW RATE

The first thing to remember about heat pumps is that the nominal capacity Q depends on the nominal flow rate m at the nominal temperature difference between the flow and return water dT of 5 K (e.g. 35/30 temperature regime).

The thermodynamic capacity (heat) formula is:

 Q = m x c x dT [kW]

 where

- m is the symbol for mass, in this case volumetric flow rate (L/sec)

- c is the specific heat capacity of water, a constant of 4.2 kJ/kgK

- dT is the temperature difference between the flow and return water, optimally 5 K

From the above formula, the flow rate can always be calculated at a given capacity.

For example, a 9 kW heat pump has a nominal flow rate of 25.8 L/min:

m = 9 / (4.2 x 5) = 0.43 L/sec —> multiplied by 60 to get L/min —> 25.8 L/min

This is the optimal case in which the nominal capacity will be achieved. If the flow rate is 50% lower, 12.9 L/min, dT must be twice as large (10 K) to achieve 9 kW capacity, which means lower comfort and longer compressor operation (higher consumption) to achieve capacity.

It is crucial to:

1. Properly size the system and piping to ensure nominal flow rate;

2. Correctly select the circulation pump with sufficient delivery height (pressure drop) depending on the flow rate.

P.S. For calculating the flow rate in L/min at a given capacity for dT = 5 K, the quick formula is m = Q x 60 / 21 (~ 3 x Q)