Energy balance in evaporator. Thermal load and heat transfer rate

[mrr]

Hi,

I have a question regarding energy balance in an evaporator. We have an experimental setup of a single stage vapor compression system. The evaporator is a B15THx20 SWEP heat exchanger, and the fluid to cool is a glycol solution in a small tank where we have a variable resistance to control the thermal load. The glycol solution is pumped constantly from the tank to the evaporator at a rate of about 12 l/min.

I want to calculate the heat transfer rate. To my knowledge, provided the system reach a stable point, the energy balance should be the following:

Q = m'_ref * deltaH = m'_glycol * c * delta_T = resistancePower

where:
Q = heat transfer rate (kW)
m'_ref = mass flowrate of the refrigerant (kg/s)
deltaH = specific enthalpy difference between the outlet and inlet of the evaporator (refrigerant side) (kJ/kg)

m'_glycol = mass flowrate of the refrigerant (kg/s)
c = specific heat capacity (kJ/kg/K)
delta_T = temperature difference between inlet and outlet of the evaporator (glycol side) (K)

resistancePower =imposed thermal load (kW)

I have done several (a lot) of tests by analyzing a lot of stable points, modifying the thermal load, the evaporating temperature, condensing pressures, etc.

Given the test I did a data logging and I calculated Q based on the previous equations. I was expecting to get similar values to the imposed thermal load, but that does not happen. For example, if I impose 1 kW, I was expecting the term m'_ref * deltaH to be about 1 kW, and the same for the glycol side. However, I always get about 80-100% greater value of the calculated heat transfer rate, i.e, if I impose a thermal load of 1 kW I get about 2 kW from the refrigerant side calculation (m'_ref * deltaH). Regarding the glicol side the measurement is more noisy (my delta_T is between 1 and 3 ºC and I have NTC probes which can give a +-0.5 ºC ) but I also get higher values.

Thus, There is always a positive offset in the calculation of the heat transfer rate in the evaporator (refrigerant and glycol side) with respect to the imposed thermal load. The calculated heat transfer rate seems to be bigger than the imposed thermal load. What really intrigues me is that the aforementioned offset happens in both sides of the evaporator, and I am pretty sure we are controlling well the imposed thermal load in the resistance since we have measured the voltage and current across it (it is a three-phase resistance controlled by a 4-20 mA signal). What am i missing?

Attached is a figure where I show the problem.

EnergyBalance.PNG

As you can see in the figure, and as I said before, it seems there is a constant offset. The question is, provided there are no errors in the measurement (ideal case) am I right in the energy balance equation or am I missing something else? I could expect some losses but not twice the imposed thermal load.

Looking forward to your comments.

PS: we have a direct measurement of the refrigerant flow rate and enthalpies are calculated by using pressure probes at the evaporator input and condenser output.
PS2: The hydraulic pump that recirculates the glycol solution has an electrical power of 0.2 kW. I suppose the heat it transfers due to friction to the glycol solution is negligible.