Pressure drop in an evaporator coil.

[pankajnalamwar]

Dear Technocrats,
I have designed air source heat pump for water heating, the technical details of it are as follows-

Refrigerant used = 134a
Cooling capacity = 32.4 kW
Heating capacity = 47.62 kW
Evaporating temperature = 10 deg. centigrade
Condensing temperature = 70 deg. centigrade
Suction superheat = 8 K
Subcooling = 5 K

In this system I have used fin tube type evaporator, while testing the system it is observed that the expansion valve outlet pressure (i,e evaporator coil inlet pressure) is 80 psig which is much more than the designed pressure of 46 psig and evaporator coil outlet pressure (Suction pressure) is 30 psig which is less than designed pressure.

So to check why it is happening I used a plated type heat exchanger instate of fin tube type evaporator coil then it is found that the expansion valve outlrt pressure and the suction pressure are as per the design i,e 46 psig.

Kindly guide me why the pressure is getting dropped in fin tube type evaporator coil.

[Brian_UK]

Tube resistance.

[The MG Pony]

as said above, thats why you need an externaly equalised valve when doing tube types.

[sbkvelocity]

You said its a heat pump? Water to Water system or Air to Water? since you have designed it for 10DegC Evapurating Temp, it needs be maintained across the Evp Heat exchanger to have steady Suction Pressure. Usually in Air to Water system, Variable Fan resolves this issue, for Water to Water you will need to have modulation valve to control Suction Pressures.