var headers = {
  'X-API-KEY': 'YOUR_API_KEY',
  'Content-Type': 'application/json'
};
var request = http.Request('POST', Uri.parse('https://api.swep.net/calculateSinglePhase'));
request.body = json.encode({
  "inputs": {
    "calculationMethod": "Design",
    "selectedFluidSide1": {
      "fluidId": 53,
      "isGas": false,
      "inputValue": null
    },
    "selectedFluidSide2": {
      "fluidId": 53,
      "isGas": false,
      "inputValue": null
    },
    "coCurrent": false,
    "selectedHeatExchangers": [
      {
        "heatExchangerInputType": "Parent",
        "id": 1
      }
    ],
    "heatLoad": {
      "amount": 1500,
      "unit": "HeatLoadUnit.Kilowatt"
    },
    "inletTemperatureSide1": {
      "amount": 80,
      "unit": "TemperatureUnit.DegreeCelsius"
    },
    "inletTemperatureSide2": {
      "amount": 20,
      "unit": "TemperatureUnit.DegreeCelsius"
    },
    "outletTemperatureSide1": {
      "amount": 40,
      "unit": "TemperatureUnit.DegreeCelsius"
    },
    "outletTemperatureSide2": {
      "amount": 50,
      "unit": "TemperatureUnit.DegreeCelsius"
    },
    "flowSide1": null,
    "flowSide2": null,
    "maxPressureDropSide1": {
      "amount": 20000,
      "unit": "PressureUnit.Pascal"
    },
    "maxPressureDropSide2": {
      "amount": 20000,
      "unit": "PressureUnit.Pascal"
    },
    "numberOfPasses": null,
    "numberOfPlates": null,
    "numberOfPlatesStack1": null,
    "numberOfPlatesStack2": null,
    "oversurface": null,
    "portSwitch": true,
    "autoperformance": false
  }
});
request.headers.addAll(headers);

http.StreamedResponse response = await request.send();

if (response.statusCode == 200) {
  print(await response.stream.bytesToString());
}
else {
  print(response.reasonPhrase);
}