I created two classes, the first one is a Fluid, and the second one is a Heat Exchanger. 

The Heat Exchanger has to contain 2 fluids, but I don't know how to add the fluid into the exchanger...

```
Sub test()
Dim water As cFluide
Set water = New cFluide
water.Temp_init = 49.1
water.Temp_out = 36.6
water.Flow_init = 124.4

Dim HX As cHX
Set HX = New cHX

Dim Fluide_1 As cFluide
Set Fluide_1 = New cFluide

HX.Fluide_1 = water
End Sub

```

I keep getting the following error 

> 
> '91': Object variable or With block variable not set
> 
> 
> 

Would anyone have any idea on this?

Here is the definition of the cHX class:

```
Dim HXSurface As Single
Dim HXh As Single
Dim HXType As String

Dim HXFluide_1 As cFluide
Dim HXFluide_2 As cFluide

Public Property Get Surface() As Single
    Surface = HXSurface
End Property

Public Property Let Surface(S As Single)
    HXSurface = Application.WorksheetFunction.Max(0, S)
End Property

Public Property Get h() As Single
    h = HXh
End Property

Public Property Let h(h As Single)
    HXh = Application.WorksheetFunction.Max(0, h)
End Property

Public Property Get Type_HX() As String
    Type_HX = HXType
End Property

Public Property Let Type_HX(t As String)
    HXType = Application.WorksheetFunction.Max(0, t)
End Property

Public Property Get Fluide_1() As cFluide
    Fluide_1 = HXFluide_1
End Property

Public Property Let Fluide_1(f_1 As cFluide)
    HXFluide_1 = f_1
End Property

Public Property Get Fluide_2() As cFluide
    Fluide_2 = HXFluide_2
End Property

Public Property Let Fluide_2(f_2 As cFluide)
    HXFluide_2 = f_2
End Property

```