\documentclass{scrreprt}

\usepackage{tikz}
\usetikzlibrary{external,calc}
\usepackage{ifthen}

\tikzset{external/system call={"C:/texlive/port/bin/win32/lualatex.exe" --shell-escape -halt-on-error -batchmode=interactive -jobname "\image" "\texsource"}}
\tikzexternalize[prefix=./]

\newcommand\WT[3]{%
  \begin{tikzpicture}[remember picture, scale=#2, rotate=#3]%
  \draw[
  %      show curve controls
  ] (-2,-8) -- (-2,8) .. controls +(0,1) and +(0,1) .. (2,8) node [pos=0.5, anchor=center] (#1_in) {} -- (2,-8) .. controls +(0,-1) and +(0,-1) .. (-2,-8) node [pos=0.5,anchor=center] (#1_out) {} ;
  
  \draw[fill=white] (-2.5,-6.5) -- (-2.5,-6) -- (2.5,-6) -- (2.5,-6.5) --cycle;
  \draw[fill=white] (-2.5,6.5) -- (-2.5,6) -- (2.5,6) -- (2.5,6.5) --cycle;
  
  \draw[fill=black] (-1.4,-6) -- ++(0,12);
  
  \draw[fill=black] (-0.84,-6) -- ++(0,12);
  
  \draw[fill=black] (-0.28,-6) -- ++(0,12);
  
  \draw[fill=black] ( 0.28,-6) -- ++(0,12);
  
  \draw[fill=black] ( 0.84,-6) -- ++(0,12);
  
  \draw[fill=black] ( 1.4,-6) -- ++(0,12);
  \end{tikzpicture}%
}

\begin{document}

\tikzexternaldisable
\begin{tikzpicture}[remember picture]

\def\figheight{7}
\def\figwidth{4}
\def\stages{7}

% Kolonnenumriss
\draw[
%show curve controls
] (-0.5*\figwidth, -0.5*\figheight) -- ++(0,\figheight) .. controls +(0,1.2) and +(0,1.2) .. 
node foreach[count=\i,] \k in {0,0.1,...,1,1} (Kopf_node\i) [pos=\k, anchor=center] {}
++(\figwidth,0) -- ++(0,-\figheight) ..controls +(0,-1.2) and +(0,-1.2) .. 
node foreach[count=\i,] \k in {0,0.1,...,1,1} (Sumpf_node\i) [pos=\k, anchor=center] {}
++(-\figwidth,0) --cycle;

%\foreach[count=\I,] \K/\nlabel in {-1/N,0/\dots,1/k+1,2/k,3/k-1,4/\dots,5/f,6/\dots,7/j+1,8/j,9/j-1,10/\dots,11/1} {
\foreach[count=\I,] \K/\nlabel in {-1/N,0/\dots,1/k,2/\dots,3/f,4/\dots,5/j,6/j-1,7/\dots} {
  \pgfmathsetmacro{\k}{\K*\figheight/(\stages)};
  \pgfmathsetmacro{\dk}{0.5*\figheight/(\stages)};
  \ifthenelse{\I > 1}{
    
    % Stufenböden
    \draw (-0.5*\figwidth,\k-0.5*\figheight) -- (0.5*\figwidth,\k-0.5*\figheight);
    
    % Pfeile für Dampf V und Flüssigkeit L
    \draw[->] (0.5-0.5*\figwidth,-0.35+\k-0.5*\figheight) -- (0.5-0.5*\figwidth,0.35+\k-0.5*\figheight) node[pos=1] (vlabel\K) {};
    \draw[<-] (-0.5+0.5*\figwidth,-0.35+\k-0.5*\figheight) -- (-0.5+0.5*\figwidth,0.35+\k-0.5*\figheight) node[pos=0] (llabel\K) {};
  }
  {};
  
  \node (nlabel\K) at (0,\k-0.5*\figheight+\dk) {};
  
};
%

% Kondensator Kopf
\node[anchor=west, inner sep=0pt] (KopfWT) at ($(Kopf_node6)+(0.5,0.75)$)  {\WT{KopfWT}{0.1}{-90}};

% Kondensator Sumpf
\node[anchor=east, inner sep=0pt] (SumpfWT) at ($(Sumpf_node6)+(-0.5,-0.75)$)  {\WT{SumpfWT}{0.1}{90}};

% Kopfprodukt -> Kondensator
\draw [->] (Kopf_node6.center) |- (KopfWT_out.center) node[pos=0, anchor=south east] (labelKopf) {};

% Sumpfprodukt -> Kondensator
\draw [->] (Sumpf_node6.center) |- (SumpfWT_out.center) node[pos=0, anchor=north west] (labelSumpf) {};

% Kondensator -> Produkt
\draw [->] (KopfWT_in.center) -- +(1.75,0) node[pos=1, anchor=south east] (KopfProd) {};

% Kondensator -> Produkt
\draw [->] (SumpfWT_in.center) -- +(-1.75,0) node[pos=1, anchor=south west] (SumpfProd) {};

\draw[dashed] let
\p1 = ($(SumpfProd.south west)+(0.75,-0.5)$),
\p2 = ($(KopfProd.north east)+(-0.75,+0.5)$),
\p3 = (nlabel2)
in
(\p1) -- ($(\x1, \y3)+(0,-0.35)$) -- ($(\x2, \y3)+(0,-0.35)$) -- (\x2,\y1) -- cycle;

\draw[dashed] let
\p1 = ($(SumpfProd.south west)+(0.75,-0.5)$),
\p2 = ($(KopfProd.north east)+(-0.75,+0.5)$),
\p3 = (nlabel4)
in
(\x1,\y2) -- ($(\x1, \y3)+(0,0.35)$) -- ($(\x2, \y3)+(0,0.35)$) -- (\p2) -- cycle;


\foreach[count=\I,] \K/\nlabel in {-1/N,0/\dots,1/k,2/\dots,3/f,4/\dots,5/j,6/\dots,7/1} {
  
  \node[anchor=center, fill=white, opacity=0.9, text opacity=1] at (nlabel\K.center) {\nlabel};
  
};

\end{tikzpicture}
  
\end{document}