pgfplot annotation spacing and arrow length

Question

I am trying to make the following figure using pgfplots.

\documentclass{article}

\usepackage{pgfplots}
    \pgfplotsset{width=8cm,compat=1.5.1}
    \usetikzlibrary{decorations.markings}    % Arrows

\usepackage{caption}

\begin{document}

\noindent
\begin{minipage}{\linewidth}

    \centering
    \begin{tikzpicture}[scale=1]
    \begin{axis}[ axis lines=left,
                  xmin=-.1, xmax=1.9,   ymin=-1, ymax=22,
                  xlabel={$v$},         ylabel={$p$},
                  xtick =\empty,        ytick=\empty,
                ]
        % Compression (Work in)
        \addplot[blue, domain=0.85:0.1,    samples = 20, postaction={decorate},
                decoration={markings, mark=at position 0.6 with {\arrow{>}}}
               ] { (0.85/x)^(1.4) }
               [sloped, font=\small]
               node[below, pos=0.7] {$s =$ const.};

        % Heat addition
        \addplot[ red, postaction={decorate},
                  decoration={markings, mark=at position 0.65 with {\arrow{>}}}
                ]
                coordinates{ (0.1,20) (0.2,20)}
                [ every node/.style={font=\small, text = black} ]
                    node[ left, pos = 0] {2}
                    node[right, pos = 1] {3};

        % Expansion (Work out)
        \addplot[ blue,domain=0.2:1.7, samples = 40, postaction={decorate},
                  decoration={markings, mark=at position 0.32 with {\arrow{>}}}
                ] { (1.7/x)^(1.4) }
               [sloped, font=\small]
               node[above, pos=0.3] {$s =$ const.};

        % Heat removal
        \addplot[ red, postaction={decorate},
                  decoration={markings, mark=at position 0.6 with {\arrow{>}}}
                ]
                coordinates{ (1.7,1) (0.85,1) }
                [ every node/.style={font=\small, text = black} ]
                    node[below, pos = 0] {4}
                    node[below, pos = 1] {1};

        \draw[blue, -latex, line width = 1pt] (axis cs:0.15,5.5) -- (axis cs:0.35,7)
        node[anchor = north east, pos=0.5, font=\footnotesize] {$\dot{W}_{\textrm{in}}$};

        \draw[blue, -latex, line width = 1pt] (axis cs:0.4,5.5) -- (axis cs:0.6,7)
        node[anchor = west, pos=0.9, font=\footnotesize] {$\dot{W}_{\textrm{out}}$};

    \end{axis}
    \end{tikzpicture}
    \captionof{figure}{Ideal Brayton cycle}
\end{minipage}

\end{document}

1. How can I get the annotation arrows (the one that indicate power e.g. \dot{W} to be of same length?
2. Right now I am specifying the start and end point co-ordiantes of the arrows. This means that if I need to move the arrow, I need to tweak the co-ordinates correctly. Is there a way to specify based on pos on the curve like that other annotations?
3. How can I adjust the spacing between the s = const. annotation and the corresponding curve? I would like to increase the spacing from the 3-to-4 path.

Answer 1

For 1. and 2.: using a technique exposed by Alain Matthes in one of his answers (that I cannot find right now), I placed a node with south west anchor along each one of the blue paths using the pos= key; then I used those nodes to draw normal (orthogonal) lines; the length was increased using the shorten >= and shorten <= options. In this way, there's no need to use explicit coordinates and the length for both arrows is the same.

For 3: you can use the yshift= option to the nodes control the spacing.

\documentclass{article}
\usepackage{pgfplots}
\pgfplotsset{width=8cm,compat=1.5.1}
\usetikzlibrary{decorations.markings}    % Arrows
\usepackage{caption}

\begin{document}

\noindent
\begin{minipage}{\linewidth}

\centering
\begin{tikzpicture}[scale=1]
\begin{axis}[ axis lines=left,
  xmin=-.1, xmax=1.9,   ymin=-1, ymax=22,
  xlabel={$v$},         ylabel={$p$},
  xtick =\empty,        ytick=\empty,
]
        % Compression (Work in)
\addplot[blue, domain=0.85:0.1,    samples = 20, postaction={decorate},
  decoration={markings, mark=at position 0.6 with {\arrow{>}}}
] 
{ (0.85/x)^(1.4) }
  [sloped, font=\small]
  node[below, pos=0.7,yshift=-3pt] {$s =$ const.} node[above,pos=0.15,anchor=south west] (L) {};

        % Heat addition
\addplot[ red, postaction={decorate},
  decoration={markings, mark=at position 0.65 with {\arrow{>}}}
] 
coordinates{ (0.1,20) (0.2,20)}
  [ every node/.style={font=\small, text = black} ]
  node[ left, pos = 0] {2}
  node[right, pos = 1] {3};

        % Expansion (Work out)
\addplot[ blue,domain=0.2:1.7, samples = 40, postaction={decorate},
  decoration={markings, mark=at position 0.32 with {\arrow{>}}}
] 
{ (1.7/x)^(1.4) }
  [sloped, font=\small]
  node[above, pos=0.3,yshift=5pt] {$s =$ const.} node[above,pos=0.7,anchor=south west] (M) {};

        % Heat removal
\addplot[ red, postaction={decorate},
  decoration={markings, mark=at position 0.6 with {\arrow{>}}}
]
coordinates{ (1.7,1) (0.85,1) }
  [ every node/.style={font=\small, text = black} ]
  node[below, pos = 0] {4}
  node[below, pos = 1] {1};

\draw[-latex,blue,line width = 1pt,shorten >= -5pt,shorten <=-5pt] 
  (M.south west) -- (M.north west) 
  node[anchor = west, font=\footnotesize,xshift=3pt] {$\dot{W}_{\textrm{out}}$};        

\draw[-latex,blue,line width = 1pt,shorten >= -5pt,shorten <=-5pt] 
    (L.south west) node[anchor = east, font=\footnotesize,xshift=-3pt] {$\dot{W}_{\textrm{in}}$} 
  -- (L.north west);        

\end{axis}
\end{tikzpicture}
\captionof{figure}{Ideal Brayton cycle}
\end{minipage}

\end{document}