How to create a multirow table within a longtable?

Maybe this is what you are looking for

I have used the booktabs package to typeset this table. Your question wasn't very clear, so I just tried what I felt was good.

Code

documentclass{article}
usepackage{longtable}
usepackage{makecell}
usepackage{multirow}
newcolumntype{L}[1]{>{centeringarraybackslash}p{#1}}
usepackage{lscape}
usepackage{booktabs}
usepackage{geometry}

begin{document}
begin{landscape}
    begin{longtable}{
        c 
        >{raggedright}p{2.5cm} 
        >{raggedright}p{2.5cm}
        >{raggedright}p{2.5cm}
        >{raggedright}p{2.5cm}
        >{raggedright}p{2.5cm}
        >{raggedright}p{2.5cm}
        p{2cm}}toprule
        & multicolumn{3}{c}{scshape force} 
            & Inclination 
                & Vibration
                    & Fluidization 
                        & Other cmidrule{2-4}
%
        & multicolumn{1}{c}{Centrifugal} 
            & multicolumn{1}{c}{Gravitational}
                & multicolumn{1}{c}{Inertial} 
                    &&&& Effects midrule
%
        Size 
            & multicolumn{1}{c}{Cyclones} 
                & Counterflow vertical air raise classifiers, Zig-Zag Classifiers 
                    & These kinds of issues are mostly related to the PDF viewer, they will not appear in a print version of the document.
                        & These kinds of issues are mostly related to the PDF viewer, they will not appear in a print version of the document. 
                            & Cyclones, Rotating Wheel Air-Classifiers, High-Efficiency Separator, Turbo-air Classifier, Circulating air-classifiers. 
                                & &  cmidrule{1-1}cmidrule{3-6}
        Density 
            & & aplha
                & echo 
                    & charlie 
                        & bravo 
                            & & midrule
        Shape 
            & These kinds of issues are mostly related to the PDF viewer, they will not appear in a print version of the document.
                & These kinds of issues are mostly related to the PDF viewer, they will not appear in a print version of the document.
                    & These kinds of issues are mostly related to the PDF viewer, they will not appear in a print version of the document.
                        & These kinds of issues are mostly related to the PDF viewer, they will not appear in a print version of the document.
                            & These kinds of issues are mostly related to the PDF viewer, they will not appear in a print version of the document.
                                & &  bottomrule
    end{longtable}
end{landscape}
end{document}

tip: avoid using vertical lines in tables :)

Welcome to TeX.SE

It is not clear what is your problem (beside that table is to wide for fitting on pages). You may consider the following a wee bit changes of your code:

documentclass{article}
usepackage[margin=25mm]{geometry}
usepackage{ragged2e}
usepackage{longtable, makecell, multirow}
    newcolumntype{L}[1]{>{RaggedRighthspace{0pt}}p{#1}}
usepackage[table, svgnames]{xcolor}

begin{document}
begingroup
    footnotesize
    setlengthtabcolsep{3pt}
    setlengthextrarowheight{2pt}
begin{longtable}{| c | *{7}{L{20mm}|} }
    hline
    rowcolor{Gainsboro!60}
    & makecell{Centrifugal  force}  
        & makecell{Gravitational  force} 
            & makecell{Inclination} 
                & makecell{Vibration} 
                    & makecell{Fluidization} 
                        & makecell{Inertial  force} 
                            & makecell{Other  Effects}       
    hline
endhead
% table body
Size & multirow[t]{12}{=}{Cyclones}
        & Counter flow vertical air raise classifiers, Zig-Zag Classifiers
            & These kind of issues are mostly related to the PDF viewer, they will not appear in a print version of the document.
                & These kind of issues are mostly related to the PDF viewer, they will not appear in a print version of the document.
                    & Cyclones, Rotating Wheel Air-Classifiers, High-Efficiency Separator, Turbo-air Classifier, Circulating air-classifiers.
                        &   &   
   cline{1-1}cline{3-6}
Density 
    &   & makecell{alpha} 
            & makecell{echo}
                & makecell{charlie} 
                    & makecell{bravo}
                        &   &   
  hline
Shape 
    & These kind of issues are mostly related to the PDF viewer, they will not appear in a print version of the document. 
        & These kind of issues are mostly related to the PDF viewer, they will not appear in a print version of the document. 
            & These kind of issues are mostly related to the PDF viewer, they will not appear in a print version of the document. 
                & These kind of issues are mostly related to the PDF viewer, they will not appear in a print version of the document. 
                    & These kind of issues are mostly related to the PDF viewer, they will not appear in a print version of the document. 
                        &   &   
   hline
end{longtable}
endgroup
end{document}