The following example demonstrates clearing the console window by using the screensize information passed back from the Windows API function GetConsoleScreenBufferInfo() to write several blank lines.

Example:
program Project ;
{$APPTYPE CONSOLE}
uses
  Windows;
{$R *.RES}
var
  sbi : TConsoleScreenBufferInfo;
  i : integer;
begin
  Writeln('A Console Applicaiton');
  Writeln('Press Enter To Clear The Screen');
  GetConsoleScreenBufferInfo(GetStdHandle(STD_OUTPUT_HANDLE),
                             sbi);
  Readln;
  GetConsoleScreenBufferInfo(GetStdHandle(STD_OUTPUT_HANDLE),
                             sbi);
  for i := 0 to sbi.dwSize.y do
    writeln;
  Writeln('Press Enter To End');
  Readln;
end.