diff --git a/Classes/PHPExcel.php b/Classes/PHPExcel.php
index ec98ad03..901cb16d 100644
--- a/Classes/PHPExcel.php
+++ b/Classes/PHPExcel.php
@@ -42,6 +42,13 @@ if (!defined('PHPEXCEL_ROOT')) {
  */
 class PHPExcel
 {
+    /**
+     * Unique ID
+     *
+     * @var string
+     */
+    private $_uniqueID;
+
     /**
      * Document properties
      *
@@ -63,6 +70,13 @@ class PHPExcel
      */
     private $_workSheetCollection = array();
 
+	/**
+	 * Calculation Engine
+	 *
+	 * @var PHPExcel_Calculation
+	 */
+	private $_calculationEngine = NULL;
+
     /**
      * Active sheet index
      *
@@ -103,6 +117,9 @@ class PHPExcel
      */
     public function __construct()
     {
+		$this->_uniqueID = uniqid();
+		$this->_calculationEngine	= PHPExcel_Calculation::getInstance($this);
+
         // Initialise worksheet collection and add one worksheet
         $this->_workSheetCollection = array();
         $this->_workSheetCollection[] = new PHPExcel_Worksheet($this);
@@ -127,6 +144,14 @@ class PHPExcel
     }
 
 
+    /**
+     * Destroy this workbook
+     */
+    public function __destruct() {
+        PHPExcel_Calculation::unsetInstance($this);
+        $this->disconnectWorksheets();
+    }    //    function __destruct()
+
     /**
      * Disconnect all worksheets from this PHPExcel workbook object,
      *    typically so that the PHPExcel object can be unset
@@ -141,6 +166,16 @@ class PHPExcel
         $this->_workSheetCollection = array();
     }
 
+	/**
+	 * Return the calculation engine for this worksheet
+	 *
+	 * @return PHPExcel_Calculation
+	 */
+	public function getCalculationEngine() {
+
+		return $this->_calculationEngine;
+	}	//	function getCellCacheController()
+
     /**
      * Get properties
      *
@@ -824,12 +859,14 @@ class PHPExcel
             foreach ($sheet->getColumnDimensions() as $columnDimension) {
                 $columnDimension->setXfIndex( $map[$columnDimension->getXfIndex()] );
             }
-        }
 
-        // also do garbage collection for all the sheets
-        foreach ($this->getWorksheetIterator() as $sheet) {
+        	// also do garbage collection for all the sheets
             $sheet->garbageCollect();
         }
     }
 
+    public function getID() {
+        return $this->_uniqueID;
+    }
+
 }
diff --git a/Classes/PHPExcel/Calculation.php b/Classes/PHPExcel/Calculation.php
index 254a4e9c..a2f53721 100644
--- a/Classes/PHPExcel/Calculation.php
+++ b/Classes/PHPExcel/Calculation.php
@@ -96,13 +96,29 @@ class PHPExcel_Calculation {
 	private static $_instance;
 
 
+	/**
+	 * Instance of the workbook this Calculation Engine is using
+	 *
+	 * @access	private
+	 * @var PHPExcel
+	 */
+    private $_workbook;
+
+	/**
+	 * List of instances of the calculation engine that we've instantiated
+	 *
+	 * @access	private
+	 * @var PHPExcel_Calculation[]
+	 */
+    private static $_workbookSets;
+
 	/**
 	 * Calculation cache
 	 *
 	 * @access	private
 	 * @var array
 	 */
-	private static $_calculationCache = array ();
+	private $_calculationCache = array ();
 
 
 	/**
@@ -111,7 +127,7 @@ class PHPExcel_Calculation {
 	 * @access	private
 	 * @var boolean
 	 */
-	private static $_calculationCacheEnabled = true;
+	private $_calculationCacheEnabled = true;
 
 
 	/**
@@ -1671,12 +1687,18 @@ class PHPExcel_Calculation {
 
 
 
-	private function __construct() {
+	private function __construct(PHPExcel $workbook = NULL) {
 		$setPrecision = (PHP_INT_SIZE == 4) ? 12 : 16;
 		$this->_savedPrecision = ini_get('precision');
 		if ($this->_savedPrecision < $setPrecision) {
 			ini_set('precision',$setPrecision);
 		}
+
+		if ($workbook !== NULL) {
+			self::$_workbookSets[$workbook->getID()] = $this;
+		}
+
+		$this->_workbook = $workbook;
 	}	//	function __construct()
 
 
@@ -1702,7 +1724,14 @@ class PHPExcel_Calculation {
 	 * @access	public
 	 * @return PHPExcel_Calculation
 	 */
-	public static function getInstance() {
+	public static function getInstance(PHPExcel $workbook = NULL) {
+		if ($workbook !== NULL) {
+    		if (isset(self::$_workbookSets[$workbook->getID()])) {
+    			return self::$_workbookSets[$workbook->getID()];
+    		}
+			return new PHPExcel_Calculation($workbook);
+		}
+
 		if (!isset(self::$_instance) || (self::$_instance === NULL)) {
 			self::$_instance = new PHPExcel_Calculation();
 		}
@@ -1710,6 +1739,13 @@ class PHPExcel_Calculation {
 		return self::$_instance;
 	}	//	function getInstance()
 
+	public static function unsetInstance(PHPExcel $workbook = NULL) {
+		if ($workbook !== NULL) {
+    		if (isset(self::$_workbookSets[$workbook->getID()])) {
+    			unset(self::$_workbookSets[$workbook->getID()]);
+    		}
+		}
+    }
 
 	/**
 	 * Flush the calculation cache for any existing instance of this class
@@ -1718,10 +1754,8 @@ class PHPExcel_Calculation {
 	 * @access	public
 	 * @return null
 	 */
-	public static function flushInstance() {
-		if (isset(self::$_instance) && (self::$_instance !== NULL)) {
-			self::$_instance->clearCalculationCache();
-		}
+	public function flushInstance() {
+		$this->clearCalculationCache();
 	}	//	function flushInstance()
 
 
@@ -1792,7 +1826,7 @@ class PHPExcel_Calculation {
 	 * @return boolean
 	 */
 	public function getCalculationCacheEnabled() {
-		return self::$_calculationCacheEnabled;
+		return $this->_calculationCacheEnabled;
 	}	//	function getCalculationCacheEnabled()
 
 
@@ -1803,7 +1837,7 @@ class PHPExcel_Calculation {
 	 * @param boolean $pValue
 	 */
 	public function setCalculationCacheEnabled($pValue = true) {
-		self::$_calculationCacheEnabled = $pValue;
+		$this->_calculationCacheEnabled = $pValue;
 		$this->clearCalculationCache();
 	}	//	function setCalculationCacheEnabled()
 
@@ -1828,7 +1862,7 @@ class PHPExcel_Calculation {
 	 * Clear calculation cache
 	 */
 	public function clearCalculationCache() {
-		self::$_calculationCache = array();
+		$this->_calculationCache = array();
 	}	//	function clearCalculationCache()
 
 
@@ -2206,7 +2240,7 @@ class PHPExcel_Calculation {
 		//	Disable calculation cacheing because it only applies to cell calculations, not straight formulae
 		//	But don't actually flush any cache
 		$resetCache = $this->getCalculationCacheEnabled();
-		self::$_calculationCacheEnabled = false;
+		$this->_calculationCacheEnabled = FALSE;
 		//	Execute the calculation
 		try {
 			$result = self::_unwrapResult($this->_calculateFormulaValue($formula, $cellID, $pCell));
@@ -2215,7 +2249,7 @@ class PHPExcel_Calculation {
 		}
 
 		//	Reset calculation cacheing to its previous state
-		self::$_calculationCacheEnabled = $resetCache;
+		$this->_calculationCacheEnabled = $resetCache;
 
 		return $result;
 	}	//	function calculateFormula()
@@ -2250,15 +2284,15 @@ class PHPExcel_Calculation {
 		}
 		// Is calculation cacheing enabled?
 		if ($cellID !== NULL) {
-			if (self::$_calculationCacheEnabled) {
+			if ($this->_calculationCacheEnabled) {
 				// Is the value present in calculation cache?
 				$this->_writeDebug('Testing cache value for cell ', $cellID);
 //				echo 'Testing cache value<br />';
-				if (isset(self::$_calculationCache[$wsTitle][$cellID])) {
+				if (isset($this->_calculationCache[$wsTitle][$cellID])) {
 //					echo 'Value is in cache<br />';
 					$this->_writeDebug('Retrieving value for ', $cellID, ' from cache');
 					// Return the cached result
-					$returnValue = self::$_calculationCache[$wsTitle][$cellID];
+					$returnValue = $this->_calculationCache[$wsTitle][$cellID];
 //					echo 'Retrieving data value of '.$returnValue.' for '.$cellID.' from cache<br />';
 					if (is_array($returnValue)) {
 						$returnValue = PHPExcel_Calculation_Functions::flattenArray($returnValue);
@@ -2294,8 +2328,8 @@ class PHPExcel_Calculation {
 
 		// Save to calculation cache
 		if ($cellID !== NULL) {
-			if (self::$_calculationCacheEnabled) {
-				self::$_calculationCache[$wsTitle][$cellID] = $cellValue;
+			if ($this->_calculationCacheEnabled) {
+				$this->_calculationCache[$wsTitle][$cellID] = $cellValue;
 			}
 		}
 
diff --git a/Classes/PHPExcel/Cell.php b/Classes/PHPExcel/Cell.php
index 2e9cd327..ea37c7ec 100644
--- a/Classes/PHPExcel/Cell.php
+++ b/Classes/PHPExcel/Cell.php
@@ -288,7 +288,7 @@ class PHPExcel_Cell
 		if ($this->_dataType == PHPExcel_Cell_DataType::TYPE_FORMULA) {
 			try {
 //				echo 'Cell value for '.$this->getCoordinate().' is a formula: Calculating value<br />';
-				$result = PHPExcel_Calculation::getInstance()->calculateCellValue($this,$resetLog);
+				$result = $this->getParent()->getParent()->getCalculationEngine()->calculateCellValue($this,$resetLog);
 //				echo $this->getCoordinate().' calculation result is '.$result.'<br />';
 			} catch ( PHPExcel_Exception $ex ) {
 				if (($ex->getMessage() === 'Unable to access External Workbook') && ($this->_calculatedValue !== NULL)) {
@@ -859,11 +859,11 @@ class PHPExcel_Cell
 	 */
 	public static function compareCells(PHPExcel_Cell $a, PHPExcel_Cell $b)
 	{
-		if ($a->_row < $b->_row) {
+		if ($a->getRow() < $b->getRow()) {
 			return -1;
-		} elseif ($a->_row > $b->_row) {
+		} elseif ($a->getRow() > $b->getRow()) {
 			return 1;
-		} elseif (self::columnIndexFromString($a->_column) < self::columnIndexFromString($b->_column)) {
+		} elseif (self::columnIndexFromString($a->getColumn()) < self::columnIndexFromString($b->getColumn())) {
 			return -1;
 		} else {
 			return 1;