Ventilation in Operating Theatre
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Energy Consumption of Ventilation in OT
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A Brief Note on the Energy Consumption for Operating Theatre Ventilation Systems

by Agne Nilsson

1.0 Abstract
Operating theatre ventilation systems are expensive to operate in terms of energy costs. This is due mainly to the high volumetric flow rate caused by the operating theatre ventilation systems. Minimum air volumes are usually fixed by the room loads or fresh air requirements. However, in any given operating theatre with the conventional laminar airflow (LAF) type of ventilation system installed, the air volumes have to be increased dramatically in the hope to achieve uni-directional airflow. This, obviously, has great impacts on the healthcare facility operating costs.

2.0 Introduction
Certain ventilation systems need to achieve an extremely high air change rate within the clean zone so as to ensure uni-directionality of supply airflow. This airflow type is commonly known as the laminar airflow (LAF), which is a conventional type of ventilation systems installed in many healthcare facilities.

The Johnson Medical Guided AirflowTM Ventilation system has its unique design whereby the supply air is flowing diagonally from the supply air surface towards the operating table. The supply air stream is guided with a slit injection booster fan system. Medical instrument-laying table is continuously flushed with fresh/sterile air. All “dirty” air is being exhausted out on the contaminated side of the operating theatre. The Johnson Medical Guided AirflowTM Ventilation system is designed in such a way that low bacterial count in the operating theatre can be achieved without the expense of high operating costs. The supply air volumetric flow rate is low and yet the requirements as outlined in HTM 2025 are fully complied with.

The UK standard HTM 2025: Ventilation in Healthcare Premises Design Considerations states that the energy consumption for air-conditioning ventilation systems is dependent upon external conditions, internal control bands, volume of airflow and method of zoning and control. The following investigates the energy consumption for the conventional LAF systems and the Johnson Medical Guided AirflowTM Ventilation system in relation to volume of airflow.

3.0 Energy Consumption for Operating Theatre Ventilation System
This section shows the comparison of energy consumption for conventional LAF systems and the Johnson Medical Guided AirflowTM Ventilation system. Although the factors as mentioned in Section 2.0 can significantly influence energy consumption, the main concern in this technical bulletin is to discuss on the impact of airflow rate only on energy consumption. This is the unique difference between the conventional LAF systems and the Johnson Medical Guided AirflowTM Ventilation system.

In simple terms, the cooling load can be determined by using the following equation:

Cooling Load (kW) = ...(3a)

Cooling Load (kW) = ...(3b)

= Mass of air flow rate,
= Change of enthalpy,
= Air volume flow rate,
= Air density,
= Specific heat,
= Temperature difference,
**Neglecting all other heat gains and heat losses.

From equation (3a) and (3b), it is assumed that the value of air density and specific heat will be constants. Hence, the cooling load will only be affected by the ‘air volume flow rate’ for a particular temperature difference of interest.

Reference [3], [4] and [5] reported the tests and evaluations on different ventilation systems conducted by Professor C. Allander and Professor B. Ljungkvist at the Royal Institute of Technology, Stockholm, Sweden. The results and findings have been summarized in the following graph.

Graph 1: Comparison of different ventilation systems vs. Johnson Medical Guided Airflow Ventilation System

From Graph 1, by comparing air volume flow rate for a particular temperature difference of interests, the Johnson Medical Guided AirflowTM Ventilation system will only require 1800m3/h (for a clean area coverage of ~ 33m2), which is 1/5 of the required air volume flow rate for Laminar Airflow systems (for a clean area coverage of ~ 7.8m2) in order to achieve low bacterial (cfu) count.

To achieve consistently low bacterial count independent of activity in the operating theatre, the LAF systems require a massive airflow of 9000 m3/h, which resulted in high energy consumption and consequential high operating costs. Ljungqvist and Öhlund [6] also reported that the airflow rate for LAF systems have to be increased from a normal 2000 m3/h to 9000 m3/h to obtain a steady reading of 0.2 cfu/m3. These LAF systems are not only expensive to install, they are also expensive to run, maintain and often give rise to noise problems and a non-ideal working environment for medical staff.

In contrast, the Johnson Medical Guided AirflowTM Ventilation Systems are capable to achieve a bacteria count of 0.5 cfu/m3 with an exceptionally low airflow rate of 1800 m3/h. The results were obtained irrespective of the level of activity in the operating theatre. The Johnson Medical Guided AirflowTM Ventilation System is employing the slit injection technology, which has proven to be highly efficient. In 1990, Ljungquist and Hillerbrandt [7] presented in the International Symposium on Contamination Control, Zurich, Switzerland, that among all the air diffuser systems tested, the best results were obtained with the supply-air systems with slit injection. Better adaptation to the work routines of the operating theatre has also been achieved in the slit injection supply-air systems. One example of this is that the instrument-laying table is always flushed with sterile air from the air circulation point of view.

Bibliography & References
  1. HTM 2025. Ventilation in healthcare premises design considerations.
  2. Carrier system design manual. Carrier air conditioning company, Syracuse, New York.
  3. Ljungkvist, B., öhlund, L. Tilluftssystem med spalt-inblåsning för operationssalar.
  4. Ljungkvist, B., öhlund, L. Supply air system with slit injection for operating theatres. (Translated version of reference [3])
  5. Johnson Medical Technical Bulletin (Republished 2002). Ventilation systems in operating theatres, aspects to consider.
  6. Ljungkvist, B., öhlund, L. Luftspaltsystem med spalt inblåsning för OP-salar. Tidskiften Sjukhuset4/86.
  7. Ljungquist, B., Hillerbrandt, B. Comparison between three air distribution systems for operating rooms. International Committee of Contamination Control Societies (ICCCS). 10th International Symposium on Contamination Control (ICCCS 90), Zurich, Switzerland, September 1990.

Agne Nilsson, Biomed. Eng., MTF, SFAI – Hospital and Research Engineer, Inventor

Agne Nilsson, originally from Sweden, is the Technical Director for Johnson Medical Technology (JMT). He is the founding member of the Swedish Society of Biomedical Engineering and Medical Physics. He also holds membership for the Scandinavian Society of Anaestesiologists and Swedish Society of Inventors. He is the author of the book ‘Medical Equipment Planning’.

Agne Nilsson works on hospital-related projects and has invented and designed a number of systems and medical equipment over the past 30 years. He was awarded the 1986 Development Stipendium by the National Development Foundation. He has extensive international experience on hospital planning. For the past 5 years, he has been working on providing functional and conceptual designs for different government and private projects and departments of hospitals in Malaysia.