What to Consider When Selecting DHS for Your Pharma Applications?

What is a DHS?

Dry Heat Sterilizer - DHS is an equipment used to sterilize items using hot air as a medium. The hot air at high temperatures is continuously circulated inside the chamber. When it comes in contact with the objects inside, it kills microorganisms, spores, viruses, bacteria, and other harmful bodies. 

DHS is used to sterilize objects that are sensitive to moisture and can provide temperatures around  250⁰C enough temperature to kill harmful objects. Like an autoclave, the standard sterilization temperature is preset and cannot be changed.

If temperature change is required, it can only occur after thorough tests and validation studies.

DHS consists of many sub-components that individually perform different functions to collectively sterile the items inside. It includes

• Heater - The heater is used to heat the air flowing into the chamber, increasing the chamber’s temperature.

• Chamber - The chamber is a part where items to be sterilized are placed.

• Blower - A blower, driven by motor is used to generate airflow. When the hot airflow flows into the chamber, it raises the temperature used for Sterilization.

Like an autoclave, DHS performs Sterilization in three stages: pre-, Sterilization, and post-heat. 

Pre - Heat stage is the initial stage used to increase the chamber temperature to  facilitate the sterilization process. In this stage, hot air is continuously blown into the chamber. This stage continues until sterilization temperature achieved inside the chamber.

After this stage, he DHS shifts to the sterilization phase automatically

In the Sterilization stage, the temperature is maintained for a specified period. This is achieved by activating a timer. The value of the timer is preset and depends on the product and process requirements determined through validation studies. 

When the timer elapses, the DHS shifts to –post - heat cycle.

In the Post - Heat stage, the chamber is cooled by blowing air into it. The heaters are turned – off, and only the blower operates. The primary purpose of this cycle is to cool down items and chamber for easy handling. 

This stage continues for a specific low temperature, which is also preset. When this low temperature is achieved, the DHS automatically shuts down, and items can be offloaded for further processing.

It is necessary and critical to monitor & record sterilization temperature for its efficiency during Sterilization. The Sterilization can be monitored by

• Temperature sensors, that are placed strategically throughout the chamber. They indicate the degree to which the temperature has been achieved at every corner & part of the DHS.

• Biological indicators that indicate the killing of certain microorganisms and verified through biological testing and incubation. The organism can only be killed when specific requirements are met, which are only at certain temperatures.

What can go wrong if DHS is selected wrongly

Some possible problems if a DHS is selected wrongly include

• The Sterilization will not be performed effectively, resulting in process & product damage. In some cases, the product is recalled from the warehouse or market due to un – safe product characteristics

• The selected DHS will not fulfill current production requirements

• Quality and regulatory bodies will object to its design, and all the costs will end up loss

Some guidelines for selecting a DHS

Let’s discuss some design parameters to consider when selecting a DHS.

Commercial scale vs. Lab scale

Like other equipment, Dry Heat Sterilizers are used for both production and laboratory for different purposes. In contrast to the production department, the laboratory's scale of operation and requirement is small with small volume of items to be sterilized. Because, only items related to testing procedures, such as samples and instruments, are used and requires sterilization in lab-scale DHS. On the other hand, the production department uses large volumes of batches, and large-sized instruments are utilized.

When purchasing a Dry Heat Sterilizer, it is necessary to consider whether the DHS will be used for production or lab-scale operations. 

Lab-scale DHS has smaller dimensions and low volume (capacity). Their power requirements are less than the production scale because the temperature can be achieved in a much smaller time due to its small physical dimensions. Also, lab scale DHS requires less space to installed as compared to the production scale DHS

Required temperature

The DHS should be able to quickly achieve the desired temperature, which depends on the item being sterilized for a specific period. A DHS must reach and maintain this temperature at every corner of the equipment, without any deviations.

In the pharma industry, sterilization temperatures for DHS lie in the range of 160°C to o250°C, depending upon the product being sterilized, that can be determined by quality department.

When selecting a DHS, it is necessary to communicate your sterilization temperature requirement to the DHS manufacturer and verify if it is achievable by a DHS. 

Safety Features

Safety is a significant concern for high-temperature equipment in all industries, including pharmaceuticals. The considerable risk with high-temperature equipment is that if temperatures increase abruptly and cannot be controlled, it can damage the equipment itself , harm humans and destroy other items in the vicinity of the DHS. On the other hand, if the required temperature has not been achieved, it can damage the product and process.

Safety features are added to prevent damage to equipment, processes, and products to avoid abnormal temperature behavior.

When selecting DHS for your organization's requirement, it is necessary to consider all the safety systems, which can include but are not limited to 

• High temperature - this safety is activated when the temperature becomes higher than the preset value

• Low temperature - this safety is activated when the temperature goes far below the preset value

• Door safety – to ensure that the Door is closed before the start of operations. It also ensures that the Door does not open during the operation. If the accidentally Door is opened, it should shut down the heaters and blowers.

• Heater problem - This safety is enabled when there is a problem in the heater, such as over current, short or open circuit.

Surface Finish requirements

Like other pharma equipment, the DHS material characteristics must follow surface finish requirements; since it is a part of pharma process applications. Some common surface finish requirements, include

• No dead points

• Surface roughness requirements

• Stainless Steel - 304, because the surface of DHS does not come in direct contact with the product

• Orbital welding, where welding is done

Recording devices

A critical regulatory and quality department requirement are the process records that indicate the time vs sterilization temperature relationship. These records indicate the actual process parameters and act as a proof that process has been executed satisfactorily.

To record the time vs. temperature relationship, there are two methods -controller based and standalone.

• In controller-based, DHS controller such as Programmable Logic Controller - PLC is used to measure temperature and record in its memory. The controller then displays this relationship on the local display, such as the Human Machine Interface - HMI. A printable record can also be taken if required.

• In a standalone system, a data logger is used to measure temperature values at different points, which are recorded on the device's memory. The data logger can also display temperature on it. Additionally, a printable record can also be obtained from this system.

When considering a DHS, always select the temperature record provision to fulfill regulatory requirements. Otherwise, it will be difficult to make records and will be rejected by quality and regulatory bodies.

Door mechanism - hinge vs sliding 

There are different types of doors designed on the DHS, including but not limited to 

• Sliding Door - that slides at one side 

• Hinged Door - The door is attached to the DHS with hinges

• Single Door - the DHS is loaded and offloaded from one Door

• Double Door - Items are loaded from one side and offloaded at another side

The door configuration is selected based on available area, while single & double door DHS are chosen based on application requirement.

Validation port

Validation is a crucial process to test the design parameters of pharma equipment. It is performed by the quality department and regulatory bodies review these records during their visit. 

For DHS, validation is performed by measuring the actual temperature inside the DHS chamber and determining the design temperature deviation. The external temperature probes are inserted into the DHS chamber through the validation port. The advantage of validation port is that it prevents leakage in the chamber during the time probes are inserted into the chamber.

When selecting DHS for your applications, consider the presence of a validation port in the DHS.. if it is not checked at the time of procurement, it will become challenging to perform validation. Alternatively, making validation port at the owner site by the owner can affect the DHS physical structure and sterilization process.

Power requirements 

The most common heat source in the DHS are the heaters strategically placed around the air blower. The heaters consume more energy than other components in the DHS, significantly increasing the power consumption.

When purchasing DHS for your pharma application, it is helpful to consider the power capacity of your plant. If enough power is unavailable, arranging the required power is necessary. 

About the Author:

Muhammad Asim Niazi

Muhammad Asim Niazi has a vast experience of about 11 years in a Pharmaceutical company. During his tenure he worked in their different departments and had been part of many initiatives within the company. He now uses his experience and skill to write interested content for audiences at PharmaSources.com.