CRITERIA AND OPTIONS
The four main components of a vapour compression refrigeration circuit — the evaporator, the compressor, the condenser and the expansion valve — must be selected to give a balanced system.
Each of these items must
1. be suitable for the application
2. be correctly sized for the duty
3. function as required in conjunction with the other components.
A characteristic of refrigeration system design is that there are almost always several solutions to the problem, some of which may be equally cost-effective. Individuals and companies often have their own preferred approach based on 131
Experience. The system designer must examine the options that may be available in order to determine a best selection with reference to the following criteria, and there may be others.
1. Capital cost: Obviously this criterion is of major importance, but it should be noted that reducing capital cost will almost certainly result in increased running cost and carbon emissions.
2. Running cost: The cost of electricity usually represents the major cost of ownership. Other costs may include other fuels, water, spares and operating and maintenance labour. It is probable that a small extra expenditure on some items, especially heat exchangers, will reduce running costs.
3. Environmental impact: This will be related to running cost, because the major environmental effect is associated with electricity consumption. TEWI or life cycle analyses can assist in comparing various design options.
4. Fnstallation time: Installation of a new plant may cause serious disruption of the user’s ongoing business, and the extent of this disruption should be determined prior to work commencing. Apart from the installation of the equipment itself, there is the associated builders’ work and the temporary disconnection of other services.
The use of factory-built packaged equipment helps to keep installation time to a minimum.
5. Operational requirements: Most systems are now automatic in operation, but users must be aware of the control system and have facilities to run on manual control, as far as this may be possible, in the event of a control failure. Operators must understand the function of the system. If not, they will not have the confidence to work on or with it, and the plant will not be operated at its best efficiency. Also, if it breaks down for any reason, they will be unable to put it right.
6. Maintenance: Most modern equipment is almost maintenance-free but the user must be aware of the maintenance functions required (see Chapter 29), and to what extent the maintenance can be done in-house. Where maintenance is contracted out, it is preferable that the supplier should carry this out, at least for the warranty period.
7. Life expectancy: This is normally 15-20 years for refrigeration systems, and somewhat less for small packaged equipment. Where the need is for a shorter period, such as a limited production run or for a temporary building, rental or second hand equipment could be considered.
When considering the options an analytical approach should be adopted to ensure correct selection. The principles to be applied are those of value analysis — to start with the basic need and no pre-conceived method, to consider all the different methods of satisfying the need, and to evaluate each of these objectively before moving towards a choice.
The details of such an approach will vary considerably, and the following guidelines should be taken as an indication of the factors to be considered, rather than as an exhaustive list:
1. What is the basic need?
• To cool something o a dry product
Maximum air speed? o other solid product o a liquid
What liquid? temperature range? viscosity?
• To keep something cool o a solid product
Conditions? o an enclosed space
2. What is the load and temperature?
• If at ambient, can it be done without mechanical refrigeration?
• Product cooling load?
• Heat leakage, sensible and latent?
• Convection heat gains, sensible and latent?
• Internal heat gains?
• Time required?
• Position of plant?
• Same type of equipment as existing?
4. Possible methods
• Direct expansion?
O Indirect — what medium? o Part by tower water or ambient air? o Thermal storage? o Existing plant spare capacity?
• Adjacent space?
• Within cooled space?
• Maintenance access?
• Inbuilt: o water? o air?
Availability of cooling medium?
• Maintenance access?
If these steps have been carried through in an objective manner, there will be at least three options for most projects, and possibly as many as five. Enquiries can now go out for equipment to satisfy the need, based on the options presented. No attempt should be made to reach a decision until these have been evaluated.
Posted in Refrigeration and Air Conditioning